[PATCH v5 0/8] man2: document "new" mount API

[PATCH v5 0/8] man2: document "new" mount API

[Aleksa Sarai]

Back in 2019, the new mount API was merged[1]. David Howells then set
about writing man pages for these new APIs, and sent some patches back
in 2020[2].

Unfortunately, these patches were never merged, which meant that these
APIs were practically undocumented for many years -- arguably this has
been a contributing factor to the relatively slow adoption of these new
(far better) APIs. For instance, I have often discovered that many folks
are unaware of the read(2)-based message retrieval interface provided by
filesystem context file descriptors.

In 2024, Christian Brauner adapted David Howell's original man pages
into the easier-to-edit Markdown format and published them on GitHub[3].
These have been maintained since, including updated information on new
features added since David Howells's 2020 draft pages (such as
MOVE_MOUNT_BENEATH).

While this was a welcome improvement to the previous status quo (that
had lasted over 6 years), speaking personally my experience is that not
having access to these man pages from the terminal has been a fairly
common painpoint.

So, this is a modern version of the man pages for these APIs, in the
hopes that we can finally (6 years later) get proper documentation for
these APIs in the man-pages project.

One important thing to note is that most of these were re-written by me,
with very minimal copying from the versions available from Christian[2].
The reasons for this are two-fold:

 * Both Howells's original version and Christian's maintained versions
   contain crucial mistakes that I have been bitten by in the past (the
   most obvious being that all of these APIs were merged in Linux 5.2,
   but the man pages all claim they were merged in different versions.)

 * As the man pages appear to have been written from Howells's
   perspective while implementing them, some of the wording is a little
   too tied to the implementation (or appears to describe features that
   don't really exist in the merged versions of these APIs).

 * The original versions of the man-pages lacked bigger-picture
   explanations of the reasoning behind the API, which would make it
   easier for readers to understand what operations are doing.

I decided that the best way to resolve these issues is to rewrite them
from the perspective of an actual user of these APIs (me), and check
that we do not repeat the mistakes I found in the originals. I have also
done my best to resolve the issues raised by Michael Kerrisk on the
original patchset sent by Howells[1].

In addition, I have also included a man page for open_tree_attr(2) (as a
subsection of the new open_tree(2) man page), which was merged in Linux
6.15.

[1]: https://lore.kernel.org/all/20190507204921.GL23075@ZenIV.linux.org.uk/
[2]: https://lore.kernel.org/linux-man/159680892602.29015.6551860260436544999.stgit@warthog.procyon.org.uk/
[3]: https://github.com/brauner/man-pages-md

Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
Changes in v5:
- `sed -i s|file descriptor based|file-descriptor-based|`.
  [Alejandro Colomar]
- fsconfig(2): use bullets instead of ordered list for workflow
  description. [Alejandro Colomar]
- mount_setattr(2): fix minor wording nit in new attribute-parameter
  subsection.
- fsopen(2): remove brackets around "message" for message retrieval
  interface description. [Alejandro Colomar]
- {move_mount,fspick}(2): fix remaining incorrect no-automount text.
  [Askar Safin]
- {fsmount,open_tree}(2): `sed -i s|MOUNT_DETACH|MNT_DETACH|g`.
  [Askar Safin]
- mount_setattr(2): fix copy-paste snafu in attribute-parameter
  subsection. [Askar Safin]
- *: clean `make -R build-catman-troff`. [Alejandro Colomar]
- *: switch to \[em]\c where appropriate.
- open_tree(2): clean up MNT_DETACH-on-close description and make it
  slightly more prominent. [Alejandro Colomar]
- open_tree(2): mention the distinction from open(O_PATH) with regards
  to automounts. Askar suggested it be put in the section about
  ~OPEN_TREE_CLONE, but the change in behaviour also applies to
  OPEN_TREE_CLONE and it looked awkward to include it in the
  dentry_open() case because O_PATH only gets mentioned in the following
  paragraph (where I've put the text now). [Askar Safin]
- {move_mount,open_tree{,_attr}}(2): fix column-width-related "make -R
  check" failures.
- *: fix remaining "make -R lint" failures.
- open_tree_attr(2): add example using MOUNT_ATTR_IDMAP.
- v4: <https://lore.kernel.org/r/20250919-new-mount-api-v4-0-1261201ab562@cyphar.com>

Changes in v4:
- `sed -i s|\\% |\\%|g`.
- Remove unneeded quotes in SYNOPSIS. [Alejandro Colomar]
- open_tree(2): fix leftover confusing usages of "attach" when referring
  to file descriptors being associated with mount objects.
- open_tree(2): rename "Anonymous mount namespaces" NOTES subsection to
  the far more informative "Mount propagation" and clean up the wording
  a little.
- open_tree_attr(2): add a code comment about
  <https://lore.kernel.org/all/20250808-open_tree_attr-bugfix-idmap-v1-0-0ec7bc05646c@cyphar.com/>
- {fsconfig,open_tree_attr}(2): use _Nullable.
- {fsmount,open_tree}(2): mention the the unmount-on-close behaviour is
  actually lazy (a-la MNT_DETACH).
- {fsconfig,mount_setattr}(2): improve "mount attributes and filesystem
  parameters" wording to make it clearer that superblock and mount flags
  are sibling properties, not the same thing.
- open_tree(2): mention that any mount propagation events while the
  mount object is detached are completely lost -- i.e., they don't get
  replayed once you attach the mount somewhere.
- fsconfig(2): fix minor grammatical / missing joining word issues.
- fsconfig(2): fix final leftover `.IR A " and " B` cases.
- fsconfig(2): explain that failed fsconfig(FSCONFIG_CMD_*) operations
  render the filesystem context invalid.
- fsconfig(2): rework the description of superblock reuse, as the
  previous text was very wrong. (Though there has been discussion about
  changing this behaviour...)
- fsconfig(2): remove misleading wording in FSCONFIG_CMD_CREATE_EXCL
  about how we are requesting a new filesystem instance -- in theory
  filesystems could take this request into account but in practice none
  do (and it seems unlikely any ever will).
- fsconfig(2): mention that key, value, and aux must be 0 or NULL for
  FSCONFIG_CMD_RECONF.
- fsmount(2): fix usage of "filesystem instance" in relation to
  fsmount() and open_tree() comparison. [Askar Safin]
- move_mount(2): "as attached" -> "as a detached" [Askar Safin]
- fspick(2): add note about filesystem parameter list being copied
  rather than reset with FSCONFIG_CMD_RECONFIGURE. [Askar Safin]
- v3: <https://lore.kernel.org/r/20250809-new-mount-api-v3-0-f61405c80f34@cyphar.com>

Changes in v3:
- `sed -i s|Co-developed-by|Co-authored-by|g`. [Alejandro Colomar]
  - Add Signed-off-by for co-authors. [Christian Brauner]
- `sed -i s|needs-mount|awaiting-mount|g`, to match the kernel parlance.
- Fix VERSIONS/HISTORY mixup in mount_attr(2type) that was copied from
  open_how(2type). [Alejandro Colomar]
- Fix incorrect .BR usage in SYNOPSIS.
- Some more semantic newlines fixes. [Alejandro Colomar]
- Minor fixes suggested by Alejandro. [Alejandro Colomar]
- open_tree_attr(2): heavily reword everything to be better formatted
  and more explicit about its behaviour.
- open_tree(2): write proper explanatory paragraphs for the EXAMPLES.
- mount_setattr(2): fix stray doublequote in SYNOPSIS. [Askar Safin]
- fsopen(2): rework structure of the DESCRIPTION introduction.
- fsopen(2): explicitly say that read(2) errors in the message retrieval
  interface are actual errors, not return 0. [Askar Safin]
- fsopen(2): add BUGS section to describe the unfortunate -ENODATA
  message dropping behaviour that should be fixed by
  <https://lore.kernel.org/r/20250807-fscontext-log-cleanups-v3-0-8d91d6242dc3@cyphar.com/>.
- fsconfig(2): add a NOTES subsection about generic filesystem
  parameters.
- fsconfig(2): add comment about the weirdness surrounding
  FSCONFIG_SET_PATH.
- {fspick,open_tree}(2): Correct AT_NO_AUTOMOUNT description (copied
  from David, who probably copied it from statx(2)) -- AT_NO_AUTOMOUNT
  applies to all path components, not just the final one. [Christian
  Brauner]
- statx(2): fix AT_NO_AUTOMOUNT documentation.
- open_tree(2): swap open(2) reference for openat(2) when saying that
  the result is identical. [Askar Safin]
- fsmount(2): fix DESCRIPTION introduction, and rework attr_flags
  description to better reference mount_setattr(2).
- {fsopen,fspick,fsmount,open_tree}(2): don't use "attach" when talking
  about the file descriptors we return that reference in-kernel objects,
  to avoid confusing readers with mount object attachment status.
- fsconfig(2): remove pidns argument example, as it was kind of unclear
  and referenced kernel features not yet merged.
- fsconfig(2): remove rambling FSCONFIG_SET_PATH_EMPTY text (which
  mostly describes an academic issue that doesn't apply to any existing
  filesystem), and instead add a CAVEATS section which touches on the
  weird type behaviour of fsconfig(2).
- v2: <https://lore.kernel.org/r/20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>

Changes in v2:
- `make -R lint-man`. [Alejandro Colomar]
- `sed -i s|Glibc|glibc|g`. [Alejandro Colomar]
- `sed -i s|pathname|path|g` [Alejandro Colomar]
- Clean up macro usage, example code, and synopsis. [Alejandro Colomar]
- Try to use semantic newlines. [Alejandro Colomar]
- Make sure the usage of "filesystem context", "filesystem instance",
  and "mount object" are consistent. [Askar Safin]
- Avoid referring to these syscalls without an "at" suffix as "*at()
  syscalls". [Askar Safin]
- Use \% to avoid hyphenation of constants. [Askar Safin, G. Branden Robinson]
- Add a new subsection to mount_setattr(2) to describe the distinction
  between mount attributes and filesystem parameters.
- (Under protest) double-space-after-period formatted commit messages.
- v1: <https://lore.kernel.org/r/20250806-new-mount-api-v1-0-8678f56c6ee0@cyphar.com>

---
Aleksa Sarai (8):
      man/man2/fsopen.2: document "new" mount API
      man/man2/fspick.2: document "new" mount API
      man/man2/fsconfig.2: document "new" mount API
      man/man2/fsmount.2: document "new" mount API
      man/man2/move_mount.2: document "new" mount API
      man/man2/open_tree.2: document "new" mount API
      man/man2/open_tree{,_attr}.2: document new open_tree_attr() API
      man/man2/{fsconfig,mount_setattr}.2: add note about attribute-parameter distinction

 man/man2/fsconfig.2       | 741 ++++++++++++++++++++++++++++++++++++++++++++++
 man/man2/fsmount.2        | 231 +++++++++++++++
 man/man2/fsopen.2         | 385 ++++++++++++++++++++++++
 man/man2/fspick.2         | 343 +++++++++++++++++++++
 man/man2/mount_setattr.2  |  39 +++
 man/man2/move_mount.2     | 646 ++++++++++++++++++++++++++++++++++++++++
 man/man2/open_tree.2      | 709 ++++++++++++++++++++++++++++++++++++++++++++
 man/man2/open_tree_attr.2 |   1 +
 8 files changed, 3095 insertions(+)
---
base-commit: f17990c243eafc1891ff692f90b6ce42e6449be8
change-id: 20250802-new-mount-api-436db984f432


Kind regards,
-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

[PATCH v5 1/8] man/man2/fsopen.2: document "new" mount API

[Aleksa Sarai]

This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).

Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/fsopen.2 | 385 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 385 insertions(+)

diff --git a/man/man2/fsopen.2 b/man/man2/fsopen.2
new file mode 100644
index 0000000000000000000000000000000000000000..7fbc6c3d28e2e741cd9003c105621b4242abd487
--- /dev/null
+++ b/man/man2/fsopen.2
@@ -0,0 +1,385 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsopen 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fsopen \- create a new filesystem context
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <sys/mount.h>
+.P
+.BI "int fsopen(const char *" fsname ", unsigned int " flags );
+.fi
+.SH DESCRIPTION
+The
+.BR fsopen ()
+system call is part of
+the suite of file-descriptor-based mount facilities in Linux.
+.P
+.BR fsopen ()
+creates a blank filesystem configuration context within the kernel
+for the filesystem named by
+.I fsname
+and places it into creation mode.
+A new file descriptor
+associated with the filesystem configuration context
+is then returned.
+The calling process must have the
+.B \%CAP_SYS_ADMIN
+capability in order to create a new filesystem configuration context.
+.P
+A filesystem configuration context is
+an in-kernel representation of a pending transaction,
+containing a set of configuration parameters that are to be applied
+when creating a new instance of a filesystem
+(or modifying the configuration of an existing filesystem instance,
+such as when using
+.BR fspick (2)).
+.P
+After obtaining a filesystem configuration context with
+.BR fsopen (),
+the general workflow for operating on the context looks like the following:
+.IP (1) 5
+Pass the filesystem context file descriptor to
+.BR fsconfig (2)
+to specify any desired filesystem parameters.
+This may be done as many times as necessary.
+.IP (2)
+Pass the same filesystem context file descriptor to
+.BR fsconfig (2)
+with
+.B \%FSCONFIG_CMD_CREATE
+to create an instance of the configured filesystem.
+.IP (3)
+Pass the same filesystem context file descriptor to
+.BR fsmount (2)
+to create a new detached mount object for
+the root of the filesystem instance,
+which is then attached to a new file descriptor.
+(This also places the filesystem context file descriptor into
+reconfiguration mode,
+similar to the mode produced by
+.BR fspick (2).)
+Once a mount object has been created with
+.BR fsmount (2),
+the filesystem context file descriptor can be safely closed.
+.IP (4)
+Now that a mount object has been created,
+you may
+.RS
+.IP \[bu] 3
+use the detached mount object file descriptor as a
+.I dirfd
+argument to "*at()" system calls;
+and/or
+.IP \[bu]
+attach the mount object to a mount point
+by passing the mount object file descriptor to
+.BR move_mount (2).
+This will also prevent the mount object from
+being unmounted and destroyed when
+the mount object file descriptor is closed.
+.RE
+.IP
+The mount object file descriptor will
+remain associated with the mount object
+even after doing the above operations,
+so you may repeatedly use the mount object file descriptor with
+.BR move_mount (2)
+and/or "*at()" system calls
+as many times as necessary.
+.P
+A filesystem context will move between different modes
+throughout its lifecycle
+(such as the creation phase
+when created with
+.BR fsopen (),
+the reconfiguration phase
+when an existing filesystem instance is selected with
+.BR fspick (2),
+and the intermediate "awaiting-mount" phase
+.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
+between
+.B \%FSCONFIG_CMD_CREATE
+and
+.BR fsmount (2)),
+which has an impact on
+what operations are permitted on the filesystem context.
+.P
+The file descriptor returned by
+.BR fsopen ()
+also acts as a channel for filesystem drivers to
+provide more comprehensive diagnostic information
+than is normally provided through the standard
+.BR errno (3)
+interface for system calls.
+If an error occurs at any time during the workflow mentioned above,
+calling
+.BR read (2)
+on the filesystem context file descriptor
+will retrieve any ancillary information about the encountered errors.
+(See the "Message retrieval interface" section
+for more details on the message format.)
+.P
+.I flags
+can be used to control aspects of
+the creation of the filesystem configuration context file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSOPEN_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.RE
+.P
+A list of filesystems supported by the running kernel
+(and thus a list of valid values for
+.IR fsname )
+can be obtained from
+.IR /proc/filesystems .
+(See also
+.BR proc_filesystems (5).)
+.SS Message retrieval interface
+When doing operations on a filesystem configuration context,
+the filesystem driver may choose to provide
+ancillary information to userspace
+in the form of message strings.
+.P
+The filesystem context file descriptors returned by
+.BR fsopen ()
+and
+.BR fspick (2)
+may be queried for message strings at any time by calling
+.BR read (2)
+on the file descriptor.
+Each call to
+.BR read (2)
+will return a single message,
+prefixed to indicate its class:
+.RS
+.TP
+.BI e\~ message
+An error message was logged.
+This is usually associated with an error being returned
+from the corresponding system call which triggered this message.
+.TP
+.BI w\~ message
+A warning message was logged.
+.TP
+.BI i\~ message
+An informational message was logged.
+.RE
+.P
+Messages are removed from the queue as they are read.
+Note that the message queue has limited depth,
+so it is possible for messages to get lost.
+If there are no messages in the message queue,
+.B read(2)
+will return \-1 and
+.I errno
+will be set to
+.BR \%ENODATA .
+If the
+.I buf
+argument to
+.BR read (2)
+is not large enough to contain the entire message,
+.BR read (2)
+will return \-1 and
+.I errno
+will be set to
+.BR \%EMSGSIZE .
+(See BUGS.)
+.P
+If there are multiple filesystem contexts
+referencing the same filesystem instance
+(such as if you call
+.BR fspick (2)
+multiple times for the same mount),
+each one gets its own independent message queue.
+This does not apply to multiple file descriptors that are
+tied to the same underlying open file description
+(such as those created with
+.BR dup (2)).
+.P
+Message strings will usually be prefixed by
+the name of the filesystem or kernel subsystem
+that logged the message,
+though this may not always be the case.
+See the Linux kernel source code for details.
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EFAULT
+.I fsname
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+.I flags
+had an invalid flag set.
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B ENODEV
+The filesystem named by
+.I fsname
+is not supported by the kernel.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit 24dcb3d90a1f67fe08c68a004af37df059d74005
+.\" commit 400913252d09f9cfb8cce33daee43167921fc343
+glibc 2.36.
+.SH BUGS
+.SS Message retrieval interface and \fB\%EMSGSIZE\fP
+As described in the "Message retrieval interface" subsection above,
+calling
+.BR read (2)
+with too small a buffer to contain
+the next pending message in the message queue
+for the filesystem configuration context
+will cause
+.BR read (2)
+to return \-1 and set
+.BR errno (3)
+to
+.BR \%EMSGSIZE .
+.P
+However,
+this failed operation still
+consumes the message from the message queue.
+This effectively discards the message silently,
+as no data is copied into the
+.BR read (2)
+buffer.
+.P
+Programs should take care to ensure that
+their buffers are sufficiently large
+to contain any reasonable message string,
+in order to avoid silently losing valuable diagnostic information.
+.\" Aleksa Sarai
+.\"   This unfortunate behaviour has existed since this feature was merged, but
+.\"   I have sent a patchset which will finally fix it.
+.\"   <https://lore.kernel.org/r/20250807-fscontext-log-cleanups-v3-1-8d91d6242dc3@cyphar.com/>
+.SH EXAMPLES
+To illustrate the workflow for creating a new mount,
+the following is an example of how to mount an
+.BR ext4 (5)
+filesystem stored on
+.I /dev/sdb1
+onto
+.IR /mnt .
+.P
+.in +4n
+.EX
+int fsfd, mntfd;
+\&
+fsfd = fsopen("ext4", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_PATH, "source", "/dev/sdb1", AT_FDCWD);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "noatime", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "iversion", NULL, 0)
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_RELATIME);
+move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+First,
+an ext4 configuration context is created and attached to the file descriptor
+.IR fsfd .
+Then, a series of parameters
+(such as the source of the filesystem)
+are provided using
+.BR fsconfig (2),
+followed by the filesystem instance being created with
+.BR \%FSCONFIG_CMD_CREATE .
+.BR fsmount (2)
+is then used to create a new mount object attached to the file descriptor
+.IR mntfd ,
+which is then attached to the intended mount point using
+.BR move_mount (2).
+.P
+The above procedure is functionally equivalent to
+the following mount operation using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/dev/sdb1", "/mnt", "ext4", MS_RELATIME,
+      "ro,noatime,acl,user_xattr,iversion");
+.EE
+.in
+.P
+And here's an example of creating a mount object
+of an NFS server share
+and setting a Smack security module label.
+However, instead of attaching it to a mount point,
+the program uses the mount object directly
+to open a file from the NFS share.
+.P
+.in +4n
+.EX
+int fsfd, mntfd, fd;
+\&
+fsfd = fsopen("nfs", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "example.com/pub", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "nfsvers", "3", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "rsize", "65536", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "wsize", "65536", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "smackfsdef", "foolabel", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "rdma", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, 0, MOUNT_ATTR_NODEV);
+fd = openat(mntfd, "src/linux-5.2.tar.xz", O_RDONLY);
+.EE
+.in
+.P
+Unlike the previous example,
+this operation has no trivial equivalent with
+.BR mount (2),
+as it was not previously possible to create a mount object
+that is not attached to any mount point.
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)

-- 
2.51.0

[PATCH v5 2/8] man/man2/fspick.2: document "new" mount API

[Aleksa Sarai]

This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).

Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/fspick.2 | 343 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 343 insertions(+)

diff --git a/man/man2/fspick.2 b/man/man2/fspick.2
new file mode 100644
index 0000000000000000000000000000000000000000..800aed81d38384be4563f2558e3cef846d7e7cee
--- /dev/null
+++ b/man/man2/fspick.2
@@ -0,0 +1,343 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fspick 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fspick \- select filesystem for reconfiguration
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <fcntl.h>" "          /* Definition of " AT_* " constants */"
+.B #include <sys/mount.h>
+.P
+.BI "int fspick(int " dirfd ", const char *" path ", unsigned int " flags );
+.fi
+.SH DESCRIPTION
+The
+.BR fspick ()
+system call is part of
+the suite of file-descriptor-based mount facilities in Linux.
+.P
+.BR fspick ()
+creates a new filesystem configuration context
+for the extant filesystem instance
+associated with the path described by
+.I dirfd
+and
+.IR path ,
+places it into reconfiguration mode
+(similar to
+.BR mount (8)
+with the
+.I \-o\~remount
+option).
+A new file descriptor
+associated with the filesystem configuration context
+is then returned.
+The calling process must have the
+.B \%CAP_SYS_ADMIN
+capability in order to create a new filesystem configuration context.
+.P
+The resultant file descriptor can be used with
+.BR fsconfig (2)
+to specify the desired set of changes to
+filesystem parameters of the filesystem instance.
+Once the desired set of changes have been configured,
+the changes can be effectuated by calling
+.BR fsconfig (2)
+with the
+.B \%FSCONFIG_CMD_RECONFIGURE
+command.
+In contrast to
+the behaviour of
+.B MS_REMOUNT
+with
+.BR mount (2),
+.BR fspick ()
+instantiates the filesystem configuration context
+with a copy of
+the extant filesystem's filesystem parameters;
+thus,
+subsequent
+.B \%FSCONFIG_CMD_RECONFIGURE
+operations
+will only update filesystem parameters
+explicitly modified with
+.BR fsconfig (2).
+.P
+As with "*at()" system calls,
+.BR fspick ()
+uses the
+.I dirfd
+argument in conjunction with the
+.I path
+argument to determine the path to operate on, as follows:
+.IP \[bu] 3
+If the pathname given in
+.I path
+is absolute, then
+.I dirfd
+is ignored.
+.IP \[bu]
+If the pathname given in
+.I path
+is relative and
+.I dirfd
+is the special value
+.BR \%AT_FDCWD ,
+then
+.I path
+is interpreted relative to
+the current working directory
+of the calling process (like
+.BR open (2)).
+.IP \[bu]
+If the pathname given in
+.I path
+is relative,
+then it is interpreted relative to
+the directory referred to by the file descriptor
+.I dirfd
+(rather than relative to
+the current working directory
+of the calling process,
+as is done by
+.BR open (2)
+for a relative pathname).
+In this case,
+.I dirfd
+must be a directory
+that was opened for reading
+.RB ( O_RDONLY )
+or using the
+.B O_PATH
+flag.
+.IP \[bu]
+If
+.I path
+is an empty string,
+and
+.I flags
+contains
+.BR \%FSPICK_EMPTY_PATH ,
+then the file descriptor
+.I dirfd
+is operated on directly.
+In this case,
+.I dirfd
+may refer to any type of file,
+not just a directory.
+.P
+See
+.BR openat (2)
+for an explanation of why the
+.I dirfd
+argument is useful.
+.P
+.I flags
+can be used to control aspects of how
+.I path
+is resolved and
+properties of the returned file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSPICK_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.TP
+.B FSPICK_EMPTY_PATH
+If
+.I path
+is an empty string,
+operate on the file referred to by
+.I dirfd
+(which may have been obtained from
+.BR open (2),
+.BR fsmount (2),
+or
+.BR open_tree (2)).
+In this case,
+.I dirfd
+may refer to any type of file,
+not just a directory.
+If
+.I dirfd
+is
+.BR \%AT_FDCWD ,
+.BR fspick ()
+will operate on the current working directory
+of the calling process.
+.TP
+.B FSPICK_SYMLINK_NOFOLLOW
+Do not follow symbolic links
+in the terminal component of
+.IR path .
+If
+.I path
+references a symbolic link,
+the returned filesystem context will reference
+the filesystem that the symbolic link itself resides on.
+.TP
+.B FSPICK_NO_AUTOMOUNT
+Do not automount the terminal ("basename") component of
+.I path
+if it is a directory that is an automount point.
+This allows you to reconfigure an automount point,
+rather than the location that would be mounted.
+This flag has no effect
+if the automount point has already been mounted over.
+.RE
+.P
+As with filesystem contexts created with
+.BR fsopen (2),
+the file descriptor returned by
+.BR fspick ()
+may be queried for message strings at any time by calling
+.BR read (2)
+on the file descriptor.
+(See the "Message retrieval interface" subsection in
+.BR fsopen (2)
+for more details on the message format.)
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EACCES
+Search permission is denied
+for one of the directories
+in the path prefix of
+.IR path .
+(See also
+.BR path_resolution (7).)
+.TP
+.B EBADF
+.I path
+is relative but
+.I dirfd
+is neither
+.B \%AT_FDCWD
+nor a valid file descriptor.
+.TP
+.B EFAULT
+.I path
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B ELOOP
+Too many symbolic links encountered when resolving
+.IR path .
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENAMETOOLONG
+.I path
+is longer than
+.BR PATH_MAX .
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B ENOENT
+A component of
+.I path
+does not exist,
+or is a dangling symbolic link.
+.TP
+.B ENOENT
+.I path
+is an empty string, but
+.B \%FSPICK_EMPTY_PATH
+is not specified in
+.IR flags .
+.TP
+.B ENOTDIR
+A component of the path prefix of
+.I path
+is not a directory;
+or
+.I path
+is relative and
+.I dirfd
+is a file descriptor referring to a file other than a directory.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit cf3cba4a429be43e5527a3f78859b1bfd9ebc5fb
+.\" commit 400913252d09f9cfb8cce33daee43167921fc343
+glibc 2.36.
+.SH EXAMPLES
+The following example sets the read-only flag
+on the filesystem instance referenced by
+the mount object attached at
+.IR /tmp .
+.P
+.in +4n
+.EX
+int fsfd = fspick(AT_FDCWD, "/tmp", FSPICK_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
+.EE
+.in
+.P
+The above procedure is roughly equivalent to
+the following mount operation using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount(NULL, "/tmp", NULL, MS_REMOUNT | MS_RDONLY, NULL);
+.EE
+.in
+.P
+With the notable caveat that
+in this example,
+.BR mount (2)
+will clear all other filesystem parameters
+(such as
+.B MS_DIRSYNC
+or
+.BR MS_SYNCHRONOUS );
+.BR fsconfig (2)
+will only modify the
+.I ro
+parameter.
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fsopen (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)

-- 
2.51.0

[PATCH v5 3/8] man/man2/fsconfig.2: document "new" mount API

[Aleksa Sarai]

This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).

Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 729 insertions(+)

diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
new file mode 100644
index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
--- /dev/null
+++ b/man/man2/fsconfig.2
@@ -0,0 +1,729 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsconfig 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fsconfig \- configure new or existing filesystem context
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <sys/mount.h>
+.P
+.BI "int fsconfig(int " fd ", unsigned int " cmd ,
+.BI "             const char *_Nullable " key ,
+.BI "             const void *_Nullable " value ", int " aux );
+.fi
+.SH DESCRIPTION
+The
+.BR fsconfig ()
+system call is part of
+the suite of file-descriptor-based mount facilities in Linux.
+.P
+.BR fsconfig ()
+is used to supply parameters to
+and issue commands against
+the filesystem configuration context
+associated with the file descriptor
+.IR fd .
+Filesystem configuration contexts can be created with
+.BR fsopen (2)
+or be instantiated from an extant filesystem instance with
+.BR fspick (2).
+.P
+The
+.I cmd
+argument indicates the command to be issued.
+Some commands supply parameters to the context
+(equivalent to mount options specified with
+.BR mount (8)),
+while others are meta-operations on the filesystem context.
+The list of valid
+.I cmd
+values are:
+.RS
+.TP
+.B FSCONFIG_SET_FLAG
+Set the flag parameter named by
+.IR key .
+.I value
+must be NULL,
+and
+.I aux
+must be 0.
+.TP
+.B FSCONFIG_SET_STRING
+Set the string parameter named by
+.I key
+to the value specified by
+.IR value .
+.I value
+points to a null-terminated string,
+and
+.I aux
+must be 0.
+.TP
+.B FSCONFIG_SET_BINARY
+Set the blob parameter named by
+.I key
+to the contents of the binary blob
+specified by
+.IR value .
+.I value
+points to
+the start of a buffer
+that is
+.I aux
+bytes in length.
+.TP
+.B FSCONFIG_SET_FD
+Set the file parameter named by
+.I key
+to the open file description
+referenced by the file descriptor
+.IR aux .
+.I value
+must be NULL.
+.IP
+You may also use
+.B \%FSCONFIG_SET_STRING
+for file parameters,
+with
+.I value
+set to a null-terminated string
+containing a base-10 representation
+of the file descriptor number.
+This mechanism is primarily intended for compatibility
+with older
+.BR mount (2)-based
+programs,
+and only works for parameters
+that
+.I only
+accept file descriptor arguments.
+.TP
+.B FSCONFIG_SET_PATH
+Set the path parameter named by
+.I key
+to the object at a provided path,
+resolved in a similar manner to
+.BR openat (2).
+.I value
+points to a null-terminated pathname string,
+and
+.I aux
+is equivalent to the
+.I dirfd
+argument to
+.BR openat (2).
+See
+.BR openat (2)
+for an explanation of the need for
+.BR \%FSCONFIG_SET_PATH .
+.IP
+You may also use
+.B \%FSCONFIG_SET_STRING
+for path parameters,
+the behaviour of which is equivalent to
+.B \%FSCONFIG_SET_PATH
+with
+.I aux
+set to
+.BR \%AT_FDCWD .
+.TP
+.B FSCONFIG_SET_PATH_EMPTY
+As with
+.BR \%FSCONFIG_SET_PATH ,
+except that if
+.I value
+is an empty string,
+the file descriptor specified by
+.I aux
+is operated on directly
+and may be any type of file
+(not just a directory).
+This is equivalent to the behaviour of
+.B \%AT_EMPTY_PATH
+with most "*at()" system calls.
+If
+.I aux
+is
+.BR \%AT_FDCWD ,
+the parameter will be set to
+the current working directory
+of the calling process.
+.TP
+.B FSCONFIG_CMD_CREATE
+This command instructs the filesystem driver
+to instantiate an instance of the filesystem in the kernel
+with the parameters specified in the filesystem configuration context.
+.I key
+and
+.I value
+must be NULL,
+and
+.I aux
+must be 0.
+.IP
+This command can only be issued once
+in the lifetime of a filesystem context.
+If the operation succeeds,
+the filesystem context
+associated with file descriptor
+.I fd
+now references the created filesystem instance,
+and is placed into a special "awaiting-mount" mode
+that allows you to use
+.BR fsmount (2)
+to create a mount object from the filesystem instance.
+.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
+If the operation fails,
+in most cases
+the filesystem context is placed in a failed mode
+and cannot be used for any further
+.BR fsconfig ()
+operations
+(though you may still retrieve diagnostic messages
+through the message retrieval interface,
+as described in
+the corresponding subsection of
+.BR fsopen (2)).
+.IP
+This command can only be issued against
+filesystem configuration contexts
+that were created with
+.BR fsopen (2).
+In order to create a filesystem instance,
+the calling process must have the
+.B \%CAP_SYS_ADMIN
+capability.
+.IP
+An important thing to be aware of is that
+the Linux kernel will
+.I silently
+reuse extant filesystem instances
+depending on the filesystem type
+and the configured parameters
+(each filesystem driver has
+its own policy for
+how filesystem instances are reused).
+This means that
+the filesystem instance "created" by
+.B \%FSCONFIG_CMD_CREATE
+may, in fact, be a reference
+to an extant filesystem instance in the kernel.
+(For reference,
+this behaviour also applies to
+.BR mount (2).)
+.IP
+One side-effect of this behaviour is that
+if an extant filesystem instance is reused,
+.I all
+parameters configured
+for this filesystem configuration context
+are
+.I silently ignored
+(with the exception of the
+.I ro
+and
+.I rw
+flag parameters;
+if the state of the read-only flag in the
+extant filesystem instance and the filesystem configuration context
+do not match, this operation will return
+.BR EBUSY ).
+This also means that
+.B \%FSCONFIG_CMD_RECONFIGURE
+commands issued against
+the "created" filesystem instance
+will also affect any mount objects associated with
+the extant filesystem instance.
+.IP
+Programs that need to ensure
+that they create a new filesystem instance
+with specific parameters
+(notably, security-related parameters
+such as
+.I acl
+to enable POSIX ACLs\[em]\c
+as described in
+.BR acl (5))
+should use
+.B \%FSCONFIG_CMD_CREATE_EXCL
+instead.
+.TP
+.BR FSCONFIG_CMD_CREATE_EXCL " (since Linux 6.6)"
+.\" commit 22ed7ecdaefe0cac0c6e6295e83048af60435b13
+.\" commit 84ab1277ce5a90a8d1f377707d662ac43cc0918a
+As with
+.BR \%FSCONFIG_CMD_CREATE ,
+except that the kernel is instructed
+to not reuse extant filesystem instances.
+If the operation
+would be forced to
+reuse an extant filesystem instance,
+this operation will return
+.B EBUSY
+instead.
+.IP
+As a result (unlike
+.BR \%FSCONFIG_CMD_CREATE ),
+if this operation succeeds
+then the calling process can be sure that
+all of the parameters successfully configured with
+.BR fsconfig ()
+will actually be applied
+to the created filesystem instance.
+.TP
+.B FSCONFIG_CMD_RECONFIGURE
+This command instructs the filesystem driver
+to apply the parameters specified in the filesystem configuration context
+to the extant filesystem instance
+referenced by the filesystem configuration context.
+.I key
+and
+.I value
+must be NULL,
+and
+.I aux
+must be 0.
+.IP
+This is primarily intended for use with
+.BR fspick (2),
+but may also be used to modify
+the parameters of a filesystem instance
+after
+.B \%FSCONFIG_CMD_CREATE
+was used to create it
+and a mount object was created using
+.BR fsmount (2).
+In order to reconfigure an extant filesystem instance,
+the calling process must have the
+.B CAP_SYS_ADMIN
+capability.
+.IP
+If the operation succeeds,
+the filesystem context is reset
+but remains in reconfiguration mode
+and thus can be reused for subsequent
+.B \%FSCONFIG_CMD_RECONFIGURE
+commands.
+If the operation fails,
+in most cases
+the filesystem context is placed in a failed mode
+and cannot be used for any further
+.BR fsconfig ()
+operations
+(though you may still retrieve diagnostic messages
+through the message retrieval interface,
+as described in
+the corresponding subsection of
+.BR fsopen (2)).
+.RE
+.P
+Parameters specified with
+.BI FSCONFIG_SET_ *
+do not take effect
+until a corresponding
+.B \%FSCONFIG_CMD_CREATE
+or
+.B \%FSCONFIG_CMD_RECONFIGURE
+command is issued.
+.SH RETURN VALUE
+On success,
+.BR fsconfig ()
+returns 0.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+If an error occurs, the filesystem driver may provide
+additional information about the error
+through the message retrieval interface for filesystem configuration contexts.
+This additional information can be retrieved at any time by calling
+.BR read (2)
+on the filesystem instance or filesystem configuration context
+referenced by the file descriptor
+.IR fd .
+(See the "Message retrieval interface" subsection in
+.BR fsopen (2)
+for more details on the message format.)
+.P
+Even after an error occurs,
+the filesystem configuration context is
+.I not
+invalidated,
+and thus can still be used with other
+.BR fsconfig ()
+commands.
+This means that users can probe support for filesystem parameters
+on a per-parameter basis,
+and adjust which parameters they wish to set.
+.P
+The error values given below result from
+filesystem type independent errors.
+Each filesystem type may have its own special errors
+and its own special behavior.
+See the Linux kernel source code for details.
+.TP
+.B EACCES
+A component of a path
+provided as a path parameter
+was not searchable.
+(See also
+.BR path_resolution (7).)
+.TP
+.B EACCES
+.B \%FSCONFIG_CMD_CREATE
+was attempted
+for a read-only filesystem
+without specifying the
+.RB ' ro '
+flag parameter.
+.TP
+.B EACCES
+A specified block device parameter
+is located on a filesystem
+mounted with the
+.B \%MS_NODEV
+option.
+.TP
+.B EBADF
+The file descriptor given by
+.I fd
+(or possibly by
+.IR aux ,
+depending on the command)
+is invalid.
+.TP
+.B EBUSY
+The filesystem context associated with
+.I fd
+is in the wrong state
+for the given command.
+.TP
+.B EBUSY
+The filesystem instance cannot be reconfigured as read-only
+with
+.B \%FSCONFIG_CMD_RECONFIGURE
+because some programs
+still hold files open for writing.
+.TP
+.B EBUSY
+A new filesystem instance was requested with
+.B \%FSCONFIG_CMD_CREATE_EXCL
+but a matching superblock already existed.
+.TP
+.B EFAULT
+One of the pointer arguments
+points to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+.I fd
+does not refer to
+a filesystem configuration context
+or filesystem instance.
+.TP
+.B EINVAL
+One of the values of
+.IR name ,
+.IR value ,
+and/or
+.I aux
+were set to a non-zero value when
+.I cmd
+required that they be zero
+(or NULL).
+.TP
+.B EINVAL
+The parameter named by
+.I name
+cannot be set
+using the type specified with
+.IR cmd .
+.TP
+.B EINVAL
+One of the source parameters
+referred to
+an invalid superblock.
+.TP
+.B ELOOP
+Too many links encountered
+during pathname resolution
+of a path argument.
+.TP
+.B ENAMETOOLONG
+A path argument was longer than
+.BR PATH_MAX .
+.TP
+.B ENOENT
+A path argument had a non-existent component.
+.TP
+.B ENOENT
+A path argument is an empty string,
+but
+.I cmd
+is not
+.BR \%FSCONFIG_SET_PATH_EMPTY .
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B ENOTBLK
+The parameter named by
+.I name
+must be a block device,
+but the provided parameter value was not a block device.
+.TP
+.B ENOTDIR
+A component of the path prefix
+of a path argument
+was not a directory.
+.TP
+.B EOPNOTSUPP
+The command given by
+.I cmd
+is not valid.
+.TP
+.B ENXIO
+The major number
+of a block device parameter
+is out of range.
+.TP
+.B EPERM
+The command given by
+.I cmd
+was
+.BR \%FSCONFIG_CMD_CREATE ,
+.BR \%FSCONFIG_CMD_CREATE_EXCL ,
+or
+.BR \%FSCONFIG_CMD_RECONFIGURE ,
+but the calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit ecdab150fddb42fe6a739335257949220033b782
+.\" commit 400913252d09f9cfb8cce33daee43167921fc343
+glibc 2.36.
+.SH NOTES
+.SS Generic filesystem parameters
+Each filesystem driver is responsible for
+parsing most parameters specified with
+.BR fsconfig (),
+meaning that individual filesystems
+may have very different behaviour
+when encountering parameters with the same name.
+In general,
+you should not assume that the behaviour of
+.BR fsconfig ()
+when specifying a parameter to one filesystem type
+will match the behaviour of the same parameter
+with a different filesystem type.
+.P
+However,
+the following generic parameters
+apply to all filesystems and have unified behaviour.
+They are set using the listed
+.BI \%FSCONFIG_SET_ *
+command.
+.TP
+\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+Configure whether the filesystem instance is read-only.
+.TP
+\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+Make directory changes on this filesystem instance synchronous.
+.TP
+\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+Configure whether writes on this filesystem instance
+will be made synchronous
+(as though the
+.B O_SYNC
+flag to
+.BR open (2)
+was specified for
+all file opens in this filesystem instance).
+.TP
+\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+Configure whether to reduce on-disk updates
+of inode timestamps on this filesystem instance
+(as described in the
+.B \%MS_LAZYTIME
+section of
+.BR mount (2)).
+.TP
+\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
+Configure whether the filesystem instance should permit mandatory locking.
+Since Linux 5.15,
+.\" commit f7e33bdbd6d1bdf9c3df8bba5abcf3399f957ac3
+mandatory locking has been deprecated
+and setting this flag is a no-op.
+.TP
+\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
+This parameter is equivalent to the
+.I source
+parameter passed to
+.BR mount (2)
+for the same filesystem type,
+and is usually the pathname of a block device
+containing the filesystem.
+This parameter may only be set once
+per filesystem configuration context transaction.
+.P
+In addition,
+any filesystem parameters associated with
+Linux Security Modules (LSMs)
+are also generic with respect to the underlying filesystem.
+See the documentation for the LSM you wish to configure for more details.
+.SH CAVEATS
+.SS Filesystem parameter types
+As a result of
+each filesystem driver being responsible for
+parsing most parameters specified with
+.BR fsconfig (),
+some filesystem drivers
+may have unintuitive behaviour
+with regards to which
+.BI \%FSCONFIG_SET_ *
+commands are permitted
+to configure a given parameter.
+.P
+In order for
+filesystem parameters to be backwards compatible with
+.BR mount (2),
+they must be parseable as strings;
+this almost universally means that
+.B \%FSCONFIG_SET_STRING
+can also be used to configure them.
+.\" Aleksa Sarai
+.\"   Theoretically, a filesystem could check fc->oldapi and refuse
+.\"   FSCONFIG_SET_STRING if the operation is coming from the new API, but no
+.\"   filesystems do this (and probably never will).
+However, other
+.BI \%FSCONFIG_SET_ *
+commands need to be opted into
+by each filesystem driver's parameter parser.
+.P
+One of the most user-visible instances of
+this inconsistency is that
+many filesystems do not support
+configuring path parameters with
+.B \%FSCONFIG_SET_PATH
+(despite the name),
+which can lead to somewhat confusing
+.B EINVAL
+errors.
+(For example, the generic
+.I source
+parameter\[em]\c
+which is usually a path\[em]\c
+can only be configured
+with
+.BR \%FSCONFIG_SET_STRING .)
+.P
+When writing programs that use
+.BR fsconfig ()
+to configure parameters
+with commands other than
+.BR \%FSCONFIG_SET_STRING ,
+users should verify
+that the
+.BI \%FSCONFIG_SET_ *
+commands used to configure each parameter
+are supported by the corresponding filesystem driver.
+.\" Aleksa Sarai
+.\"   While this (quite confusing) inconsistency in behaviour is true today
+.\"   (and has been true since this was merged), this appears to mostly be an
+.\"   unintended consequence of filesystem drivers hand-coding fsparam parsing.
+.\"   Path parameters are the most eggregious causes of confusion.
+.\"   Hopefully we can make this no longer the case in a future kernel.
+.SH EXAMPLES
+To illustrate the different kinds of flags that can be configured with
+.BR fsconfig (),
+here are a few examples of some different filesystems being created:
+.P
+.in +4n
+.EX
+int fsfd, mntfd;
+\&
+fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "inode64", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "uid", "1234", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "huge", "never", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "casefold", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOEXEC);
+move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
+\&
+fsfd = fsopen("erofs", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/loop0", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE_EXCL, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOSUID);
+move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+Usually,
+specifying the same parameter named by
+.I key
+multiple times with
+.BR fsconfig ()
+causes the parameter value to be replaced.
+However, some filesystems may have unique behaviour:
+.P
+.in +4n
+.EX
+\&
+int fsfd, mntfd;
+int lowerdirfd = open("/o/ctr/lower1", O_DIRECTORY | O_CLOEXEC);
+\&
+fsfd = fsopen("overlay", FSOPEN_CLOEXEC);
+/* "lowerdir+" appends to the lower dir stack each time */
+fsconfig(fsfd, FSCONFIG_SET_FD, "lowerdir+", NULL, lowerdirfd);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower2", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower3", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower4", 0);
+.\" fsconfig(fsfd, FSCONFIG_SET_PATH, "lowerdir+", "/o/ctr/lower5", AT_FDCWD);
+.\" fsconfig(fsfd, FSCONFIG_SET_PATH_EMPTY, "lowerdir+", "", lowerdirfd);
+.\" Aleksa Sarai: Hopefully these will also be supported in the future.
+fsconfig(fsfd, FSCONFIG_SET_STRING, "xino", "auto", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "nfs_export", "off", 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+And here is an example of how
+.BR fspick (2)
+can be used with
+.BR fsconfig ()
+to reconfigure the parameters
+of an extant filesystem instance
+attached to
+.IR /proc :
+.P
+.in +4n
+.EX
+int fsfd = fspick(AT_FDCWD, "/proc", FSPICK_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "ptraceable", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "subset", "pid", 0);
+fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
+.EE
+.in
+.SH SEE ALSO
+.BR fsmount (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)

-- 
2.51.0

[PATCH v5 4/8] man/man2/fsmount.2: document "new" mount API

[Aleksa Sarai]

This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).

Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/fsmount.2 | 231 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 231 insertions(+)

diff --git a/man/man2/fsmount.2 b/man/man2/fsmount.2
new file mode 100644
index 0000000000000000000000000000000000000000..b62850a68443bb8f6178389eb6cb1a5f9029ab30
--- /dev/null
+++ b/man/man2/fsmount.2
@@ -0,0 +1,231 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsmount 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fsmount \- instantiate mount object from filesystem context
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <sys/mount.h>
+.P
+.BI "int fsmount(int " fsfd ", unsigned int " flags \
+", unsigned int " attr_flags );
+.fi
+.SH DESCRIPTION
+The
+.BR fsmount ()
+system call is part of
+the suite of file-descriptor-based mount facilities in Linux.
+.P
+.BR fsmount ()
+creates a new detached mount object
+for the root of the new filesystem instance
+referenced by the filesystem context file descriptor
+.IR fsfd .
+A new file descriptor
+associated with the detached mount object
+is then returned.
+In order to create a mount object with
+.BR fsmount (),
+the calling process must have the
+.B \%CAP_SYS_ADMIN
+capability.
+.P
+The filesystem context must have been created with a call to
+.BR fsopen (2)
+and then had a filesystem instance instantiated with a call to
+.BR fsconfig (2)
+with
+.B \%FSCONFIG_CMD_CREATE
+or
+.B \%FSCONFIG_CMD_CREATE_EXCL
+in order to be in the correct state
+for this operation
+(the "awaiting-mount" mode in kernel-developer parlance).
+.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
+Unlike
+.BR open_tree (2)
+with
+.BR \%OPEN_TREE_CLONE ,
+.BR fsmount ()
+can only be called once
+in the lifetime of a filesystem context
+to produce a mount object.
+.P
+As with file descriptors returned from
+.BR open_tree (2)
+called with
+.BR OPEN_TREE_CLONE ,
+the returned file descriptor
+can then be used with
+.BR move_mount (2),
+.BR mount_setattr (2),
+or other such system calls to do further mount operations.
+This mount object will be unmounted and destroyed
+when the file descriptor is closed
+if it was not otherwise attached to a mount point
+by calling
+.BR move_mount (2).
+(Note that the unmount operation on
+.BR close (2)
+is lazy\[em]akin to calling
+.BR umount2 (2)
+with
+.BR MNT_DETACH ;
+any existing open references to files
+from the mount object
+will continue to work,
+and the mount object will only be completely destroyed
+once it ceases to be busy.)
+The returned file descriptor
+also acts the same as one produced by
+.BR open (2)
+with
+.BR O_PATH ,
+meaning it can also be used as a
+.I dirfd
+argument
+to "*at()" system calls.
+.P
+.I flags
+controls the creation of the returned file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSMOUNT_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.RE
+.P
+.I attr_flags
+specifies mount attributes
+which will be applied to the created mount object,
+in the form of
+.BI \%MOUNT_ATTR_ *
+flags.
+The flags are interpreted as though
+.BR mount_setattr (2)
+was called with
+.I attr.attr_set
+set to the same value as
+.IR attr_flags .
+.BI \%MOUNT_ATTR_ *
+flags which would require
+specifying additional fields in
+.BR mount_attr (2type)
+(such as
+.BR \%MOUNT_ATTR_IDMAP )
+are not valid flag values for
+.IR attr_flags .
+.P
+If the
+.BR fsmount ()
+operation is successful,
+the filesystem context
+associated with the file descriptor
+.I fsfd
+is reset
+and placed into reconfiguration mode,
+as if it were just returned by
+.BR fspick (2).
+You may continue to use
+.BR fsconfig (2)
+with the now-reset filesystem context,
+including issuing the
+.B \%FSCONFIG_CMD_RECONFIGURE
+command
+to reconfigure the filesystem instance.
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EBUSY
+The filesystem context associated with
+.I fsfd
+is not in the right state
+to be used by
+.BR fsmount ().
+.TP
+.B EINVAL
+.I flags
+had an invalid flag set.
+.TP
+.B EINVAL
+.I attr_flags
+had an invalid
+.BI MOUNT_ATTR_ *
+flag set.
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B ENOSPC
+The "anonymous" mount namespace
+necessary to contain the new mount object
+could not be allocated,
+as doing so would exceed
+the configured per-user limit on
+the number of mount namespaces in the current user namespace.
+(See also
+.BR namespaces (7).)
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit 93766fbd2696c2c4453dd8e1070977e9cd4e6b6d
+.\" commit 400913252d09f9cfb8cce33daee43167921fc343
+glibc 2.36.
+.SH EXAMPLES
+.in +4n
+.EX
+int fsfd, mntfd, tmpfd;
+\&
+fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC,
+                MOUNT_ATTR_NODEV | MOUNT_ATTR_NOEXEC);
+\&
+/* Create a new file without attaching the mount object */
+tmpfd = openat(mntfd, "tmpfile", O_CREAT | O_EXCL | O_RDWR, 0600);
+unlinkat(mntfd, "tmpfile", 0);
+\&
+/* Attach the mount object to "/tmp" */
+move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)

-- 
2.51.0

[PATCH v5 5/8] man/man2/move_mount.2: document "new" mount API

[Aleksa Sarai]

This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).

Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/move_mount.2 | 646 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 646 insertions(+)

diff --git a/man/man2/move_mount.2 b/man/man2/move_mount.2
new file mode 100644
index 0000000000000000000000000000000000000000..f954f36c43c444afb167088cc665607dfeb10676
--- /dev/null
+++ b/man/man2/move_mount.2
@@ -0,0 +1,646 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH move_mount 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+move_mount \- move or attach mount object to filesystem
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <fcntl.h>" "          /* Definition of " AT_* " constants */"
+.B #include <sys/mount.h>
+.P
+.BI "int move_mount(int " from_dirfd ", const char *" from_path ,
+.BI "               int " to_dirfd ", const char *" to_path ,
+.BI "               unsigned int " flags );
+.fi
+.SH DESCRIPTION
+The
+.BR move_mount ()
+system call is part of
+the suite of file-descriptor-based mount facilities in Linux.
+.P
+.BR move_mount ()
+moves the mount object indicated by
+.I from_dirfd
+and
+.I from_path
+to the path indicated by
+.I to_dirfd
+and
+.IR to_path .
+The mount object being moved
+can be an existing mount point in the current mount namespace,
+or a detached mount object created by
+.BR fsmount (2)
+or
+.BR open_tree (2)
+with
+.BR \%OPEN_TREE_CLONE .
+.P
+To access the source mount object
+or the destination mount point,
+no permissions are required on the object itself,
+but if either pathname is supplied,
+execute (search) permission is required
+on all of the directories specified in
+.I from_path
+or
+.IR to_path .
+.P
+The calling process must have the
+.B \%CAP_SYS_ADMIN
+capability in order to move or attach a mount object.
+.P
+As with "*at()" system calls,
+.BR move_mount ()
+uses the
+.I from_dirfd
+and
+.I to_dirfd
+arguments
+in conjunction with the
+.I from_path
+and
+.I to_path
+arguments to determine the source and destination objects to operate on
+(respectively), as follows:
+.IP \[bu] 3
+If the pathname given in
+.I *_path
+is absolute, then
+the corresponding
+.I *_dirfd
+is ignored.
+.IP \[bu]
+If the pathname given in
+.I *_path
+is relative and
+the corresponding
+.I *_dirfd
+is the special value
+.BR \%AT_FDCWD ,
+then
+.I *_path
+is interpreted relative to
+the current working directory
+of the calling process (like
+.BR open (2)).
+.IP \[bu]
+If the pathname given in
+.I *_path
+is relative,
+then it is interpreted relative to
+the directory referred to by
+the corresponding file descriptor
+.I *_dirfd
+(rather than relative to
+the current working directory
+of the calling process,
+as is done by
+.BR open (2)
+for a relative pathname).
+In this case,
+the corresponding
+.I *_dirfd
+must be a directory
+that was opened for reading
+.RB ( O_RDONLY )
+or using the
+.B O_PATH
+flag.
+.IP \[bu]
+If
+.I *_path
+is an empty string,
+and
+.I flags
+contains the appropriate
+.BI \%MOVE_MOUNT_ * _EMPTY_PATH
+flag,
+then the corresponding file descriptor
+.I *_dirfd
+is operated on directly.
+In this case,
+the corresponding
+.I *_dirfd
+may refer to any type of file,
+not just a directory.
+.P
+See
+.BR openat (2)
+for an explanation of why the
+.I *_dirfd
+arguments are useful.
+.P
+.I flags
+can be used to control aspects of the path lookup
+for both the source and destination objects,
+as well as other properties of the mount operation.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B MOVE_MOUNT_F_EMPTY_PATH
+If
+.I from_path
+is an empty string, operate on the file referred to by
+.I from_dirfd
+(which may have been obtained from
+.BR open (2),
+.BR fsmount (2),
+or
+.BR open_tree (2)).
+In this case,
+.I from_dirfd
+may refer to any type of file,
+not just a directory.
+If
+.I from_dirfd
+is
+.BR \%AT_FDCWD ,
+.BR move_mount ()
+will operate on the current working directory
+of the calling process.
+.IP
+This is the most common mechanism
+used to attach detached mount objects
+produced by
+.BR fsmount (2)
+and
+.BR open_tree (2)
+to a mount point.
+.TP
+.B MOVE_MOUNT_T_EMPTY_PATH
+As with
+.BR \%MOVE_MOUNT_F_EMPTY_PATH ,
+except operating on
+.I to_dirfd
+and
+.IR to_path .
+.TP
+.B MOVE_MOUNT_F_SYMLINKS
+If
+.I from_path
+references a symbolic link,
+then dereference it.
+The default behaviour for
+.BR move_mount ()
+is to
+.I not follow
+symbolic links.
+.TP
+.B MOVE_MOUNT_T_SYMLINKS
+As with
+.BR \%MOVE_MOUNT_F_SYMLINKS ,
+except operating on
+.I to_dirfd
+and
+.IR to_path .
+.TP
+.B MOVE_MOUNT_F_NO_AUTOMOUNT
+Do not automount the terminal ("basename") component of
+.I \%from_path
+if it is a directory that is an automount point.
+This allows a mount object
+that has an automount point at its root
+to be moved
+and prevents unintended triggering of an automount point.
+This flag has no effect
+if the automount point has already been mounted over.
+.TP
+.B MOVE_MOUNT_T_NO_AUTOMOUNT
+As with
+.BR \%MOVE_MOUNT_F_NO_AUTOMOUNT ,
+except operating on
+.I to_dirfd
+and
+.IR to_path .
+This allows an automount point to be manually mounted over.
+.TP
+.BR MOVE_MOUNT_SET_GROUP " (since Linux 5.15)"
+Add the attached private-propagation mount object indicated by
+.I to_dirfd
+and
+.I to_path
+into the mount propagation "peer group"
+of the attached non-private-propagation mount object indicated by
+.I from_dirfd
+and
+.IR from_path .
+.IP
+Unlike other
+.BR move_mount ()
+operations,
+this operation does not move or attach any mount objects.
+Instead, it only updates the metadata
+of attached mount objects.
+(Also, take careful note of
+the argument order\[em]\c
+the mount object being modified
+by this operation is the one specified by
+.I to_dirfd
+and
+.IR to_path .)
+.IP
+This makes it possible to first create a mount tree
+consisting only of private mounts
+and then configure the desired propagation layout afterwards.
+(See the "SHARED SUBTREES" section of
+.BR mount_namespaces (7)
+for more information about mount propagation and peer groups.)
+.TP
+.BR MOVE_MOUNT_BENEATH " (since Linux 6.5)"
+If the path indicated by
+.I to_dirfd
+and
+.I to_path
+is an existing mount object,
+rather than attaching or moving the mount object
+indicated by
+.I from_dirfd
+and
+.I from_path
+on top of the mount stack,
+attach or move it beneath the current top mount
+on the mount stack.
+.IP
+After using
+.BR \%MOVE_MOUNT_BENEATH ,
+it is possible to
+.BR umount (2)
+the top mount
+in order to reveal the mount object
+which was attached beneath it earlier.
+This allows for the seamless (and atomic) replacement
+of intricate mount trees,
+which can further be used
+to "upgrade" a mount tree with a newer version.
+.IP
+This operation has several restrictions:
+.RS
+.IP \[bu] 3
+Mount objects cannot be attached beneath the filesystem root,
+including cases where
+the filesystem root was configured by
+.BR chroot (2)
+or
+.BR pivot_root (2).
+To mount beneath the filesystem root,
+.BR pivot_root (2)
+must be used.
+.IP \[bu]
+The target path indicated by
+.I to_dirfd
+and
+.I to_path
+must not be a detached mount object,
+such as those produced by
+.BR open_tree (2)
+with
+.B \%OPEN_TREE_CLONE
+or
+.BR fsmount (2).
+.IP \[bu]
+The current top mount
+of the target path's mount stack
+and its parent mount
+must be in the calling process's mount namespace.
+.IP \[bu]
+The caller must have sufficient privileges
+to unmount the top mount
+of the target path's mount stack,
+to prove they have privileges
+to reveal the underlying mount.
+.IP \[bu]
+Mount propagation events triggered by this
+.BR move_mount ()
+operation
+(as described in
+.BR mount_namespaces (7))
+are calculated based on the parent mount
+of the current top mount
+of the target path's mount stack.
+.IP \[bu]
+The target path's mount
+cannot be an ancestor in the mount tree of
+the source mount object.
+.IP \[bu]
+The source mount object
+must not have any overmounts,
+otherwise it would be possible to create "shadow mounts"
+(i.e., two mounts mounted on the same parent mount at the same mount point).
+.IP \[bu]
+It is not possible to move a mount
+beneath a top mount
+if the parent mount
+of the current top mount
+propagates to the top mount itself.
+Otherwise,
+.B \%MOVE_MOUNT_BENEATH
+would cause the mount object
+to be propagated
+to the top mount
+from the parent mount,
+defeating the purpose of using
+.BR \%MOVE_MOUNT_BENEATH .
+.IP \[bu]
+It is not possible to move a mount
+beneath a top mount
+if the parent mount
+of the current top mount
+propagates to the mount object
+being mounted beneath.
+Otherwise, this would cause a similar propagation issue
+to the previous point,
+also defeating the purpose of using
+.BR \%MOVE_MOUNT_BENEATH .
+.RE
+.RE
+.P
+If
+.I from_dirfd
+is a mount object file descriptor and
+.BR move_mount ()
+is operating on it directly,
+.I from_dirfd
+will remain associated with the mount object after
+.BR move_mount ()
+succeeds,
+so you may repeatedly use
+.I from_dirfd
+with
+.BR move_mount (2)
+and/or "*at()" system calls
+as many times as necessary.
+.SH RETURN VALUE
+On success,
+.BR move_mount ()
+returns 0.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EACCES
+Search permission is denied
+for one of the directories
+in the path prefix of one of
+.I from_path
+or
+.IR to_path .
+(See also
+.BR path_resolution (7).)
+.TP
+.B EBADF
+One of
+.I from_dirfd
+or
+.I to_dirfd
+is not a valid file descriptor.
+.TP
+.B EFAULT
+One of
+.I from_path
+or
+.I to_path
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B EINVAL
+The path indicated by
+.I from_dirfd
+and
+.I from_path
+is not a mount object.
+.TP
+.B EINVAL
+The mount object type
+of the source mount object and target inode
+are not compatible
+(i.e., the source is a file but the target is a directory, or vice-versa).
+.TP
+.B EINVAL
+The source mount object or target path
+are not in the calling process's mount namespace
+(or an anonymous mount namespace of the calling process).
+.TP
+.B EINVAL
+The source mount object's parent mount
+has shared mount propagation,
+and thus cannot be moved
+(as described in
+.BR mount_namespaces (7)).
+.TP
+.B EINVAL
+The source mount has
+.B MS_UNBINDABLE
+child mounts
+but the target path
+resides on a mount tree with shared mount propagation,
+which would otherwise cause the unbindable mounts to be propagated
+(as described in
+.BR mount_namespaces (7)).
+.TP
+.B EINVAL
+.B \%MOVE_MOUNT_BENEATH
+was attempted,
+but one of the listed restrictions was violated.
+.TP
+.B ELOOP
+Too many symbolic links encountered
+when resolving one of
+.I from_path
+or
+.IR to_path .
+.TP
+.B ENAMETOOLONG
+One of
+.I from_path
+or
+.I to_path
+is longer than
+.BR PATH_MAX .
+.TP
+.B ENOENT
+A component of one of
+.I from_path
+or
+.I to_path
+does not exist.
+.TP
+.B ENOENT
+One of
+.I from_path
+or
+.I to_path
+is an empty string,
+but the corresponding
+.BI MOVE_MOUNT_ * _EMPTY_PATH
+flag is not specified in
+.IR flags .
+.TP
+.B ENOTDIR
+A component of the path prefix of one of
+.I from_path
+or
+.I to_path
+is not a directory,
+or one of
+.I from_path
+or
+.I to_path
+is relative
+and the corresponding
+.I from_dirfd
+or
+.I to_dirfd
+is a file descriptor referring to a file other than a directory.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit 2db154b3ea8e14b04fee23e3fdfd5e9d17fbc6ae
+.\" commit 400913252d09f9cfb8cce33daee43167921fc343
+glibc 2.36.
+.SH EXAMPLES
+.BR move_mount ()
+can be used to move attached mounts like the following:
+.P
+.in +4n
+.EX
+move_mount(AT_FDCWD, "/a", AT_FDCWD, "/b", 0);
+.EE
+.in
+.P
+This would move the mount object mounted on
+.I /a
+to
+.IR /b .
+The above procedure is functionally equivalent to
+the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/a", "/b", NULL, MS_MOVE, NULL);
+.EE
+.in
+.P
+.BR move_mount ()
+can also be used in conjunction with file descriptors returned from
+.BR open_tree (2)
+or
+.BR open (2):
+.P
+.in +4n
+.EX
+int fd = open_tree(AT_FDCWD, "/mnt", 0); /* open("/mnt", O_PATH); */
+move_mount(fd, "", AT_FDCWD, "/mnt2", MOVE_MOUNT_F_EMPTY_PATH);
+move_mount(fd, "", AT_FDCWD, "/mnt3", MOVE_MOUNT_F_EMPTY_PATH);
+move_mount(fd, "", AT_FDCWD, "/mnt4", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+This would move the mount object mounted at
+.I /mnt
+to
+.IR /mnt2 ,
+then
+.IR /mnt3 ,
+and then
+.IR /mnt4 .
+.P
+If the source mount object
+indicated by
+.I from_dirfd
+and
+.I from_path
+is a detached mount object,
+.BR move_mount ()
+can be used to attach it to a mount point:
+.P
+.in +4n
+.EX
+int fsfd, mntfd;
+\&
+fsfd = fsopen("ext4", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/sda1", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NODEV);
+move_mount(mntfd, "", AT_FDCWD, "/home", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+This would create a new filesystem configuration context for ext4,
+configure it,
+create a detached mount object,
+and then attach it to
+.IR /home .
+The above procedure is functionally equivalent to
+the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/dev/sda1", "/home", "ext4", MS_NODEV, "user_xattr");
+.EE
+.in
+.P
+The same operation also works with detached bind-mounts created with
+.BR open_tree (2)
+with
+.BR OPEN_TREE_CLONE :
+.P
+.in +4n
+.EX
+int mntfd = open_tree(AT_FDCWD, "/home/cyphar", OPEN_TREE_CLONE);
+move_mount(mntfd, "", AT_FDCWD, "/root", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+This would create a new bind-mount of
+.I /home/cyphar
+as a detached mount object,
+and then attach it to
+.IR /root .
+The above procedure is functionally equivalent to
+the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/home/cyphar", "/root", NULL, MS_BIND, NULL);
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)

-- 
2.51.0

[PATCH v5 6/8] man/man2/open_tree.2: document "new" mount API

[Aleksa Sarai]

This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).

Co-authored-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Co-authored-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/open_tree.2 | 518 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 518 insertions(+)

diff --git a/man/man2/open_tree.2 b/man/man2/open_tree.2
new file mode 100644
index 0000000000000000000000000000000000000000..6b04a80927a8b6a394cf7ab341b8d6b29d42d304
--- /dev/null
+++ b/man/man2/open_tree.2
@@ -0,0 +1,518 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH open_tree 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+open_tree \- open path or create detached mount object and attach to fd
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#define _GNU_SOURCE         " "/* See feature_test_macros(7) */"
+.BR "#include <fcntl.h>" "          /* Definition of " AT_* " constants */"
+.B #include <sys/mount.h>
+.P
+.BI "int open_tree(int " dirfd ", const char *" path ", unsigned int " flags );
+.fi
+.SH DESCRIPTION
+The
+.BR open_tree ()
+system call is part of
+the suite of file-descriptor-based mount facilities in Linux.
+.IP \[bu] 3
+If
+.I flags
+contains
+.BR \%OPEN_TREE_CLONE ,
+.BR open_tree ()
+creates a detached mount object
+which consists of a bind-mount of
+the path specified by the
+.IR path .
+A new file descriptor
+associated with the detached mount object
+is then returned.
+The mount object is equivalent to a bind-mount
+that would be created by
+.BR mount (2)
+called with
+.BR \%MS_BIND ,
+except that it is tied to a file descriptor
+and is not mounted onto the filesystem.
+.IP
+As with file descriptors returned from
+.BR fsmount (2),
+the resultant file descriptor can then be used with
+.BR move_mount (2),
+.BR mount_setattr (2),
+or other such system calls to do further mount operations.
+.IP
+This mount object will be unmounted and destroyed
+when the file descriptor is closed
+if it was not otherwise attached to a mount point
+by calling
+.BR move_mount (2).
+This implicit unmount operation is lazy\[em]\c
+akin to calling
+.BR umount2 (2)
+with
+.BR \%MNT_DETACH ;
+thus,
+any existing open references to files
+from the mount object
+will continue to work,
+and the mount object will only be completely destroyed
+once it ceases to be busy.
+.IP \[bu]
+If
+.I flags
+does not contain
+.BR \%OPEN_TREE_CLONE ,
+.BR open_tree ()
+returns a file descriptor
+that is exactly equivalent to
+one produced by
+.BR openat (2)
+when called with the same
+.I dirfd
+and
+.IR path .
+.P
+In either case, the resultant file descriptor
+acts the same as one produced by
+.BR open (2)
+with
+.BR O_PATH ,
+meaning it can also be used as a
+.I dirfd
+argument to
+"*at()" system calls.
+However,
+unlike
+.BR open (2)
+called with
+.BR O_PATH ,
+automounts will
+by default
+be triggered by
+.BR open_tree ()
+unless
+.B \%AT_NO_AUTOMOUNT
+is included in
+.IR flags .
+.P
+As with "*at()" system calls,
+.BR open_tree ()
+uses the
+.I dirfd
+argument in conjunction with the
+.I path
+argument to determine the path to operate on, as follows:
+.IP \[bu] 3
+If the pathname given in
+.I path
+is absolute, then
+.I dirfd
+is ignored.
+.IP \[bu]
+If the pathname given in
+.I path
+is relative and
+.I dirfd
+is the special value
+.BR \%AT_FDCWD ,
+then
+.I path
+is interpreted relative to
+the current working directory
+of the calling process (like
+.BR open (2)).
+.IP \[bu]
+If the pathname given in
+.I path
+is relative,
+then it is interpreted relative to
+the directory referred to by the file descriptor
+.I dirfd
+(rather than relative to
+the current working directory
+of the calling process,
+as is done by
+.BR open (2)
+for a relative pathname).
+In this case,
+.I dirfd
+must be a directory
+that was opened for reading
+.RB ( \%O_RDONLY )
+or using the
+.B O_PATH
+flag.
+.IP \[bu]
+If
+.I path
+is an empty string,
+and
+.I flags
+contains
+.BR \%AT_EMPTY_PATH ,
+then the file descriptor
+.I dirfd
+is operated on directly.
+In this case,
+.I dirfd
+may refer to any type of file,
+not just a directory.
+.P
+See
+.BR openat (2)
+for an explanation of why the
+.I dirfd
+argument is useful.
+.P
+.I flags
+can be used to control aspects of the path lookup
+and properties of the returned file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B \%AT_EMPTY_PATH
+If
+.I path
+is an empty string, operate on the file referred to by
+.I dirfd
+(which may have been obtained from
+.BR open (2),
+.BR fsmount (2),
+or from another
+.BR open_tree ()
+call).
+In this case,
+.I dirfd
+may refer to any type of file, not just a directory.
+If
+.I dirfd
+is
+.BR \%AT_FDCWD ,
+.BR open_tree ()
+will operate on the current working directory
+of the calling process.
+This flag is Linux-specific;
+define
+.B \%_GNU_SOURCE
+to obtain its definition.
+.TP
+.B \%AT_NO_AUTOMOUNT
+Do not automount the terminal ("basename") component of
+.I path
+if it is a directory that is an automount point.
+This allows you to create a handle to the automount point itself,
+rather than the location it would mount.
+This flag has no effect if the mount point has already been mounted over.
+This flag is Linux-specific;
+define
+.B \%_GNU_SOURCE
+to obtain its definition.
+.TP
+.B \%AT_SYMLINK_NOFOLLOW
+If
+.I path
+is a symbolic link, do not dereference it;
+instead,
+create either a handle to the link itself
+or a bind-mount of it.
+The resultant file descriptor is indistinguishable from one produced by
+.BR openat (2)
+with
+.BR \%O_PATH | O_NOFOLLLOW .
+.TP
+.B \%OPEN_TREE_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.TP
+.B \%OPEN_TREE_CLONE
+Rather than creating an
+.BR openat (2)-style
+.B O_PATH
+file descriptor,
+create a bind-mount of
+.I path
+(akin to
+.IR \%mount\~\-\-bind )
+as a detached mount object.
+In order to do this operation,
+the calling process must have the
+.B \%CAP_SYS_ADMIN
+capability.
+.TP
+.B \%AT_RECURSIVE
+Create a recursive bind-mount of the path
+(akin to
+.IR \%mount\~\-\-rbind )
+as a detached mount object.
+This flag is only permitted in conjunction with
+.BR \%OPEN_TREE_CLONE .
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EACCES
+Search permission is denied for one of the directories
+in the path prefix of
+.IR path .
+(See also
+.BR path_resolution (7).)
+.TP
+.B EBADF
+.I path
+is relative but
+.I dirfd
+is neither
+.B \%AT_FDCWD
+nor a valid file descriptor.
+.TP
+.B EFAULT
+.I path
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B ELOOP
+Too many symbolic links encountered when resolving
+.IR path .
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENAMETOOLONG
+.I path
+is longer than
+.BR PATH_MAX .
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B ENOENT
+A component of
+.I path
+does not exist, or is a dangling symbolic link.
+.TP
+.B ENOENT
+.I path
+is an empty string, but
+.B AT_EMPTY_PATH
+is not specified in
+.IR flags .
+.TP
+.B ENOTDIR
+A component of the path prefix of
+.I path
+is not a directory, or
+.I path
+is relative and
+.I dirfd
+is a file descriptor referring to a file other than a directory.
+.TP
+.B ENOSPC
+The "anonymous" mount namespace
+necessary to contain the
+.B \%OPEN_TREE_CLONE
+detached bind-mount mount object
+could not be allocated,
+as doing so would exceed
+the configured per-user limit on
+the number of mount namespaces in the current user namespace.
+(See also
+.BR namespaces (7).)
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+.I flags
+contains
+.B \%OPEN_TREE_CLONE
+but the calling process does not have the required
+.B CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit a07b20004793d8926f78d63eb5980559f7813404
+.\" commit 400913252d09f9cfb8cce33daee43167921fc343
+glibc 2.36.
+.SH NOTES
+.SS Mount propagation
+The bind-mount mount objects created by
+.BR open_tree ()
+with
+.B \%OPEN_TREE_CLONE
+are not associated with
+the mount namespace of the calling process.
+Instead, each mount object is placed
+in a newly allocated "anonymous" mount namespace
+associated with the calling process.
+.P
+One of the side-effects of this is that
+(unlike bind-mounts created with
+.BR mount (2)),
+mount propagation
+(as described in
+.BR mount_namespaces (7))
+will not be applied to bind-mounts created by
+.BR open_tree ()
+until the bind-mount is attached with
+.BR move_mount (2),
+at which point the mount object
+will be associated with the mount namespace
+where it was attached
+and mount propagation will resume.
+Note that any mount propagation events that occurred
+before the mount object was attached
+will
+.I not
+be propagated to the mount object,
+even after it is attached.
+.SH EXAMPLES
+The following examples show how
+.BR open_tree ()
+can be used in place of more traditional
+.BR mount (2)
+calls with
+.BR MS_BIND .
+.P
+.in +4n
+.EX
+int srcfd = open_tree(AT_FDCWD, "/var", OPEN_TREE_CLONE);
+move_mount(srcfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+First,
+a detached bind-mount mount object of
+.I /var
+is created
+and associated with the file descriptor
+.IR srcfd .
+Then, the mount object is attached to
+.I /mnt
+using
+.BR move_mount (2)
+with
+.B \%MOVE_MOUNT_F_EMPTY_PATH
+to request that the detached mount object
+associated with the file descriptor
+.I srcfd
+be moved (and thus attached) to
+.IR /mnt .
+.P
+The above procedure is functionally equivalent to
+the following mount operation using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/var", "/mnt", NULL, MS_BIND, NULL);
+.EE
+.in
+.P
+.B \%OPEN_TREE_CLONE
+can be combined with
+.B \%AT_RECURSIVE
+to create recursive detached bind-mount mount objects,
+which in turn can be attached to mount points
+to create recursive bind-mounts.
+.P
+.in +4n
+.EX
+int srcfd = open_tree(AT_FDCWD, "/var",
+                      OPEN_TREE_CLONE | AT_RECURSIVE);
+move_mount(srcfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+The above procedure is functionally equivalent to
+the following mount operation using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/var", "/mnt", NULL, MS_BIND | MS_REC, NULL);
+.EE
+.in
+.P
+One of the primary benefits of using
+.BR open_tree ()
+and
+.BR move_mount (2)
+over the traditional
+.BR mount (2)
+is that operating with
+.IR dirfd -style
+file descriptors is far easier and more intuitive.
+.P
+.in +4n
+.EX
+int srcfd = open_tree(100, "", AT_EMPTY_PATH | OPEN_TREE_CLONE);
+move_mount(srcfd, "", 200, "foo", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+The above procedure is roughly equivalent to
+the following mount operation using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/proc/self/fd/100",
+      "/proc/self/fd/200/foo",
+      NULL, MS_BIND, NULL);
+.EE
+.in
+.P
+In addition, you can use the file descriptor returned by
+.BR open_tree ()
+as the
+.I dirfd
+argument to any "*at()" system calls:
+.P
+.in +4n
+.EX
+int dirfd, fd;
+\&
+dirfd = open_tree(AT_FDCWD, "/etc", OPEN_TREE_CLONE);
+fd = openat(dirfd, "passwd", O_RDONLY);
+fchmodat(dirfd, "shadow", 0000, 0);
+close(dirfd);
+close(fd);
+/* The bind-mount is now destroyed */
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR mount_namespaces (7)

-- 
2.51.0

[PATCH v5 7/8] man/man2/open_tree{,_attr}.2: document new open_tree_attr() API

[Aleksa Sarai]

This is a new API added in Linux 6.15, and is effectively just a minor
expansion of open_tree(2) in order to allow for MOUNT_ATTR_IDMAP to be
changed for an existing ID-mapped mount.  glibc does not yet have a
wrapper for this.

While working on this man-page, I discovered a bug in open_tree_attr(2)
that accidentally permitted changing MOUNT_ATTR_IDMAP for extant
detached ID-mapped mount objects.  This is definitely a bug, but there
is no need to add this to BUGS because the patch to fix this has already
been accepted (slated for 6.18, and will be backported to 6.15+).

Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/open_tree.2      | 191 ++++++++++++++++++++++++++++++++++++++++++++++
 man/man2/open_tree_attr.2 |   1 +
 2 files changed, 192 insertions(+)

diff --git a/man/man2/open_tree.2 b/man/man2/open_tree.2
index 6b04a80927a8b6a394cf7ab341b8d6b29d42d304..8b48f3b782bbb8d017ff50ae6624707cc1db992b 100644
--- a/man/man2/open_tree.2
+++ b/man/man2/open_tree.2
@@ -15,7 +15,19 @@ .SH SYNOPSIS
 .B #include <sys/mount.h>
 .P
 .BI "int open_tree(int " dirfd ", const char *" path ", unsigned int " flags );
+.P
+.BR "#include <sys/syscall.h>" "    /* Definition of " SYS_* " constants */"
+.P
+.B int syscall(SYS_open_tree_attr,
+.BI "            int " dirfd ", const char *" path ", unsigned int " flags ,
+.BI "            struct mount_attr *_Nullable " attr ", size_t " size );
 .fi
+.P
+.IR Note :
+glibc provides no wrapper for
+.BR open_tree_attr (),
+necessitating the use of
+.BR syscall (2).
 .SH DESCRIPTION
 The
 .BR open_tree ()
@@ -263,6 +275,129 @@ .SH DESCRIPTION
 as a detached mount object.
 This flag is only permitted in conjunction with
 .BR \%OPEN_TREE_CLONE .
+.SS open_tree_attr()
+The
+.BR open_tree_attr ()
+system call operates in exactly the same way as
+.BR open_tree (),
+except for the differences described here.
+.P
+After performing the same operation as with
+.BR open_tree (),
+.BR open_tree_attr ()
+will apply the mount attribute changes described in
+.I attr
+to the file descriptor before it is returned.
+(See
+.BR mount_attr (2type)
+for a description of the
+.I \%mount_attr
+structure.
+As described in
+.BR mount_setattr (2),
+.I size
+must be set to
+.I \%sizeof(struct mount_attr)
+in order to support future extensions.)
+If
+.I attr
+is NULL,
+or has
+.IR \%attr.attr_clr ,
+.IR \%attr.attr_set ,
+and
+.I \%attr.propagation
+all set to zero,
+then
+.BR open_tree_attr ()
+has identical behaviour to
+.BR open_tree ().
+.P
+The application of
+.I attr
+to the resultant file descriptor
+has identical semantics to
+.BR mount_setattr (2),
+except for the following extensions and general caveats:
+.IP \[bu] 3
+Unlike
+.BR mount_setattr (2)
+called with a regular
+.B OPEN_TREE_CLONE
+detached mount object from
+.BR open_tree (),
+.BR open_tree_attr ()
+can specify a different setting for
+.B \%MOUNT_ATTR_IDMAP
+to the original mount object cloned with
+.BR \%OPEN_TREE_CLONE .
+.IP
+Adding
+.B \%MOUNT_ATTR_IDMAP
+to
+.I \%attr.attr_clr
+will disable ID-mapping for the new mount object;
+adding
+.B \%MOUNT_ATTR_IDMAP
+to
+.I \%attr.attr_set
+will configure the mount object to have the ID-mapping defined by
+the user namespace referenced by the file descriptor
+.IR \%attr.userns_fd .
+(The semantics of which are identical to when
+.BR mount_setattr (2)
+is used to configure
+.BR \%MOUNT_ATTR_IDMAP .)
+.IP
+Changing or removing the mapping
+of an ID-mapped mount is only permitted
+if a new detached mount object is being created with
+.I flags
+including
+.BR \%OPEN_TREE_CLONE .
+.\" Aleksa Sarai
+.\"  At time of writing, this is not actually true because of a bug where
+.\"  open_tree_attr() would accidentally permit changing MOUNT_ATTR_IDMAP for
+.\"  existing detached mount objects without setting OPEN_TREE_CLONE, but a
+.\"  patch to fix it has been slated for 6.18 and will be backported to 6.15+.
+.\"  <https://lore.kernel.org/r/20250808-open_tree_attr-bugfix-idmap-v1-0-0ec7bc05646c@cyphar.com/>
+.IP \[bu]
+If
+.I flags
+contains
+.BR \%AT_RECURSIVE ,
+then the attributes described in
+.I attr
+are applied recursively
+(just as when
+.BR mount_setattr (2)
+is called with
+.BR \%AT_RECURSIVE ).
+However, this applies in addition to the
+.BR open_tree ()-specific
+behaviour regarding
+.BR \%AT_RECURSIVE ,
+and thus
+.I flags
+must also contain
+.BR \%OPEN_TREE_CLONE .
+.P
+Note that if
+.I flags
+does not contain
+.BR \%OPEN_TREE_CLONE ,
+.BR open_tree_attr ()
+will attempt to modify the mount attributes of
+the mount object attached at
+the path described by
+.I dirfd
+and
+.IR path .
+As with
+.BR mount_setattr (2),
+if said path is not a mount point,
+.BR open_tree_attr ()
+will return an error.
 .SH RETURN VALUE
 On success, a new file descriptor is returned.
 On error, \-1 is returned, and
@@ -356,10 +491,15 @@ .SH ERRORS
 .SH STANDARDS
 Linux.
 .SH HISTORY
+.SS open_tree()
 Linux 5.2.
 .\" commit a07b20004793d8926f78d63eb5980559f7813404
 .\" commit 400913252d09f9cfb8cce33daee43167921fc343
 glibc 2.36.
+.SS open_tree_attr()
+Linux 6.15.
+.\" commit c4a16820d90199409c9bf01c4f794e1e9e8d8fd8
+.\" commit 7a54947e727b6df840780a66c970395ed9734ebe
 .SH NOTES
 .SS Mount propagation
 The bind-mount mount objects created by
@@ -507,6 +647,57 @@ .SH EXAMPLES
 /* The bind-mount is now destroyed */
 .EE
 .in
+.SS open_tree_attr()
+The following is an example of how
+.BR open_tree_attr ()
+can be used to
+take an existing id-mapped mount and
+construct a new bind-mount mount object
+with a different
+.B \%MOUNT_ATTR_IDMAP
+attribute.
+The resultant detached mount object
+can be used
+like any other mount object
+returned by
+.BR open_tree ().
+.P
+.in +4n
+.EX
+int nsfd1, nsfd2;
+int mntfd1, mntfd2, mntfd3;
+struct mount_attr attr;
+mntfd1 = open_tree(AT_FDCWD, "/foo", OPEN_TREE_CLONE);
+\&
+/* Configure the id-mapping of mntfd1 */
+nsfd1 = open("/proc/1234/ns/user", O_RDONLY);
+memset(&attr, 0, sizeof(attr));
+attr.attr_set = MOUNT_ATTR_IDMAP;
+attr.userns_fd = nsfd1;
+mount_setattr(mntfd1, "", AT_EMPTY_PATH, &attr, sizeof(attr));
+\&
+/* Create a new copy with a different id-mapping */
+nsfd2 = open("/proc/5678/ns/user", O_RDONLY);
+memset(&attr, 0, sizeof(attr));
+attr.attr_clr = MOUNT_ATTR_IDMAP;
+.\" Using .attr_clr is not strictly necessary but makes the intent clearer.
+attr.attr_set = MOUNT_ATTR_IDMAP;
+attr.userns_fd = nsfd2;
+mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
+                   &attr, sizeof(attr));
+\&
+/* Create a new copy with the id-mapping cleared */
+memset(&attr, 0, sizeof(attr));
+attr.attr_clr = MOUNT_ATTR_IDMAP;
+mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
+                   &attr, sizeof(attr));
+.EE
+.in
+.P
+.BR open_tree_attr ()
+can also be used
+with attached mount objects;
+the above example is only intended to be illustrative.
 .SH SEE ALSO
 .BR fsconfig (2),
 .BR fsmount (2),
diff --git a/man/man2/open_tree_attr.2 b/man/man2/open_tree_attr.2
new file mode 100644
index 0000000000000000000000000000000000000000..e57269bbd269bcce0b0a974425644ba75e379f2f
--- /dev/null
+++ b/man/man2/open_tree_attr.2
@@ -0,0 +1 @@
+.so man2/open_tree.2

-- 
2.51.0

[PATCH v5 8/8] man/man2/{fsconfig,mount_setattr}.2: add note about attribute-parameter distinction

[Aleksa Sarai]

This was not particularly well documented in mount(8) nor mount(2), and
since this is a fairly notable aspect of the new mount API, we should
probably add some words about it.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 man/man2/fsconfig.2      | 12 ++++++++++++
 man/man2/mount_setattr.2 | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
index a2d844a105c74f17af640d6991046dbd5fa69cf0..3b972761196b9c1577a6f324a2f4135471dd0ab3 100644
--- a/man/man2/fsconfig.2
+++ b/man/man2/fsconfig.2
@@ -580,6 +580,18 @@ .SS Generic filesystem parameters
 Linux Security Modules (LSMs)
 are also generic with respect to the underlying filesystem.
 See the documentation for the LSM you wish to configure for more details.
+.SS Mount attributes and filesystem parameters
+Some filesystem parameters
+(traditionally associated with
+.BR mount (8)-style
+options)
+have a sibling mount attribute
+with superficially similar user-facing behaviour.
+.P
+For a description of the distinction between
+mount attributes and filesystem parameters,
+see the "Mount attributes and filesystem parameters" subsection of
+.BR mount_setattr (2).
 .SH CAVEATS
 .SS Filesystem parameter types
 As a result of
diff --git a/man/man2/mount_setattr.2 b/man/man2/mount_setattr.2
index 2f8a79dfde722b7b58b80797d89798076af94f55..efe22496be95383b986d9a3623324d472a76c189 100644
--- a/man/man2/mount_setattr.2
+++ b/man/man2/mount_setattr.2
@@ -792,6 +792,45 @@ .SS ID-mapped mounts
 .BR chown (2)
 system call changes the ownership globally and permanently.
 .\"
+.SS Mount attributes and filesystem parameters
+Some mount attributes
+(traditionally associated with
+.BR mount (8)-style
+options)
+have a sibling filesystem parameter
+with superficially similar user-facing behaviour.
+For example, the
+.I \-o\~ro
+option to
+.BR mount (8)
+can refer to the
+"read-only" filesystem parameter,
+or the "read-only" mount attribute.
+Both of these result in mount objects becoming read-only,
+but they do have different behaviour.
+.P
+The distinction between these two kinds of option is that
+mount object attributes are applied per-mount-object
+(allowing different mount objects
+derived from a given filesystem instance
+to have different attributes),
+while filesystem instance parameters
+("superblock flags" in kernel-developer parlance)
+apply to all mount objects
+derived from the same filesystem instance.
+.P
+When using
+.BR mount (2),
+the line between these two types of mount options was blurred.
+However, with
+.BR mount_setattr ()
+and
+.BR fsconfig (2),
+the distinction is made much clearer.
+Mount attributes are configured with
+.BR mount_setattr (),
+while filesystem parameters are configured using
+.BR fsconfig (2).
 .SS Extensibility
 In order to allow for future extensibility,
 .BR mount_setattr ()

-- 
2.51.0

Re: [PATCH v5 1/8] man/man2/fsopen.2: document "new" mount API

[Alejandro Colomar]

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Hi Aleksa,

On Thu, Sep 25, 2025 at 01:31:23AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Patch applied.  Thanks!


Have a lovely day!
Alex

> ---
>  man/man2/fsopen.2 | 385 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 385 insertions(+)
> 
> diff --git a/man/man2/fsopen.2 b/man/man2/fsopen.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..7fbc6c3d28e2e741cd9003c105621b4242abd487
> --- /dev/null
> +++ b/man/man2/fsopen.2
> @@ -0,0 +1,385 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH fsopen 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +fsopen \- create a new filesystem context
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ,\~ \-lc )
> +.SH SYNOPSIS
> +.nf
> +.B #include <sys/mount.h>
> +.P
> +.BI "int fsopen(const char *" fsname ", unsigned int " flags );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR fsopen ()
> +system call is part of
> +the suite of file-descriptor-based mount facilities in Linux.
> +.P
> +.BR fsopen ()
> +creates a blank filesystem configuration context within the kernel
> +for the filesystem named by
> +.I fsname
> +and places it into creation mode.
> +A new file descriptor
> +associated with the filesystem configuration context
> +is then returned.
> +The calling process must have the
> +.B \%CAP_SYS_ADMIN
> +capability in order to create a new filesystem configuration context.
> +.P
> +A filesystem configuration context is
> +an in-kernel representation of a pending transaction,
> +containing a set of configuration parameters that are to be applied
> +when creating a new instance of a filesystem
> +(or modifying the configuration of an existing filesystem instance,
> +such as when using
> +.BR fspick (2)).
> +.P
> +After obtaining a filesystem configuration context with
> +.BR fsopen (),
> +the general workflow for operating on the context looks like the following:
> +.IP (1) 5
> +Pass the filesystem context file descriptor to
> +.BR fsconfig (2)
> +to specify any desired filesystem parameters.
> +This may be done as many times as necessary.
> +.IP (2)
> +Pass the same filesystem context file descriptor to
> +.BR fsconfig (2)
> +with
> +.B \%FSCONFIG_CMD_CREATE
> +to create an instance of the configured filesystem.
> +.IP (3)
> +Pass the same filesystem context file descriptor to
> +.BR fsmount (2)
> +to create a new detached mount object for
> +the root of the filesystem instance,
> +which is then attached to a new file descriptor.
> +(This also places the filesystem context file descriptor into
> +reconfiguration mode,
> +similar to the mode produced by
> +.BR fspick (2).)
> +Once a mount object has been created with
> +.BR fsmount (2),
> +the filesystem context file descriptor can be safely closed.
> +.IP (4)
> +Now that a mount object has been created,
> +you may
> +.RS
> +.IP \[bu] 3
> +use the detached mount object file descriptor as a
> +.I dirfd
> +argument to "*at()" system calls;
> +and/or
> +.IP \[bu]
> +attach the mount object to a mount point
> +by passing the mount object file descriptor to
> +.BR move_mount (2).
> +This will also prevent the mount object from
> +being unmounted and destroyed when
> +the mount object file descriptor is closed.
> +.RE
> +.IP
> +The mount object file descriptor will
> +remain associated with the mount object
> +even after doing the above operations,
> +so you may repeatedly use the mount object file descriptor with
> +.BR move_mount (2)
> +and/or "*at()" system calls
> +as many times as necessary.
> +.P
> +A filesystem context will move between different modes
> +throughout its lifecycle
> +(such as the creation phase
> +when created with
> +.BR fsopen (),
> +the reconfiguration phase
> +when an existing filesystem instance is selected with
> +.BR fspick (2),
> +and the intermediate "awaiting-mount" phase
> +.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
> +between
> +.B \%FSCONFIG_CMD_CREATE
> +and
> +.BR fsmount (2)),
> +which has an impact on
> +what operations are permitted on the filesystem context.
> +.P
> +The file descriptor returned by
> +.BR fsopen ()
> +also acts as a channel for filesystem drivers to
> +provide more comprehensive diagnostic information
> +than is normally provided through the standard
> +.BR errno (3)
> +interface for system calls.
> +If an error occurs at any time during the workflow mentioned above,
> +calling
> +.BR read (2)
> +on the filesystem context file descriptor
> +will retrieve any ancillary information about the encountered errors.
> +(See the "Message retrieval interface" section
> +for more details on the message format.)
> +.P
> +.I flags
> +can be used to control aspects of
> +the creation of the filesystem configuration context file descriptor.
> +A value for
> +.I flags
> +is constructed by bitwise ORing
> +zero or more of the following constants:
> +.RS
> +.TP
> +.B FSOPEN_CLOEXEC
> +Set the close-on-exec
> +.RB ( FD_CLOEXEC )
> +flag on the new file descriptor.
> +See the description of the
> +.B O_CLOEXEC
> +flag in
> +.BR open (2)
> +for reasons why this may be useful.
> +.RE
> +.P
> +A list of filesystems supported by the running kernel
> +(and thus a list of valid values for
> +.IR fsname )
> +can be obtained from
> +.IR /proc/filesystems .
> +(See also
> +.BR proc_filesystems (5).)
> +.SS Message retrieval interface
> +When doing operations on a filesystem configuration context,
> +the filesystem driver may choose to provide
> +ancillary information to userspace
> +in the form of message strings.
> +.P
> +The filesystem context file descriptors returned by
> +.BR fsopen ()
> +and
> +.BR fspick (2)
> +may be queried for message strings at any time by calling
> +.BR read (2)
> +on the file descriptor.
> +Each call to
> +.BR read (2)
> +will return a single message,
> +prefixed to indicate its class:
> +.RS
> +.TP
> +.BI e\~ message
> +An error message was logged.
> +This is usually associated with an error being returned
> +from the corresponding system call which triggered this message.
> +.TP
> +.BI w\~ message
> +A warning message was logged.
> +.TP
> +.BI i\~ message
> +An informational message was logged.
> +.RE
> +.P
> +Messages are removed from the queue as they are read.
> +Note that the message queue has limited depth,
> +so it is possible for messages to get lost.
> +If there are no messages in the message queue,
> +.B read(2)
> +will return \-1 and
> +.I errno
> +will be set to
> +.BR \%ENODATA .
> +If the
> +.I buf
> +argument to
> +.BR read (2)
> +is not large enough to contain the entire message,
> +.BR read (2)
> +will return \-1 and
> +.I errno
> +will be set to
> +.BR \%EMSGSIZE .
> +(See BUGS.)
> +.P
> +If there are multiple filesystem contexts
> +referencing the same filesystem instance
> +(such as if you call
> +.BR fspick (2)
> +multiple times for the same mount),
> +each one gets its own independent message queue.
> +This does not apply to multiple file descriptors that are
> +tied to the same underlying open file description
> +(such as those created with
> +.BR dup (2)).
> +.P
> +Message strings will usually be prefixed by
> +the name of the filesystem or kernel subsystem
> +that logged the message,
> +though this may not always be the case.
> +See the Linux kernel source code for details.
> +.SH RETURN VALUE
> +On success, a new file descriptor is returned.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EFAULT
> +.I fsname
> +is NULL
> +or a pointer to a location
> +outside the calling process's accessible address space.
> +.TP
> +.B EINVAL
> +.I flags
> +had an invalid flag set.
> +.TP
> +.B EMFILE
> +The calling process has too many open files to create more.
> +.TP
> +.B ENFILE
> +The system has too many open files to create more.
> +.TP
> +.B ENODEV
> +The filesystem named by
> +.I fsname
> +is not supported by the kernel.
> +.TP
> +.B ENOMEM
> +The kernel could not allocate sufficient memory to complete the operation.
> +.TP
> +.B EPERM
> +The calling process does not have the required
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.SH STANDARDS
> +Linux.
> +.SH HISTORY
> +Linux 5.2.
> +.\" commit 24dcb3d90a1f67fe08c68a004af37df059d74005
> +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> +glibc 2.36.
> +.SH BUGS
> +.SS Message retrieval interface and \fB\%EMSGSIZE\fP
> +As described in the "Message retrieval interface" subsection above,
> +calling
> +.BR read (2)
> +with too small a buffer to contain
> +the next pending message in the message queue
> +for the filesystem configuration context
> +will cause
> +.BR read (2)
> +to return \-1 and set
> +.BR errno (3)
> +to
> +.BR \%EMSGSIZE .
> +.P
> +However,
> +this failed operation still
> +consumes the message from the message queue.
> +This effectively discards the message silently,
> +as no data is copied into the
> +.BR read (2)
> +buffer.
> +.P
> +Programs should take care to ensure that
> +their buffers are sufficiently large
> +to contain any reasonable message string,
> +in order to avoid silently losing valuable diagnostic information.
> +.\" Aleksa Sarai
> +.\"   This unfortunate behaviour has existed since this feature was merged, but
> +.\"   I have sent a patchset which will finally fix it.
> +.\"   <https://lore.kernel.org/r/20250807-fscontext-log-cleanups-v3-1-8d91d6242dc3@cyphar.com/>
> +.SH EXAMPLES
> +To illustrate the workflow for creating a new mount,
> +the following is an example of how to mount an
> +.BR ext4 (5)
> +filesystem stored on
> +.I /dev/sdb1
> +onto
> +.IR /mnt .
> +.P
> +.in +4n
> +.EX
> +int fsfd, mntfd;
> +\&
> +fsfd = fsopen("ext4", FSOPEN_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_PATH, "source", "/dev/sdb1", AT_FDCWD);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "noatime", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "iversion", NULL, 0)
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_RELATIME);
> +move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +First,
> +an ext4 configuration context is created and attached to the file descriptor
> +.IR fsfd .
> +Then, a series of parameters
> +(such as the source of the filesystem)
> +are provided using
> +.BR fsconfig (2),
> +followed by the filesystem instance being created with
> +.BR \%FSCONFIG_CMD_CREATE .
> +.BR fsmount (2)
> +is then used to create a new mount object attached to the file descriptor
> +.IR mntfd ,
> +which is then attached to the intended mount point using
> +.BR move_mount (2).
> +.P
> +The above procedure is functionally equivalent to
> +the following mount operation using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount("/dev/sdb1", "/mnt", "ext4", MS_RELATIME,
> +      "ro,noatime,acl,user_xattr,iversion");
> +.EE
> +.in
> +.P
> +And here's an example of creating a mount object
> +of an NFS server share
> +and setting a Smack security module label.
> +However, instead of attaching it to a mount point,
> +the program uses the mount object directly
> +to open a file from the NFS share.
> +.P
> +.in +4n
> +.EX
> +int fsfd, mntfd, fd;
> +\&
> +fsfd = fsopen("nfs", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "example.com/pub", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "nfsvers", "3", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "rsize", "65536", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "wsize", "65536", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "smackfsdef", "foolabel", 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "rdma", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, 0, MOUNT_ATTR_NODEV);
> +fd = openat(mntfd, "src/linux-5.2.tar.xz", O_RDONLY);
> +.EE
> +.in
> +.P
> +Unlike the previous example,
> +this operation has no trivial equivalent with
> +.BR mount (2),
> +as it was not previously possible to create a mount object
> +that is not attached to any mount point.
> +.SH SEE ALSO
> +.BR fsconfig (2),
> +.BR fsmount (2),
> +.BR fspick (2),
> +.BR mount (2),
> +.BR mount_setattr (2),
> +.BR move_mount (2),
> +.BR open_tree (2),
> +.BR mount_namespaces (7)
> 
> -- 
> 2.51.0
> 
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 2/8] man/man2/fspick.2: document "new" mount API

[Alejandro Colomar]

[-- Attachment #1: Type: text/plain, Size: 9639 bytes --]

Hi Aleksa,

On Thu, Sep 25, 2025 at 01:31:24AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Patch applied (with minor amends; see below).  Thanks!

> ---
>  man/man2/fspick.2 | 343 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 343 insertions(+)
> 
> diff --git a/man/man2/fspick.2 b/man/man2/fspick.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..800aed81d38384be4563f2558e3cef846d7e7cee
> --- /dev/null
> +++ b/man/man2/fspick.2
> @@ -0,0 +1,343 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH fspick 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +fspick \- select filesystem for reconfiguration
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ,\~ \-lc )
> +.SH SYNOPSIS
> +.nf
> +.BR "#include <fcntl.h>" "          /* Definition of " AT_* " constants */"
> +.B #include <sys/mount.h>
> +.P
> +.BI "int fspick(int " dirfd ", const char *" path ", unsigned int " flags );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR fspick ()
> +system call is part of
> +the suite of file-descriptor-based mount facilities in Linux.
> +.P
> +.BR fspick ()
> +creates a new filesystem configuration context
> +for the extant filesystem instance
> +associated with the path described by
> +.I dirfd
> +and
> +.IR path ,
> +places it into reconfiguration mode
> +(similar to
> +.BR mount (8)
> +with the
> +.I \-o\~remount

I've changed this to bold.  Command options are literal, and thus we
document them with bold (see for example ldd(1)).

> +option).
> +A new file descriptor
> +associated with the filesystem configuration context
> +is then returned.
> +The calling process must have the
> +.B \%CAP_SYS_ADMIN
> +capability in order to create a new filesystem configuration context.
> +.P
> +The resultant file descriptor can be used with
> +.BR fsconfig (2)
> +to specify the desired set of changes to
> +filesystem parameters of the filesystem instance.
> +Once the desired set of changes have been configured,
> +the changes can be effectuated by calling
> +.BR fsconfig (2)
> +with the
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +command.
> +In contrast to
> +the behaviour of
> +.B MS_REMOUNT
> +with
> +.BR mount (2),
> +.BR fspick ()
> +instantiates the filesystem configuration context
> +with a copy of
> +the extant filesystem's filesystem parameters;
> +thus,
> +subsequent
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +operations
> +will only update filesystem parameters
> +explicitly modified with
> +.BR fsconfig (2).
> +.P
> +As with "*at()" system calls,
> +.BR fspick ()
> +uses the
> +.I dirfd
> +argument in conjunction with the
> +.I path
> +argument to determine the path to operate on, as follows:
> +.IP \[bu] 3
> +If the pathname given in
> +.I path
> +is absolute, then
> +.I dirfd
> +is ignored.
> +.IP \[bu]
> +If the pathname given in
> +.I path
> +is relative and
> +.I dirfd
> +is the special value
> +.BR \%AT_FDCWD ,
> +then
> +.I path
> +is interpreted relative to
> +the current working directory
> +of the calling process (like
> +.BR open (2)).
> +.IP \[bu]
> +If the pathname given in
> +.I path
> +is relative,
> +then it is interpreted relative to
> +the directory referred to by the file descriptor
> +.I dirfd
> +(rather than relative to
> +the current working directory
> +of the calling process,
> +as is done by
> +.BR open (2)
> +for a relative pathname).
> +In this case,
> +.I dirfd
> +must be a directory
> +that was opened for reading
> +.RB ( O_RDONLY )
> +or using the
> +.B O_PATH
> +flag.
> +.IP \[bu]
> +If
> +.I path
> +is an empty string,
> +and
> +.I flags
> +contains
> +.BR \%FSPICK_EMPTY_PATH ,
> +then the file descriptor
> +.I dirfd
> +is operated on directly.
> +In this case,
> +.I dirfd
> +may refer to any type of file,
> +not just a directory.
> +.P
> +See
> +.BR openat (2)
> +for an explanation of why the
> +.I dirfd
> +argument is useful.
> +.P
> +.I flags
> +can be used to control aspects of how
> +.I path
> +is resolved and
> +properties of the returned file descriptor.
> +A value for
> +.I flags
> +is constructed by bitwise ORing
> +zero or more of the following constants:
> +.RS
> +.TP
> +.B FSPICK_CLOEXEC
> +Set the close-on-exec
> +.RB ( FD_CLOEXEC )
> +flag on the new file descriptor.
> +See the description of the
> +.B O_CLOEXEC
> +flag in
> +.BR open (2)
> +for reasons why this may be useful.
> +.TP
> +.B FSPICK_EMPTY_PATH
> +If
> +.I path
> +is an empty string,
> +operate on the file referred to by
> +.I dirfd
> +(which may have been obtained from
> +.BR open (2),
> +.BR fsmount (2),
> +or
> +.BR open_tree (2)).
> +In this case,
> +.I dirfd
> +may refer to any type of file,
> +not just a directory.
> +If
> +.I dirfd
> +is
> +.BR \%AT_FDCWD ,
> +.BR fspick ()
> +will operate on the current working directory
> +of the calling process.
> +.TP
> +.B FSPICK_SYMLINK_NOFOLLOW
> +Do not follow symbolic links
> +in the terminal component of
> +.IR path .
> +If
> +.I path
> +references a symbolic link,
> +the returned filesystem context will reference
> +the filesystem that the symbolic link itself resides on.
> +.TP
> +.B FSPICK_NO_AUTOMOUNT
> +Do not automount the terminal ("basename") component of
> +.I path
> +if it is a directory that is an automount point.
> +This allows you to reconfigure an automount point,
> +rather than the location that would be mounted.
> +This flag has no effect
> +if the automount point has already been mounted over.
> +.RE
> +.P
> +As with filesystem contexts created with
> +.BR fsopen (2),
> +the file descriptor returned by
> +.BR fspick ()
> +may be queried for message strings at any time by calling
> +.BR read (2)
> +on the file descriptor.
> +(See the "Message retrieval interface" subsection in
> +.BR fsopen (2)
> +for more details on the message format.)
> +.SH RETURN VALUE
> +On success, a new file descriptor is returned.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EACCES
> +Search permission is denied
> +for one of the directories
> +in the path prefix of
> +.IR path .
> +(See also
> +.BR path_resolution (7).)
> +.TP
> +.B EBADF
> +.I path
> +is relative but
> +.I dirfd
> +is neither
> +.B \%AT_FDCWD
> +nor a valid file descriptor.
> +.TP
> +.B EFAULT
> +.I path
> +is NULL
> +or a pointer to a location
> +outside the calling process's accessible address space.
> +.TP
> +.B EINVAL
> +Invalid flag specified in
> +.IR flags .
> +.TP
> +.B ELOOP
> +Too many symbolic links encountered when resolving
> +.IR path .
> +.TP
> +.B EMFILE
> +The calling process has too many open files to create more.
> +.TP
> +.B ENAMETOOLONG
> +.I path
> +is longer than
> +.BR PATH_MAX .
> +.TP
> +.B ENFILE
> +The system has too many open files to create more.
> +.TP
> +.B ENOENT
> +A component of
> +.I path
> +does not exist,
> +or is a dangling symbolic link.
> +.TP
> +.B ENOENT
> +.I path
> +is an empty string, but
> +.B \%FSPICK_EMPTY_PATH
> +is not specified in
> +.IR flags .
> +.TP
> +.B ENOTDIR
> +A component of the path prefix of
> +.I path
> +is not a directory;
> +or
> +.I path
> +is relative and
> +.I dirfd
> +is a file descriptor referring to a file other than a directory.
> +.TP
> +.B ENOMEM
> +The kernel could not allocate sufficient memory to complete the operation.
> +.TP
> +.B EPERM
> +The calling process does not have the required
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.SH STANDARDS
> +Linux.
> +.SH HISTORY
> +Linux 5.2.
> +.\" commit cf3cba4a429be43e5527a3f78859b1bfd9ebc5fb
> +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> +glibc 2.36.
> +.SH EXAMPLES
> +The following example sets the read-only flag
> +on the filesystem instance referenced by
> +the mount object attached at
> +.IR /tmp .
> +.P
> +.in +4n
> +.EX
> +int fsfd = fspick(AT_FDCWD, "/tmp", FSPICK_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
> +.EE
> +.in
> +.P
> +The above procedure is roughly equivalent to
> +the following mount operation using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount(NULL, "/tmp", NULL, MS_REMOUNT | MS_RDONLY, NULL);
> +.EE
> +.in
> +.P
> +With the notable caveat that
> +in this example,
> +.BR mount (2)
> +will clear all other filesystem parameters
> +(such as
> +.B MS_DIRSYNC
> +or
> +.BR MS_SYNCHRONOUS );
> +.BR fsconfig (2)
> +will only modify the
> +.I ro

I've also changed this to bold, as it's something literal.


Cheers,
Alex

> +parameter.
> +.SH SEE ALSO
> +.BR fsconfig (2),
> +.BR fsmount (2),
> +.BR fsopen (2),
> +.BR mount (2),
> +.BR mount_setattr (2),
> +.BR move_mount (2),
> +.BR open_tree (2),
> +.BR mount_namespaces (7)

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 3/8] man/man2/fsconfig.2: document "new" mount API

[Alejandro Colomar]

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Hi Aleksa,

On Thu, Sep 25, 2025 at 01:31:25AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 729 insertions(+)
> 
> diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
> --- /dev/null
> +++ b/man/man2/fsconfig.2
> @@ -0,0 +1,729 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH fsconfig 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +fsconfig \- configure new or existing filesystem context
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ,\~ \-lc )
> +.SH SYNOPSIS
> +.nf
> +.B #include <sys/mount.h>
> +.P
> +.BI "int fsconfig(int " fd ", unsigned int " cmd ,
> +.BI "             const char *_Nullable " key ,
> +.BI "             const void *_Nullable " value ", int " aux );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR fsconfig ()
> +system call is part of
> +the suite of file-descriptor-based mount facilities in Linux.
> +.P
> +.BR fsconfig ()
> +is used to supply parameters to
> +and issue commands against
> +the filesystem configuration context
> +associated with the file descriptor
> +.IR fd .
> +Filesystem configuration contexts can be created with
> +.BR fsopen (2)
> +or be instantiated from an extant filesystem instance with
> +.BR fspick (2).
> +.P
> +The
> +.I cmd
> +argument indicates the command to be issued.
> +Some commands supply parameters to the context
> +(equivalent to mount options specified with
> +.BR mount (8)),
> +while others are meta-operations on the filesystem context.
> +The list of valid
> +.I cmd
> +values are:
> +.RS
> +.TP
> +.B FSCONFIG_SET_FLAG
> +Set the flag parameter named by
> +.IR key .
> +.I value
> +must be NULL,
> +and
> +.I aux
> +must be 0.
> +.TP
> +.B FSCONFIG_SET_STRING
> +Set the string parameter named by
> +.I key
> +to the value specified by
> +.IR value .
> +.I value
> +points to a null-terminated string,
> +and
> +.I aux
> +must be 0.
> +.TP
> +.B FSCONFIG_SET_BINARY
> +Set the blob parameter named by
> +.I key
> +to the contents of the binary blob
> +specified by
> +.IR value .
> +.I value
> +points to
> +the start of a buffer
> +that is
> +.I aux
> +bytes in length.
> +.TP
> +.B FSCONFIG_SET_FD
> +Set the file parameter named by
> +.I key
> +to the open file description
> +referenced by the file descriptor
> +.IR aux .
> +.I value
> +must be NULL.
> +.IP
> +You may also use
> +.B \%FSCONFIG_SET_STRING
> +for file parameters,
> +with
> +.I value
> +set to a null-terminated string
> +containing a base-10 representation
> +of the file descriptor number.
> +This mechanism is primarily intended for compatibility
> +with older
> +.BR mount (2)-based
> +programs,
> +and only works for parameters
> +that
> +.I only
> +accept file descriptor arguments.
> +.TP
> +.B FSCONFIG_SET_PATH
> +Set the path parameter named by
> +.I key
> +to the object at a provided path,
> +resolved in a similar manner to
> +.BR openat (2).
> +.I value
> +points to a null-terminated pathname string,
> +and
> +.I aux
> +is equivalent to the
> +.I dirfd
> +argument to
> +.BR openat (2).
> +See
> +.BR openat (2)
> +for an explanation of the need for
> +.BR \%FSCONFIG_SET_PATH .
> +.IP
> +You may also use
> +.B \%FSCONFIG_SET_STRING
> +for path parameters,
> +the behaviour of which is equivalent to
> +.B \%FSCONFIG_SET_PATH
> +with
> +.I aux
> +set to
> +.BR \%AT_FDCWD .
> +.TP
> +.B FSCONFIG_SET_PATH_EMPTY
> +As with
> +.BR \%FSCONFIG_SET_PATH ,
> +except that if
> +.I value
> +is an empty string,
> +the file descriptor specified by
> +.I aux
> +is operated on directly
> +and may be any type of file
> +(not just a directory).
> +This is equivalent to the behaviour of
> +.B \%AT_EMPTY_PATH
> +with most "*at()" system calls.
> +If
> +.I aux
> +is
> +.BR \%AT_FDCWD ,
> +the parameter will be set to
> +the current working directory
> +of the calling process.
> +.TP
> +.B FSCONFIG_CMD_CREATE
> +This command instructs the filesystem driver
> +to instantiate an instance of the filesystem in the kernel
> +with the parameters specified in the filesystem configuration context.
> +.I key
> +and
> +.I value
> +must be NULL,
> +and
> +.I aux
> +must be 0.
> +.IP
> +This command can only be issued once
> +in the lifetime of a filesystem context.
> +If the operation succeeds,
> +the filesystem context
> +associated with file descriptor
> +.I fd
> +now references the created filesystem instance,
> +and is placed into a special "awaiting-mount" mode
> +that allows you to use
> +.BR fsmount (2)
> +to create a mount object from the filesystem instance.
> +.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
> +If the operation fails,
> +in most cases
> +the filesystem context is placed in a failed mode
> +and cannot be used for any further
> +.BR fsconfig ()
> +operations
> +(though you may still retrieve diagnostic messages
> +through the message retrieval interface,
> +as described in
> +the corresponding subsection of
> +.BR fsopen (2)).
> +.IP
> +This command can only be issued against
> +filesystem configuration contexts
> +that were created with
> +.BR fsopen (2).
> +In order to create a filesystem instance,
> +the calling process must have the
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.IP
> +An important thing to be aware of is that
> +the Linux kernel will
> +.I silently
> +reuse extant filesystem instances
> +depending on the filesystem type
> +and the configured parameters
> +(each filesystem driver has
> +its own policy for
> +how filesystem instances are reused).
> +This means that
> +the filesystem instance "created" by
> +.B \%FSCONFIG_CMD_CREATE
> +may, in fact, be a reference
> +to an extant filesystem instance in the kernel.
> +(For reference,
> +this behaviour also applies to
> +.BR mount (2).)
> +.IP
> +One side-effect of this behaviour is that
> +if an extant filesystem instance is reused,
> +.I all
> +parameters configured
> +for this filesystem configuration context
> +are
> +.I silently ignored
> +(with the exception of the
> +.I ro
> +and
> +.I rw
> +flag parameters;
> +if the state of the read-only flag in the
> +extant filesystem instance and the filesystem configuration context
> +do not match, this operation will return
> +.BR EBUSY ).
> +This also means that
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +commands issued against
> +the "created" filesystem instance
> +will also affect any mount objects associated with
> +the extant filesystem instance.
> +.IP
> +Programs that need to ensure
> +that they create a new filesystem instance
> +with specific parameters
> +(notably, security-related parameters
> +such as
> +.I acl
> +to enable POSIX ACLs\[em]\c
> +as described in
> +.BR acl (5))
> +should use
> +.B \%FSCONFIG_CMD_CREATE_EXCL
> +instead.
> +.TP
> +.BR FSCONFIG_CMD_CREATE_EXCL " (since Linux 6.6)"
> +.\" commit 22ed7ecdaefe0cac0c6e6295e83048af60435b13
> +.\" commit 84ab1277ce5a90a8d1f377707d662ac43cc0918a
> +As with
> +.BR \%FSCONFIG_CMD_CREATE ,
> +except that the kernel is instructed
> +to not reuse extant filesystem instances.
> +If the operation
> +would be forced to
> +reuse an extant filesystem instance,
> +this operation will return
> +.B EBUSY
> +instead.
> +.IP
> +As a result (unlike
> +.BR \%FSCONFIG_CMD_CREATE ),
> +if this operation succeeds
> +then the calling process can be sure that
> +all of the parameters successfully configured with
> +.BR fsconfig ()
> +will actually be applied
> +to the created filesystem instance.
> +.TP
> +.B FSCONFIG_CMD_RECONFIGURE
> +This command instructs the filesystem driver
> +to apply the parameters specified in the filesystem configuration context
> +to the extant filesystem instance
> +referenced by the filesystem configuration context.
> +.I key
> +and
> +.I value
> +must be NULL,
> +and
> +.I aux
> +must be 0.
> +.IP
> +This is primarily intended for use with
> +.BR fspick (2),
> +but may also be used to modify
> +the parameters of a filesystem instance
> +after
> +.B \%FSCONFIG_CMD_CREATE
> +was used to create it
> +and a mount object was created using
> +.BR fsmount (2).
> +In order to reconfigure an extant filesystem instance,
> +the calling process must have the
> +.B CAP_SYS_ADMIN
> +capability.
> +.IP
> +If the operation succeeds,
> +the filesystem context is reset
> +but remains in reconfiguration mode
> +and thus can be reused for subsequent
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +commands.
> +If the operation fails,
> +in most cases
> +the filesystem context is placed in a failed mode
> +and cannot be used for any further
> +.BR fsconfig ()
> +operations
> +(though you may still retrieve diagnostic messages
> +through the message retrieval interface,
> +as described in
> +the corresponding subsection of
> +.BR fsopen (2)).
> +.RE
> +.P
> +Parameters specified with
> +.BI FSCONFIG_SET_ *
> +do not take effect
> +until a corresponding
> +.B \%FSCONFIG_CMD_CREATE
> +or
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +command is issued.
> +.SH RETURN VALUE
> +On success,
> +.BR fsconfig ()
> +returns 0.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +If an error occurs, the filesystem driver may provide
> +additional information about the error
> +through the message retrieval interface for filesystem configuration contexts.
> +This additional information can be retrieved at any time by calling
> +.BR read (2)
> +on the filesystem instance or filesystem configuration context
> +referenced by the file descriptor
> +.IR fd .
> +(See the "Message retrieval interface" subsection in
> +.BR fsopen (2)
> +for more details on the message format.)
> +.P
> +Even after an error occurs,
> +the filesystem configuration context is
> +.I not
> +invalidated,
> +and thus can still be used with other
> +.BR fsconfig ()
> +commands.
> +This means that users can probe support for filesystem parameters
> +on a per-parameter basis,
> +and adjust which parameters they wish to set.
> +.P
> +The error values given below result from
> +filesystem type independent errors.
> +Each filesystem type may have its own special errors
> +and its own special behavior.
> +See the Linux kernel source code for details.
> +.TP
> +.B EACCES
> +A component of a path
> +provided as a path parameter
> +was not searchable.
> +(See also
> +.BR path_resolution (7).)
> +.TP
> +.B EACCES
> +.B \%FSCONFIG_CMD_CREATE
> +was attempted
> +for a read-only filesystem
> +without specifying the
> +.RB ' ro '
> +flag parameter.
> +.TP
> +.B EACCES
> +A specified block device parameter
> +is located on a filesystem
> +mounted with the
> +.B \%MS_NODEV
> +option.
> +.TP
> +.B EBADF
> +The file descriptor given by
> +.I fd
> +(or possibly by
> +.IR aux ,
> +depending on the command)
> +is invalid.
> +.TP
> +.B EBUSY
> +The filesystem context associated with
> +.I fd
> +is in the wrong state
> +for the given command.
> +.TP
> +.B EBUSY
> +The filesystem instance cannot be reconfigured as read-only
> +with
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +because some programs
> +still hold files open for writing.
> +.TP
> +.B EBUSY
> +A new filesystem instance was requested with
> +.B \%FSCONFIG_CMD_CREATE_EXCL
> +but a matching superblock already existed.
> +.TP
> +.B EFAULT
> +One of the pointer arguments
> +points to a location
> +outside the calling process's accessible address space.
> +.TP
> +.B EINVAL
> +.I fd
> +does not refer to
> +a filesystem configuration context
> +or filesystem instance.
> +.TP
> +.B EINVAL
> +One of the values of
> +.IR name ,
> +.IR value ,
> +and/or
> +.I aux
> +were set to a non-zero value when
> +.I cmd
> +required that they be zero
> +(or NULL).
> +.TP
> +.B EINVAL
> +The parameter named by
> +.I name
> +cannot be set
> +using the type specified with
> +.IR cmd .
> +.TP
> +.B EINVAL
> +One of the source parameters
> +referred to
> +an invalid superblock.
> +.TP
> +.B ELOOP
> +Too many links encountered
> +during pathname resolution
> +of a path argument.
> +.TP
> +.B ENAMETOOLONG
> +A path argument was longer than
> +.BR PATH_MAX .
> +.TP
> +.B ENOENT
> +A path argument had a non-existent component.
> +.TP
> +.B ENOENT
> +A path argument is an empty string,
> +but
> +.I cmd
> +is not
> +.BR \%FSCONFIG_SET_PATH_EMPTY .
> +.TP
> +.B ENOMEM
> +The kernel could not allocate sufficient memory to complete the operation.
> +.TP
> +.B ENOTBLK
> +The parameter named by
> +.I name

There's no such parameter.  (I guess you meant 'key'?)


Cheers,
Alex

> +must be a block device,
> +but the provided parameter value was not a block device.
> +.TP
> +.B ENOTDIR
> +A component of the path prefix
> +of a path argument
> +was not a directory.
> +.TP
> +.B EOPNOTSUPP
> +The command given by
> +.I cmd
> +is not valid.
> +.TP
> +.B ENXIO
> +The major number
> +of a block device parameter
> +is out of range.
> +.TP
> +.B EPERM
> +The command given by
> +.I cmd
> +was
> +.BR \%FSCONFIG_CMD_CREATE ,
> +.BR \%FSCONFIG_CMD_CREATE_EXCL ,
> +or
> +.BR \%FSCONFIG_CMD_RECONFIGURE ,
> +but the calling process does not have the required
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.SH STANDARDS
> +Linux.
> +.SH HISTORY
> +Linux 5.2.
> +.\" commit ecdab150fddb42fe6a739335257949220033b782
> +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> +glibc 2.36.
> +.SH NOTES
> +.SS Generic filesystem parameters
> +Each filesystem driver is responsible for
> +parsing most parameters specified with
> +.BR fsconfig (),
> +meaning that individual filesystems
> +may have very different behaviour
> +when encountering parameters with the same name.
> +In general,
> +you should not assume that the behaviour of
> +.BR fsconfig ()
> +when specifying a parameter to one filesystem type
> +will match the behaviour of the same parameter
> +with a different filesystem type.
> +.P
> +However,
> +the following generic parameters
> +apply to all filesystems and have unified behaviour.
> +They are set using the listed
> +.BI \%FSCONFIG_SET_ *
> +command.
> +.TP
> +\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether the filesystem instance is read-only.
> +.TP
> +\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Make directory changes on this filesystem instance synchronous.
> +.TP
> +\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether writes on this filesystem instance
> +will be made synchronous
> +(as though the
> +.B O_SYNC
> +flag to
> +.BR open (2)
> +was specified for
> +all file opens in this filesystem instance).
> +.TP
> +\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether to reduce on-disk updates
> +of inode timestamps on this filesystem instance
> +(as described in the
> +.B \%MS_LAZYTIME
> +section of
> +.BR mount (2)).
> +.TP
> +\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether the filesystem instance should permit mandatory locking.
> +Since Linux 5.15,
> +.\" commit f7e33bdbd6d1bdf9c3df8bba5abcf3399f957ac3
> +mandatory locking has been deprecated
> +and setting this flag is a no-op.
> +.TP
> +\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
> +This parameter is equivalent to the
> +.I source
> +parameter passed to
> +.BR mount (2)
> +for the same filesystem type,
> +and is usually the pathname of a block device
> +containing the filesystem.
> +This parameter may only be set once
> +per filesystem configuration context transaction.
> +.P
> +In addition,
> +any filesystem parameters associated with
> +Linux Security Modules (LSMs)
> +are also generic with respect to the underlying filesystem.
> +See the documentation for the LSM you wish to configure for more details.
> +.SH CAVEATS
> +.SS Filesystem parameter types
> +As a result of
> +each filesystem driver being responsible for
> +parsing most parameters specified with
> +.BR fsconfig (),
> +some filesystem drivers
> +may have unintuitive behaviour
> +with regards to which
> +.BI \%FSCONFIG_SET_ *
> +commands are permitted
> +to configure a given parameter.
> +.P
> +In order for
> +filesystem parameters to be backwards compatible with
> +.BR mount (2),
> +they must be parseable as strings;
> +this almost universally means that
> +.B \%FSCONFIG_SET_STRING
> +can also be used to configure them.
> +.\" Aleksa Sarai
> +.\"   Theoretically, a filesystem could check fc->oldapi and refuse
> +.\"   FSCONFIG_SET_STRING if the operation is coming from the new API, but no
> +.\"   filesystems do this (and probably never will).
> +However, other
> +.BI \%FSCONFIG_SET_ *
> +commands need to be opted into
> +by each filesystem driver's parameter parser.
> +.P
> +One of the most user-visible instances of
> +this inconsistency is that
> +many filesystems do not support
> +configuring path parameters with
> +.B \%FSCONFIG_SET_PATH
> +(despite the name),
> +which can lead to somewhat confusing
> +.B EINVAL
> +errors.
> +(For example, the generic
> +.I source
> +parameter\[em]\c
> +which is usually a path\[em]\c
> +can only be configured
> +with
> +.BR \%FSCONFIG_SET_STRING .)
> +.P
> +When writing programs that use
> +.BR fsconfig ()
> +to configure parameters
> +with commands other than
> +.BR \%FSCONFIG_SET_STRING ,
> +users should verify
> +that the
> +.BI \%FSCONFIG_SET_ *
> +commands used to configure each parameter
> +are supported by the corresponding filesystem driver.
> +.\" Aleksa Sarai
> +.\"   While this (quite confusing) inconsistency in behaviour is true today
> +.\"   (and has been true since this was merged), this appears to mostly be an
> +.\"   unintended consequence of filesystem drivers hand-coding fsparam parsing.
> +.\"   Path parameters are the most eggregious causes of confusion.
> +.\"   Hopefully we can make this no longer the case in a future kernel.
> +.SH EXAMPLES
> +To illustrate the different kinds of flags that can be configured with
> +.BR fsconfig (),
> +here are a few examples of some different filesystems being created:
> +.P
> +.in +4n
> +.EX
> +int fsfd, mntfd;
> +\&
> +fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "inode64", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "uid", "1234", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "huge", "never", 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "casefold", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOEXEC);
> +move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
> +\&
> +fsfd = fsopen("erofs", FSOPEN_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/loop0", 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE_EXCL, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOSUID);
> +move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +Usually,
> +specifying the same parameter named by
> +.I key
> +multiple times with
> +.BR fsconfig ()
> +causes the parameter value to be replaced.
> +However, some filesystems may have unique behaviour:
> +.P
> +.in +4n
> +.EX
> +\&
> +int fsfd, mntfd;
> +int lowerdirfd = open("/o/ctr/lower1", O_DIRECTORY | O_CLOEXEC);
> +\&
> +fsfd = fsopen("overlay", FSOPEN_CLOEXEC);
> +/* "lowerdir+" appends to the lower dir stack each time */
> +fsconfig(fsfd, FSCONFIG_SET_FD, "lowerdir+", NULL, lowerdirfd);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower2", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower3", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower4", 0);
> +.\" fsconfig(fsfd, FSCONFIG_SET_PATH, "lowerdir+", "/o/ctr/lower5", AT_FDCWD);
> +.\" fsconfig(fsfd, FSCONFIG_SET_PATH_EMPTY, "lowerdir+", "", lowerdirfd);
> +.\" Aleksa Sarai: Hopefully these will also be supported in the future.
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "xino", "auto", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "nfs_export", "off", 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
> +move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +And here is an example of how
> +.BR fspick (2)
> +can be used with
> +.BR fsconfig ()
> +to reconfigure the parameters
> +of an extant filesystem instance
> +attached to
> +.IR /proc :
> +.P
> +.in +4n
> +.EX
> +int fsfd = fspick(AT_FDCWD, "/proc", FSPICK_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "ptraceable", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "subset", "pid", 0);
> +fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
> +.EE
> +.in
> +.SH SEE ALSO
> +.BR fsmount (2),
> +.BR fsopen (2),
> +.BR fspick (2),
> +.BR mount (2),
> +.BR mount_setattr (2),
> +.BR move_mount (2),
> +.BR open_tree (2),
> +.BR mount_namespaces (7)
> 
> -- 
> 2.51.0
> 
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 3/8] man/man2/fsconfig.2: document "new" mount API

[Aleksa Sarai]

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On 2025-09-25, Alejandro Colomar <alx@kernel.org> wrote:
> Hi Aleksa,
> 
> On Thu, Sep 25, 2025 at 01:31:25AM +1000, Aleksa Sarai wrote:
> > This is loosely based on the original documentation written by David
> > Howells and later maintained by Christian Brauner, but has been
> > rewritten to be more from a user perspective (as well as fixing a few
> > critical mistakes).
> > 
> > Co-authored-by: David Howells <dhowells@redhat.com>
> > Signed-off-by: David Howells <dhowells@redhat.com>
> > Co-authored-by: Christian Brauner <brauner@kernel.org>
> > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > ---
> >  man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 729 insertions(+)
> > 
> > diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
> > --- /dev/null
> > +++ b/man/man2/fsconfig.2
> > @@ -0,0 +1,729 @@
> > +.\" Copyright, the authors of the Linux man-pages project
> > +.\"
> > +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> > +.\"
> > +.TH fsconfig 2 (date) "Linux man-pages (unreleased)"
> > +.SH NAME
> > +fsconfig \- configure new or existing filesystem context
> > +.SH LIBRARY
> > +Standard C library
> > +.RI ( libc ,\~ \-lc )
> > +.SH SYNOPSIS
> > +.nf
> > +.B #include <sys/mount.h>
> > +.P
> > +.BI "int fsconfig(int " fd ", unsigned int " cmd ,
> > +.BI "             const char *_Nullable " key ,
> > +.BI "             const void *_Nullable " value ", int " aux );
> > +.fi
> > +.SH DESCRIPTION
> > +The
> > +.BR fsconfig ()
> > +system call is part of
> > +the suite of file-descriptor-based mount facilities in Linux.
> > +.P
> > +.BR fsconfig ()
> > +is used to supply parameters to
> > +and issue commands against
> > +the filesystem configuration context
> > +associated with the file descriptor
> > +.IR fd .
> > +Filesystem configuration contexts can be created with
> > +.BR fsopen (2)
> > +or be instantiated from an extant filesystem instance with
> > +.BR fspick (2).
> > +.P
> > +The
> > +.I cmd
> > +argument indicates the command to be issued.
> > +Some commands supply parameters to the context
> > +(equivalent to mount options specified with
> > +.BR mount (8)),
> > +while others are meta-operations on the filesystem context.
> > +The list of valid
> > +.I cmd
> > +values are:
> > +.RS
> > +.TP
> > +.B FSCONFIG_SET_FLAG
> > +Set the flag parameter named by
> > +.IR key .
> > +.I value
> > +must be NULL,
> > +and
> > +.I aux
> > +must be 0.
> > +.TP
> > +.B FSCONFIG_SET_STRING
> > +Set the string parameter named by
> > +.I key
> > +to the value specified by
> > +.IR value .
> > +.I value
> > +points to a null-terminated string,
> > +and
> > +.I aux
> > +must be 0.
> > +.TP
> > +.B FSCONFIG_SET_BINARY
> > +Set the blob parameter named by
> > +.I key
> > +to the contents of the binary blob
> > +specified by
> > +.IR value .
> > +.I value
> > +points to
> > +the start of a buffer
> > +that is
> > +.I aux
> > +bytes in length.
> > +.TP
> > +.B FSCONFIG_SET_FD
> > +Set the file parameter named by
> > +.I key
> > +to the open file description
> > +referenced by the file descriptor
> > +.IR aux .
> > +.I value
> > +must be NULL.
> > +.IP
> > +You may also use
> > +.B \%FSCONFIG_SET_STRING
> > +for file parameters,
> > +with
> > +.I value
> > +set to a null-terminated string
> > +containing a base-10 representation
> > +of the file descriptor number.
> > +This mechanism is primarily intended for compatibility
> > +with older
> > +.BR mount (2)-based
> > +programs,
> > +and only works for parameters
> > +that
> > +.I only
> > +accept file descriptor arguments.
> > +.TP
> > +.B FSCONFIG_SET_PATH
> > +Set the path parameter named by
> > +.I key
> > +to the object at a provided path,
> > +resolved in a similar manner to
> > +.BR openat (2).
> > +.I value
> > +points to a null-terminated pathname string,
> > +and
> > +.I aux
> > +is equivalent to the
> > +.I dirfd
> > +argument to
> > +.BR openat (2).
> > +See
> > +.BR openat (2)
> > +for an explanation of the need for
> > +.BR \%FSCONFIG_SET_PATH .
> > +.IP
> > +You may also use
> > +.B \%FSCONFIG_SET_STRING
> > +for path parameters,
> > +the behaviour of which is equivalent to
> > +.B \%FSCONFIG_SET_PATH
> > +with
> > +.I aux
> > +set to
> > +.BR \%AT_FDCWD .
> > +.TP
> > +.B FSCONFIG_SET_PATH_EMPTY
> > +As with
> > +.BR \%FSCONFIG_SET_PATH ,
> > +except that if
> > +.I value
> > +is an empty string,
> > +the file descriptor specified by
> > +.I aux
> > +is operated on directly
> > +and may be any type of file
> > +(not just a directory).
> > +This is equivalent to the behaviour of
> > +.B \%AT_EMPTY_PATH
> > +with most "*at()" system calls.
> > +If
> > +.I aux
> > +is
> > +.BR \%AT_FDCWD ,
> > +the parameter will be set to
> > +the current working directory
> > +of the calling process.
> > +.TP
> > +.B FSCONFIG_CMD_CREATE
> > +This command instructs the filesystem driver
> > +to instantiate an instance of the filesystem in the kernel
> > +with the parameters specified in the filesystem configuration context.
> > +.I key
> > +and
> > +.I value
> > +must be NULL,
> > +and
> > +.I aux
> > +must be 0.
> > +.IP
> > +This command can only be issued once
> > +in the lifetime of a filesystem context.
> > +If the operation succeeds,
> > +the filesystem context
> > +associated with file descriptor
> > +.I fd
> > +now references the created filesystem instance,
> > +and is placed into a special "awaiting-mount" mode
> > +that allows you to use
> > +.BR fsmount (2)
> > +to create a mount object from the filesystem instance.
> > +.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
> > +If the operation fails,
> > +in most cases
> > +the filesystem context is placed in a failed mode
> > +and cannot be used for any further
> > +.BR fsconfig ()
> > +operations
> > +(though you may still retrieve diagnostic messages
> > +through the message retrieval interface,
> > +as described in
> > +the corresponding subsection of
> > +.BR fsopen (2)).
> > +.IP
> > +This command can only be issued against
> > +filesystem configuration contexts
> > +that were created with
> > +.BR fsopen (2).
> > +In order to create a filesystem instance,
> > +the calling process must have the
> > +.B \%CAP_SYS_ADMIN
> > +capability.
> > +.IP
> > +An important thing to be aware of is that
> > +the Linux kernel will
> > +.I silently
> > +reuse extant filesystem instances
> > +depending on the filesystem type
> > +and the configured parameters
> > +(each filesystem driver has
> > +its own policy for
> > +how filesystem instances are reused).
> > +This means that
> > +the filesystem instance "created" by
> > +.B \%FSCONFIG_CMD_CREATE
> > +may, in fact, be a reference
> > +to an extant filesystem instance in the kernel.
> > +(For reference,
> > +this behaviour also applies to
> > +.BR mount (2).)
> > +.IP
> > +One side-effect of this behaviour is that
> > +if an extant filesystem instance is reused,
> > +.I all
> > +parameters configured
> > +for this filesystem configuration context
> > +are
> > +.I silently ignored
> > +(with the exception of the
> > +.I ro
> > +and
> > +.I rw
> > +flag parameters;
> > +if the state of the read-only flag in the
> > +extant filesystem instance and the filesystem configuration context
> > +do not match, this operation will return
> > +.BR EBUSY ).
> > +This also means that
> > +.B \%FSCONFIG_CMD_RECONFIGURE
> > +commands issued against
> > +the "created" filesystem instance
> > +will also affect any mount objects associated with
> > +the extant filesystem instance.
> > +.IP
> > +Programs that need to ensure
> > +that they create a new filesystem instance
> > +with specific parameters
> > +(notably, security-related parameters
> > +such as
> > +.I acl
> > +to enable POSIX ACLs\[em]\c
> > +as described in
> > +.BR acl (5))
> > +should use
> > +.B \%FSCONFIG_CMD_CREATE_EXCL
> > +instead.
> > +.TP
> > +.BR FSCONFIG_CMD_CREATE_EXCL " (since Linux 6.6)"
> > +.\" commit 22ed7ecdaefe0cac0c6e6295e83048af60435b13
> > +.\" commit 84ab1277ce5a90a8d1f377707d662ac43cc0918a
> > +As with
> > +.BR \%FSCONFIG_CMD_CREATE ,
> > +except that the kernel is instructed
> > +to not reuse extant filesystem instances.
> > +If the operation
> > +would be forced to
> > +reuse an extant filesystem instance,
> > +this operation will return
> > +.B EBUSY
> > +instead.
> > +.IP
> > +As a result (unlike
> > +.BR \%FSCONFIG_CMD_CREATE ),
> > +if this operation succeeds
> > +then the calling process can be sure that
> > +all of the parameters successfully configured with
> > +.BR fsconfig ()
> > +will actually be applied
> > +to the created filesystem instance.
> > +.TP
> > +.B FSCONFIG_CMD_RECONFIGURE
> > +This command instructs the filesystem driver
> > +to apply the parameters specified in the filesystem configuration context
> > +to the extant filesystem instance
> > +referenced by the filesystem configuration context.
> > +.I key
> > +and
> > +.I value
> > +must be NULL,
> > +and
> > +.I aux
> > +must be 0.
> > +.IP
> > +This is primarily intended for use with
> > +.BR fspick (2),
> > +but may also be used to modify
> > +the parameters of a filesystem instance
> > +after
> > +.B \%FSCONFIG_CMD_CREATE
> > +was used to create it
> > +and a mount object was created using
> > +.BR fsmount (2).
> > +In order to reconfigure an extant filesystem instance,
> > +the calling process must have the
> > +.B CAP_SYS_ADMIN
> > +capability.
> > +.IP
> > +If the operation succeeds,
> > +the filesystem context is reset
> > +but remains in reconfiguration mode
> > +and thus can be reused for subsequent
> > +.B \%FSCONFIG_CMD_RECONFIGURE
> > +commands.
> > +If the operation fails,
> > +in most cases
> > +the filesystem context is placed in a failed mode
> > +and cannot be used for any further
> > +.BR fsconfig ()
> > +operations
> > +(though you may still retrieve diagnostic messages
> > +through the message retrieval interface,
> > +as described in
> > +the corresponding subsection of
> > +.BR fsopen (2)).
> > +.RE
> > +.P
> > +Parameters specified with
> > +.BI FSCONFIG_SET_ *
> > +do not take effect
> > +until a corresponding
> > +.B \%FSCONFIG_CMD_CREATE
> > +or
> > +.B \%FSCONFIG_CMD_RECONFIGURE
> > +command is issued.
> > +.SH RETURN VALUE
> > +On success,
> > +.BR fsconfig ()
> > +returns 0.
> > +On error, \-1 is returned, and
> > +.I errno
> > +is set to indicate the error.
> > +.SH ERRORS
> > +If an error occurs, the filesystem driver may provide
> > +additional information about the error
> > +through the message retrieval interface for filesystem configuration contexts.
> > +This additional information can be retrieved at any time by calling
> > +.BR read (2)
> > +on the filesystem instance or filesystem configuration context
> > +referenced by the file descriptor
> > +.IR fd .
> > +(See the "Message retrieval interface" subsection in
> > +.BR fsopen (2)
> > +for more details on the message format.)
> > +.P
> > +Even after an error occurs,
> > +the filesystem configuration context is
> > +.I not
> > +invalidated,
> > +and thus can still be used with other
> > +.BR fsconfig ()
> > +commands.
> > +This means that users can probe support for filesystem parameters
> > +on a per-parameter basis,
> > +and adjust which parameters they wish to set.
> > +.P
> > +The error values given below result from
> > +filesystem type independent errors.
> > +Each filesystem type may have its own special errors
> > +and its own special behavior.
> > +See the Linux kernel source code for details.
> > +.TP
> > +.B EACCES
> > +A component of a path
> > +provided as a path parameter
> > +was not searchable.
> > +(See also
> > +.BR path_resolution (7).)
> > +.TP
> > +.B EACCES
> > +.B \%FSCONFIG_CMD_CREATE
> > +was attempted
> > +for a read-only filesystem
> > +without specifying the
> > +.RB ' ro '
> > +flag parameter.
> > +.TP
> > +.B EACCES
> > +A specified block device parameter
> > +is located on a filesystem
> > +mounted with the
> > +.B \%MS_NODEV
> > +option.
> > +.TP
> > +.B EBADF
> > +The file descriptor given by
> > +.I fd
> > +(or possibly by
> > +.IR aux ,
> > +depending on the command)
> > +is invalid.
> > +.TP
> > +.B EBUSY
> > +The filesystem context associated with
> > +.I fd
> > +is in the wrong state
> > +for the given command.
> > +.TP
> > +.B EBUSY
> > +The filesystem instance cannot be reconfigured as read-only
> > +with
> > +.B \%FSCONFIG_CMD_RECONFIGURE
> > +because some programs
> > +still hold files open for writing.
> > +.TP
> > +.B EBUSY
> > +A new filesystem instance was requested with
> > +.B \%FSCONFIG_CMD_CREATE_EXCL
> > +but a matching superblock already existed.
> > +.TP
> > +.B EFAULT
> > +One of the pointer arguments
> > +points to a location
> > +outside the calling process's accessible address space.
> > +.TP
> > +.B EINVAL
> > +.I fd
> > +does not refer to
> > +a filesystem configuration context
> > +or filesystem instance.
> > +.TP
> > +.B EINVAL
> > +One of the values of
> > +.IR name ,
> > +.IR value ,
> > +and/or
> > +.I aux
> > +were set to a non-zero value when
> > +.I cmd
> > +required that they be zero
> > +(or NULL).
> > +.TP
> > +.B EINVAL
> > +The parameter named by
> > +.I name
> > +cannot be set
> > +using the type specified with
> > +.IR cmd .
> > +.TP
> > +.B EINVAL
> > +One of the source parameters
> > +referred to
> > +an invalid superblock.
> > +.TP
> > +.B ELOOP
> > +Too many links encountered
> > +during pathname resolution
> > +of a path argument.
> > +.TP
> > +.B ENAMETOOLONG
> > +A path argument was longer than
> > +.BR PATH_MAX .
> > +.TP
> > +.B ENOENT
> > +A path argument had a non-existent component.
> > +.TP
> > +.B ENOENT
> > +A path argument is an empty string,
> > +but
> > +.I cmd
> > +is not
> > +.BR \%FSCONFIG_SET_PATH_EMPTY .
> > +.TP
> > +.B ENOMEM
> > +The kernel could not allocate sufficient memory to complete the operation.
> > +.TP
> > +.B ENOTBLK
> > +The parameter named by
> > +.I name
> 
> There's no such parameter.  (I guess you meant 'key'?)

Ah yes, I did mean "key". The same mistake was repeated for two EINVAL
cases above as well:

    EINVAL One of the values of *name*, value, and/or aux were set to a
           non-zero value when cmd required that they be zero (or NULL).

    EINVAL The parameter named by *name* cannot be set using the type
           specified with cmd.

Do you want me to send another version or would you able to fix it when
you apply?

Thanks.

> Cheers,
> Alex
> 
> > +must be a block device,
> > +but the provided parameter value was not a block device.
> > +.TP
> > +.B ENOTDIR
> > +A component of the path prefix
> > +of a path argument
> > +was not a directory.
> > +.TP
> > +.B EOPNOTSUPP
> > +The command given by
> > +.I cmd
> > +is not valid.
> > +.TP
> > +.B ENXIO
> > +The major number
> > +of a block device parameter
> > +is out of range.
> > +.TP
> > +.B EPERM
> > +The command given by
> > +.I cmd
> > +was
> > +.BR \%FSCONFIG_CMD_CREATE ,
> > +.BR \%FSCONFIG_CMD_CREATE_EXCL ,
> > +or
> > +.BR \%FSCONFIG_CMD_RECONFIGURE ,
> > +but the calling process does not have the required
> > +.B \%CAP_SYS_ADMIN
> > +capability.
> > +.SH STANDARDS
> > +Linux.
> > +.SH HISTORY
> > +Linux 5.2.
> > +.\" commit ecdab150fddb42fe6a739335257949220033b782
> > +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> > +glibc 2.36.
> > +.SH NOTES
> > +.SS Generic filesystem parameters
> > +Each filesystem driver is responsible for
> > +parsing most parameters specified with
> > +.BR fsconfig (),
> > +meaning that individual filesystems
> > +may have very different behaviour
> > +when encountering parameters with the same name.
> > +In general,
> > +you should not assume that the behaviour of
> > +.BR fsconfig ()
> > +when specifying a parameter to one filesystem type
> > +will match the behaviour of the same parameter
> > +with a different filesystem type.
> > +.P
> > +However,
> > +the following generic parameters
> > +apply to all filesystems and have unified behaviour.
> > +They are set using the listed
> > +.BI \%FSCONFIG_SET_ *
> > +command.
> > +.TP
> > +\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +Configure whether the filesystem instance is read-only.
> > +.TP
> > +\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +Make directory changes on this filesystem instance synchronous.
> > +.TP
> > +\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +Configure whether writes on this filesystem instance
> > +will be made synchronous
> > +(as though the
> > +.B O_SYNC
> > +flag to
> > +.BR open (2)
> > +was specified for
> > +all file opens in this filesystem instance).
> > +.TP
> > +\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +Configure whether to reduce on-disk updates
> > +of inode timestamps on this filesystem instance
> > +(as described in the
> > +.B \%MS_LAZYTIME
> > +section of
> > +.BR mount (2)).
> > +.TP
> > +\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> > +Configure whether the filesystem instance should permit mandatory locking.
> > +Since Linux 5.15,
> > +.\" commit f7e33bdbd6d1bdf9c3df8bba5abcf3399f957ac3
> > +mandatory locking has been deprecated
> > +and setting this flag is a no-op.
> > +.TP
> > +\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
> > +This parameter is equivalent to the
> > +.I source
> > +parameter passed to
> > +.BR mount (2)
> > +for the same filesystem type,
> > +and is usually the pathname of a block device
> > +containing the filesystem.
> > +This parameter may only be set once
> > +per filesystem configuration context transaction.
> > +.P
> > +In addition,
> > +any filesystem parameters associated with
> > +Linux Security Modules (LSMs)
> > +are also generic with respect to the underlying filesystem.
> > +See the documentation for the LSM you wish to configure for more details.
> > +.SH CAVEATS
> > +.SS Filesystem parameter types
> > +As a result of
> > +each filesystem driver being responsible for
> > +parsing most parameters specified with
> > +.BR fsconfig (),
> > +some filesystem drivers
> > +may have unintuitive behaviour
> > +with regards to which
> > +.BI \%FSCONFIG_SET_ *
> > +commands are permitted
> > +to configure a given parameter.
> > +.P
> > +In order for
> > +filesystem parameters to be backwards compatible with
> > +.BR mount (2),
> > +they must be parseable as strings;
> > +this almost universally means that
> > +.B \%FSCONFIG_SET_STRING
> > +can also be used to configure them.
> > +.\" Aleksa Sarai
> > +.\"   Theoretically, a filesystem could check fc->oldapi and refuse
> > +.\"   FSCONFIG_SET_STRING if the operation is coming from the new API, but no
> > +.\"   filesystems do this (and probably never will).
> > +However, other
> > +.BI \%FSCONFIG_SET_ *
> > +commands need to be opted into
> > +by each filesystem driver's parameter parser.
> > +.P
> > +One of the most user-visible instances of
> > +this inconsistency is that
> > +many filesystems do not support
> > +configuring path parameters with
> > +.B \%FSCONFIG_SET_PATH
> > +(despite the name),
> > +which can lead to somewhat confusing
> > +.B EINVAL
> > +errors.
> > +(For example, the generic
> > +.I source
> > +parameter\[em]\c
> > +which is usually a path\[em]\c
> > +can only be configured
> > +with
> > +.BR \%FSCONFIG_SET_STRING .)
> > +.P
> > +When writing programs that use
> > +.BR fsconfig ()
> > +to configure parameters
> > +with commands other than
> > +.BR \%FSCONFIG_SET_STRING ,
> > +users should verify
> > +that the
> > +.BI \%FSCONFIG_SET_ *
> > +commands used to configure each parameter
> > +are supported by the corresponding filesystem driver.
> > +.\" Aleksa Sarai
> > +.\"   While this (quite confusing) inconsistency in behaviour is true today
> > +.\"   (and has been true since this was merged), this appears to mostly be an
> > +.\"   unintended consequence of filesystem drivers hand-coding fsparam parsing.
> > +.\"   Path parameters are the most eggregious causes of confusion.
> > +.\"   Hopefully we can make this no longer the case in a future kernel.
> > +.SH EXAMPLES
> > +To illustrate the different kinds of flags that can be configured with
> > +.BR fsconfig (),
> > +here are a few examples of some different filesystems being created:
> > +.P
> > +.in +4n
> > +.EX
> > +int fsfd, mntfd;
> > +\&
> > +fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
> > +fsconfig(fsfd, FSCONFIG_SET_FLAG, "inode64", NULL, 0);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "uid", "1234", 0);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "huge", "never", 0);
> > +fsconfig(fsfd, FSCONFIG_SET_FLAG, "casefold", NULL, 0);
> > +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> > +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOEXEC);
> > +move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
> > +\&
> > +fsfd = fsopen("erofs", FSOPEN_CLOEXEC);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/loop0", 0);
> > +fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
> > +fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
> > +fsconfig(fsfd, FSCONFIG_CMD_CREATE_EXCL, NULL, NULL, 0);
> > +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOSUID);
> > +move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> > +.EE
> > +.in
> > +.P
> > +Usually,
> > +specifying the same parameter named by
> > +.I key
> > +multiple times with
> > +.BR fsconfig ()
> > +causes the parameter value to be replaced.
> > +However, some filesystems may have unique behaviour:
> > +.P
> > +.in +4n
> > +.EX
> > +\&
> > +int fsfd, mntfd;
> > +int lowerdirfd = open("/o/ctr/lower1", O_DIRECTORY | O_CLOEXEC);
> > +\&
> > +fsfd = fsopen("overlay", FSOPEN_CLOEXEC);
> > +/* "lowerdir+" appends to the lower dir stack each time */
> > +fsconfig(fsfd, FSCONFIG_SET_FD, "lowerdir+", NULL, lowerdirfd);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower2", 0);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower3", 0);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower4", 0);
> > +.\" fsconfig(fsfd, FSCONFIG_SET_PATH, "lowerdir+", "/o/ctr/lower5", AT_FDCWD);
> > +.\" fsconfig(fsfd, FSCONFIG_SET_PATH_EMPTY, "lowerdir+", "", lowerdirfd);
> > +.\" Aleksa Sarai: Hopefully these will also be supported in the future.
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "xino", "auto", 0);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "nfs_export", "off", 0);
> > +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> > +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
> > +move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> > +.EE
> > +.in
> > +.P
> > +And here is an example of how
> > +.BR fspick (2)
> > +can be used with
> > +.BR fsconfig ()
> > +to reconfigure the parameters
> > +of an extant filesystem instance
> > +attached to
> > +.IR /proc :
> > +.P
> > +.in +4n
> > +.EX
> > +int fsfd = fspick(AT_FDCWD, "/proc", FSPICK_CLOEXEC);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "ptraceable", 0);
> > +fsconfig(fsfd, FSCONFIG_SET_STRING, "subset", "pid", 0);
> > +fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
> > +.EE
> > +.in
> > +.SH SEE ALSO
> > +.BR fsmount (2),
> > +.BR fsopen (2),
> > +.BR fspick (2),
> > +.BR mount (2),
> > +.BR mount_setattr (2),
> > +.BR move_mount (2),
> > +.BR open_tree (2),
> > +.BR mount_namespaces (7)
> > 
> > -- 
> > 2.51.0
> > 
> > 
> 
> -- 
> <https://www.alejandro-colomar.es>
> Use port 80 (that is, <...:80/>).



-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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Re: [PATCH v5 3/8] man/man2/fsconfig.2: document "new" mount API

[Alejandro Colomar]

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On Fri, Sep 26, 2025 at 01:15:14AM +1000, Aleksa Sarai wrote:
> > > +.TP
> > > +.B ENOTBLK
> > > +The parameter named by
> > > +.I name
> > 
> > There's no such parameter.  (I guess you meant 'key'?)
> 
> Ah yes, I did mean "key". The same mistake was repeated for two EINVAL
> cases above as well:

Thanks!

> 
>     EINVAL One of the values of *name*, value, and/or aux were set to a
>            non-zero value when cmd required that they be zero (or NULL).
> 
>     EINVAL The parameter named by *name* cannot be set using the type
>            specified with cmd.
> 
> Do you want me to send another version or would you able to fix it when
> you apply?

I can fix it.

Cheers,
Alex

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 3/8] man/man2/fsconfig.2: document "new" mount API

[Alejandro Colomar]

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Hi Aleksa,

On Thu, Sep 25, 2025 at 01:31:25AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

I've applied this patch, with a few amendments.  Thanks!
<https://www.alejandro-colomar.es:80/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=d0b3cd8a5399f734037e87b8f93d5b536af20d7d>
(Use port :80/)


Have a lovely day!
Alex

> ---
>  man/man2/fsconfig.2 | 729 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 729 insertions(+)
> 
> diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..a2d844a105c74f17af640d6991046dbd5fa69cf0
> --- /dev/null
> +++ b/man/man2/fsconfig.2
> @@ -0,0 +1,729 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH fsconfig 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +fsconfig \- configure new or existing filesystem context
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ,\~ \-lc )
> +.SH SYNOPSIS
> +.nf
> +.B #include <sys/mount.h>
> +.P
> +.BI "int fsconfig(int " fd ", unsigned int " cmd ,
> +.BI "             const char *_Nullable " key ,
> +.BI "             const void *_Nullable " value ", int " aux );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR fsconfig ()
> +system call is part of
> +the suite of file-descriptor-based mount facilities in Linux.
> +.P
> +.BR fsconfig ()
> +is used to supply parameters to
> +and issue commands against
> +the filesystem configuration context
> +associated with the file descriptor
> +.IR fd .
> +Filesystem configuration contexts can be created with
> +.BR fsopen (2)
> +or be instantiated from an extant filesystem instance with
> +.BR fspick (2).
> +.P
> +The
> +.I cmd
> +argument indicates the command to be issued.
> +Some commands supply parameters to the context
> +(equivalent to mount options specified with
> +.BR mount (8)),
> +while others are meta-operations on the filesystem context.
> +The list of valid
> +.I cmd
> +values are:
> +.RS
> +.TP
> +.B FSCONFIG_SET_FLAG
> +Set the flag parameter named by
> +.IR key .
> +.I value
> +must be NULL,
> +and
> +.I aux
> +must be 0.
> +.TP
> +.B FSCONFIG_SET_STRING
> +Set the string parameter named by
> +.I key
> +to the value specified by
> +.IR value .
> +.I value
> +points to a null-terminated string,
> +and
> +.I aux
> +must be 0.
> +.TP
> +.B FSCONFIG_SET_BINARY
> +Set the blob parameter named by
> +.I key
> +to the contents of the binary blob
> +specified by
> +.IR value .
> +.I value
> +points to
> +the start of a buffer
> +that is
> +.I aux
> +bytes in length.
> +.TP
> +.B FSCONFIG_SET_FD
> +Set the file parameter named by
> +.I key
> +to the open file description
> +referenced by the file descriptor
> +.IR aux .
> +.I value
> +must be NULL.
> +.IP
> +You may also use
> +.B \%FSCONFIG_SET_STRING
> +for file parameters,
> +with
> +.I value
> +set to a null-terminated string
> +containing a base-10 representation
> +of the file descriptor number.
> +This mechanism is primarily intended for compatibility
> +with older
> +.BR mount (2)-based
> +programs,
> +and only works for parameters
> +that
> +.I only
> +accept file descriptor arguments.
> +.TP
> +.B FSCONFIG_SET_PATH
> +Set the path parameter named by
> +.I key
> +to the object at a provided path,
> +resolved in a similar manner to
> +.BR openat (2).
> +.I value
> +points to a null-terminated pathname string,
> +and
> +.I aux
> +is equivalent to the
> +.I dirfd
> +argument to
> +.BR openat (2).
> +See
> +.BR openat (2)
> +for an explanation of the need for
> +.BR \%FSCONFIG_SET_PATH .
> +.IP
> +You may also use
> +.B \%FSCONFIG_SET_STRING
> +for path parameters,
> +the behaviour of which is equivalent to
> +.B \%FSCONFIG_SET_PATH
> +with
> +.I aux
> +set to
> +.BR \%AT_FDCWD .
> +.TP
> +.B FSCONFIG_SET_PATH_EMPTY
> +As with
> +.BR \%FSCONFIG_SET_PATH ,
> +except that if
> +.I value
> +is an empty string,
> +the file descriptor specified by
> +.I aux
> +is operated on directly
> +and may be any type of file
> +(not just a directory).
> +This is equivalent to the behaviour of
> +.B \%AT_EMPTY_PATH
> +with most "*at()" system calls.
> +If
> +.I aux
> +is
> +.BR \%AT_FDCWD ,
> +the parameter will be set to
> +the current working directory
> +of the calling process.
> +.TP
> +.B FSCONFIG_CMD_CREATE
> +This command instructs the filesystem driver
> +to instantiate an instance of the filesystem in the kernel
> +with the parameters specified in the filesystem configuration context.
> +.I key
> +and
> +.I value
> +must be NULL,
> +and
> +.I aux
> +must be 0.
> +.IP
> +This command can only be issued once
> +in the lifetime of a filesystem context.
> +If the operation succeeds,
> +the filesystem context
> +associated with file descriptor
> +.I fd
> +now references the created filesystem instance,
> +and is placed into a special "awaiting-mount" mode
> +that allows you to use
> +.BR fsmount (2)
> +to create a mount object from the filesystem instance.
> +.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
> +If the operation fails,
> +in most cases
> +the filesystem context is placed in a failed mode
> +and cannot be used for any further
> +.BR fsconfig ()
> +operations
> +(though you may still retrieve diagnostic messages
> +through the message retrieval interface,
> +as described in
> +the corresponding subsection of
> +.BR fsopen (2)).
> +.IP
> +This command can only be issued against
> +filesystem configuration contexts
> +that were created with
> +.BR fsopen (2).
> +In order to create a filesystem instance,
> +the calling process must have the
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.IP
> +An important thing to be aware of is that
> +the Linux kernel will
> +.I silently
> +reuse extant filesystem instances
> +depending on the filesystem type
> +and the configured parameters
> +(each filesystem driver has
> +its own policy for
> +how filesystem instances are reused).
> +This means that
> +the filesystem instance "created" by
> +.B \%FSCONFIG_CMD_CREATE
> +may, in fact, be a reference
> +to an extant filesystem instance in the kernel.
> +(For reference,
> +this behaviour also applies to
> +.BR mount (2).)
> +.IP
> +One side-effect of this behaviour is that
> +if an extant filesystem instance is reused,
> +.I all
> +parameters configured
> +for this filesystem configuration context
> +are
> +.I silently ignored
> +(with the exception of the
> +.I ro
> +and
> +.I rw
> +flag parameters;
> +if the state of the read-only flag in the
> +extant filesystem instance and the filesystem configuration context
> +do not match, this operation will return
> +.BR EBUSY ).
> +This also means that
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +commands issued against
> +the "created" filesystem instance
> +will also affect any mount objects associated with
> +the extant filesystem instance.
> +.IP
> +Programs that need to ensure
> +that they create a new filesystem instance
> +with specific parameters
> +(notably, security-related parameters
> +such as
> +.I acl
> +to enable POSIX ACLs\[em]\c
> +as described in
> +.BR acl (5))
> +should use
> +.B \%FSCONFIG_CMD_CREATE_EXCL
> +instead.
> +.TP
> +.BR FSCONFIG_CMD_CREATE_EXCL " (since Linux 6.6)"
> +.\" commit 22ed7ecdaefe0cac0c6e6295e83048af60435b13
> +.\" commit 84ab1277ce5a90a8d1f377707d662ac43cc0918a
> +As with
> +.BR \%FSCONFIG_CMD_CREATE ,
> +except that the kernel is instructed
> +to not reuse extant filesystem instances.
> +If the operation
> +would be forced to
> +reuse an extant filesystem instance,
> +this operation will return
> +.B EBUSY
> +instead.
> +.IP
> +As a result (unlike
> +.BR \%FSCONFIG_CMD_CREATE ),
> +if this operation succeeds
> +then the calling process can be sure that
> +all of the parameters successfully configured with
> +.BR fsconfig ()
> +will actually be applied
> +to the created filesystem instance.
> +.TP
> +.B FSCONFIG_CMD_RECONFIGURE
> +This command instructs the filesystem driver
> +to apply the parameters specified in the filesystem configuration context
> +to the extant filesystem instance
> +referenced by the filesystem configuration context.
> +.I key
> +and
> +.I value
> +must be NULL,
> +and
> +.I aux
> +must be 0.
> +.IP
> +This is primarily intended for use with
> +.BR fspick (2),
> +but may also be used to modify
> +the parameters of a filesystem instance
> +after
> +.B \%FSCONFIG_CMD_CREATE
> +was used to create it
> +and a mount object was created using
> +.BR fsmount (2).
> +In order to reconfigure an extant filesystem instance,
> +the calling process must have the
> +.B CAP_SYS_ADMIN
> +capability.
> +.IP
> +If the operation succeeds,
> +the filesystem context is reset
> +but remains in reconfiguration mode
> +and thus can be reused for subsequent
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +commands.
> +If the operation fails,
> +in most cases
> +the filesystem context is placed in a failed mode
> +and cannot be used for any further
> +.BR fsconfig ()
> +operations
> +(though you may still retrieve diagnostic messages
> +through the message retrieval interface,
> +as described in
> +the corresponding subsection of
> +.BR fsopen (2)).
> +.RE
> +.P
> +Parameters specified with
> +.BI FSCONFIG_SET_ *
> +do not take effect
> +until a corresponding
> +.B \%FSCONFIG_CMD_CREATE
> +or
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +command is issued.
> +.SH RETURN VALUE
> +On success,
> +.BR fsconfig ()
> +returns 0.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +If an error occurs, the filesystem driver may provide
> +additional information about the error
> +through the message retrieval interface for filesystem configuration contexts.
> +This additional information can be retrieved at any time by calling
> +.BR read (2)
> +on the filesystem instance or filesystem configuration context
> +referenced by the file descriptor
> +.IR fd .
> +(See the "Message retrieval interface" subsection in
> +.BR fsopen (2)
> +for more details on the message format.)
> +.P
> +Even after an error occurs,
> +the filesystem configuration context is
> +.I not
> +invalidated,
> +and thus can still be used with other
> +.BR fsconfig ()
> +commands.
> +This means that users can probe support for filesystem parameters
> +on a per-parameter basis,
> +and adjust which parameters they wish to set.
> +.P
> +The error values given below result from
> +filesystem type independent errors.
> +Each filesystem type may have its own special errors
> +and its own special behavior.
> +See the Linux kernel source code for details.
> +.TP
> +.B EACCES
> +A component of a path
> +provided as a path parameter
> +was not searchable.
> +(See also
> +.BR path_resolution (7).)
> +.TP
> +.B EACCES
> +.B \%FSCONFIG_CMD_CREATE
> +was attempted
> +for a read-only filesystem
> +without specifying the
> +.RB ' ro '
> +flag parameter.
> +.TP
> +.B EACCES
> +A specified block device parameter
> +is located on a filesystem
> +mounted with the
> +.B \%MS_NODEV
> +option.
> +.TP
> +.B EBADF
> +The file descriptor given by
> +.I fd
> +(or possibly by
> +.IR aux ,
> +depending on the command)
> +is invalid.
> +.TP
> +.B EBUSY
> +The filesystem context associated with
> +.I fd
> +is in the wrong state
> +for the given command.
> +.TP
> +.B EBUSY
> +The filesystem instance cannot be reconfigured as read-only
> +with
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +because some programs
> +still hold files open for writing.
> +.TP
> +.B EBUSY
> +A new filesystem instance was requested with
> +.B \%FSCONFIG_CMD_CREATE_EXCL
> +but a matching superblock already existed.
> +.TP
> +.B EFAULT
> +One of the pointer arguments
> +points to a location
> +outside the calling process's accessible address space.
> +.TP
> +.B EINVAL
> +.I fd
> +does not refer to
> +a filesystem configuration context
> +or filesystem instance.
> +.TP
> +.B EINVAL
> +One of the values of
> +.IR name ,
> +.IR value ,
> +and/or
> +.I aux
> +were set to a non-zero value when
> +.I cmd
> +required that they be zero
> +(or NULL).
> +.TP
> +.B EINVAL
> +The parameter named by
> +.I name
> +cannot be set
> +using the type specified with
> +.IR cmd .
> +.TP
> +.B EINVAL
> +One of the source parameters
> +referred to
> +an invalid superblock.
> +.TP
> +.B ELOOP
> +Too many links encountered
> +during pathname resolution
> +of a path argument.
> +.TP
> +.B ENAMETOOLONG
> +A path argument was longer than
> +.BR PATH_MAX .
> +.TP
> +.B ENOENT
> +A path argument had a non-existent component.
> +.TP
> +.B ENOENT
> +A path argument is an empty string,
> +but
> +.I cmd
> +is not
> +.BR \%FSCONFIG_SET_PATH_EMPTY .
> +.TP
> +.B ENOMEM
> +The kernel could not allocate sufficient memory to complete the operation.
> +.TP
> +.B ENOTBLK
> +The parameter named by
> +.I name
> +must be a block device,
> +but the provided parameter value was not a block device.
> +.TP
> +.B ENOTDIR
> +A component of the path prefix
> +of a path argument
> +was not a directory.
> +.TP
> +.B EOPNOTSUPP
> +The command given by
> +.I cmd
> +is not valid.
> +.TP
> +.B ENXIO
> +The major number
> +of a block device parameter
> +is out of range.
> +.TP
> +.B EPERM
> +The command given by
> +.I cmd
> +was
> +.BR \%FSCONFIG_CMD_CREATE ,
> +.BR \%FSCONFIG_CMD_CREATE_EXCL ,
> +or
> +.BR \%FSCONFIG_CMD_RECONFIGURE ,
> +but the calling process does not have the required
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.SH STANDARDS
> +Linux.
> +.SH HISTORY
> +Linux 5.2.
> +.\" commit ecdab150fddb42fe6a739335257949220033b782
> +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> +glibc 2.36.
> +.SH NOTES
> +.SS Generic filesystem parameters
> +Each filesystem driver is responsible for
> +parsing most parameters specified with
> +.BR fsconfig (),
> +meaning that individual filesystems
> +may have very different behaviour
> +when encountering parameters with the same name.
> +In general,
> +you should not assume that the behaviour of
> +.BR fsconfig ()
> +when specifying a parameter to one filesystem type
> +will match the behaviour of the same parameter
> +with a different filesystem type.
> +.P
> +However,
> +the following generic parameters
> +apply to all filesystems and have unified behaviour.
> +They are set using the listed
> +.BI \%FSCONFIG_SET_ *
> +command.
> +.TP
> +\fIro\fP and \fIrw\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether the filesystem instance is read-only.
> +.TP
> +\fIdirsync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Make directory changes on this filesystem instance synchronous.
> +.TP
> +\fIsync\fP and \fIasync\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether writes on this filesystem instance
> +will be made synchronous
> +(as though the
> +.B O_SYNC
> +flag to
> +.BR open (2)
> +was specified for
> +all file opens in this filesystem instance).
> +.TP
> +\fIlazytime\fP and \fInolazytime\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether to reduce on-disk updates
> +of inode timestamps on this filesystem instance
> +(as described in the
> +.B \%MS_LAZYTIME
> +section of
> +.BR mount (2)).
> +.TP
> +\fImand\fP and \fInomand\fP (\fB\%FSCONFIG_SET_FLAG\fP)
> +Configure whether the filesystem instance should permit mandatory locking.
> +Since Linux 5.15,
> +.\" commit f7e33bdbd6d1bdf9c3df8bba5abcf3399f957ac3
> +mandatory locking has been deprecated
> +and setting this flag is a no-op.
> +.TP
> +\fIsource\fP (\fB\%FSCONFIG_SET_STRING\fP)
> +This parameter is equivalent to the
> +.I source
> +parameter passed to
> +.BR mount (2)
> +for the same filesystem type,
> +and is usually the pathname of a block device
> +containing the filesystem.
> +This parameter may only be set once
> +per filesystem configuration context transaction.
> +.P
> +In addition,
> +any filesystem parameters associated with
> +Linux Security Modules (LSMs)
> +are also generic with respect to the underlying filesystem.
> +See the documentation for the LSM you wish to configure for more details.
> +.SH CAVEATS
> +.SS Filesystem parameter types
> +As a result of
> +each filesystem driver being responsible for
> +parsing most parameters specified with
> +.BR fsconfig (),
> +some filesystem drivers
> +may have unintuitive behaviour
> +with regards to which
> +.BI \%FSCONFIG_SET_ *
> +commands are permitted
> +to configure a given parameter.
> +.P
> +In order for
> +filesystem parameters to be backwards compatible with
> +.BR mount (2),
> +they must be parseable as strings;
> +this almost universally means that
> +.B \%FSCONFIG_SET_STRING
> +can also be used to configure them.
> +.\" Aleksa Sarai
> +.\"   Theoretically, a filesystem could check fc->oldapi and refuse
> +.\"   FSCONFIG_SET_STRING if the operation is coming from the new API, but no
> +.\"   filesystems do this (and probably never will).
> +However, other
> +.BI \%FSCONFIG_SET_ *
> +commands need to be opted into
> +by each filesystem driver's parameter parser.
> +.P
> +One of the most user-visible instances of
> +this inconsistency is that
> +many filesystems do not support
> +configuring path parameters with
> +.B \%FSCONFIG_SET_PATH
> +(despite the name),
> +which can lead to somewhat confusing
> +.B EINVAL
> +errors.
> +(For example, the generic
> +.I source
> +parameter\[em]\c
> +which is usually a path\[em]\c
> +can only be configured
> +with
> +.BR \%FSCONFIG_SET_STRING .)
> +.P
> +When writing programs that use
> +.BR fsconfig ()
> +to configure parameters
> +with commands other than
> +.BR \%FSCONFIG_SET_STRING ,
> +users should verify
> +that the
> +.BI \%FSCONFIG_SET_ *
> +commands used to configure each parameter
> +are supported by the corresponding filesystem driver.
> +.\" Aleksa Sarai
> +.\"   While this (quite confusing) inconsistency in behaviour is true today
> +.\"   (and has been true since this was merged), this appears to mostly be an
> +.\"   unintended consequence of filesystem drivers hand-coding fsparam parsing.
> +.\"   Path parameters are the most eggregious causes of confusion.
> +.\"   Hopefully we can make this no longer the case in a future kernel.
> +.SH EXAMPLES
> +To illustrate the different kinds of flags that can be configured with
> +.BR fsconfig (),
> +here are a few examples of some different filesystems being created:
> +.P
> +.in +4n
> +.EX
> +int fsfd, mntfd;
> +\&
> +fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "inode64", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "uid", "1234", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "huge", "never", 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "casefold", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOEXEC);
> +move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
> +\&
> +fsfd = fsopen("erofs", FSOPEN_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/loop0", 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE_EXCL, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOSUID);
> +move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +Usually,
> +specifying the same parameter named by
> +.I key
> +multiple times with
> +.BR fsconfig ()
> +causes the parameter value to be replaced.
> +However, some filesystems may have unique behaviour:
> +.P
> +.in +4n
> +.EX
> +\&
> +int fsfd, mntfd;
> +int lowerdirfd = open("/o/ctr/lower1", O_DIRECTORY | O_CLOEXEC);
> +\&
> +fsfd = fsopen("overlay", FSOPEN_CLOEXEC);
> +/* "lowerdir+" appends to the lower dir stack each time */
> +fsconfig(fsfd, FSCONFIG_SET_FD, "lowerdir+", NULL, lowerdirfd);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower2", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower3", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower4", 0);
> +.\" fsconfig(fsfd, FSCONFIG_SET_PATH, "lowerdir+", "/o/ctr/lower5", AT_FDCWD);
> +.\" fsconfig(fsfd, FSCONFIG_SET_PATH_EMPTY, "lowerdir+", "", lowerdirfd);
> +.\" Aleksa Sarai: Hopefully these will also be supported in the future.
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "xino", "auto", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "nfs_export", "off", 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
> +move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +And here is an example of how
> +.BR fspick (2)
> +can be used with
> +.BR fsconfig ()
> +to reconfigure the parameters
> +of an extant filesystem instance
> +attached to
> +.IR /proc :
> +.P
> +.in +4n
> +.EX
> +int fsfd = fspick(AT_FDCWD, "/proc", FSPICK_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "ptraceable", 0);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "subset", "pid", 0);
> +fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
> +.EE
> +.in
> +.SH SEE ALSO
> +.BR fsmount (2),
> +.BR fsopen (2),
> +.BR fspick (2),
> +.BR mount (2),
> +.BR mount_setattr (2),
> +.BR move_mount (2),
> +.BR open_tree (2),
> +.BR mount_namespaces (7)
> 
> -- 
> 2.51.0
> 
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 4/8] man/man2/fsmount.2: document "new" mount API

[Alejandro Colomar]

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Hi Aleksa,

On Thu, Sep 25, 2025 at 01:31:26AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Thanks!  I've applied this patch.
<https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=24243cc66e191fd917c9c13a01b7ac037ce0972e>


Cheers,
Alex

> ---
>  man/man2/fsmount.2 | 231 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 231 insertions(+)
> 
> diff --git a/man/man2/fsmount.2 b/man/man2/fsmount.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..b62850a68443bb8f6178389eb6cb1a5f9029ab30
> --- /dev/null
> +++ b/man/man2/fsmount.2
> @@ -0,0 +1,231 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH fsmount 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +fsmount \- instantiate mount object from filesystem context
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ,\~ \-lc )
> +.SH SYNOPSIS
> +.nf
> +.B #include <sys/mount.h>
> +.P
> +.BI "int fsmount(int " fsfd ", unsigned int " flags \
> +", unsigned int " attr_flags );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR fsmount ()
> +system call is part of
> +the suite of file-descriptor-based mount facilities in Linux.
> +.P
> +.BR fsmount ()
> +creates a new detached mount object
> +for the root of the new filesystem instance
> +referenced by the filesystem context file descriptor
> +.IR fsfd .
> +A new file descriptor
> +associated with the detached mount object
> +is then returned.
> +In order to create a mount object with
> +.BR fsmount (),
> +the calling process must have the
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.P
> +The filesystem context must have been created with a call to
> +.BR fsopen (2)
> +and then had a filesystem instance instantiated with a call to
> +.BR fsconfig (2)
> +with
> +.B \%FSCONFIG_CMD_CREATE
> +or
> +.B \%FSCONFIG_CMD_CREATE_EXCL
> +in order to be in the correct state
> +for this operation
> +(the "awaiting-mount" mode in kernel-developer parlance).
> +.\" FS_CONTEXT_AWAITING_MOUNT is the term the kernel uses for this.
> +Unlike
> +.BR open_tree (2)
> +with
> +.BR \%OPEN_TREE_CLONE ,
> +.BR fsmount ()
> +can only be called once
> +in the lifetime of a filesystem context
> +to produce a mount object.
> +.P
> +As with file descriptors returned from
> +.BR open_tree (2)
> +called with
> +.BR OPEN_TREE_CLONE ,
> +the returned file descriptor
> +can then be used with
> +.BR move_mount (2),
> +.BR mount_setattr (2),
> +or other such system calls to do further mount operations.
> +This mount object will be unmounted and destroyed
> +when the file descriptor is closed
> +if it was not otherwise attached to a mount point
> +by calling
> +.BR move_mount (2).
> +(Note that the unmount operation on
> +.BR close (2)
> +is lazy\[em]akin to calling
> +.BR umount2 (2)
> +with
> +.BR MNT_DETACH ;
> +any existing open references to files
> +from the mount object
> +will continue to work,
> +and the mount object will only be completely destroyed
> +once it ceases to be busy.)
> +The returned file descriptor
> +also acts the same as one produced by
> +.BR open (2)
> +with
> +.BR O_PATH ,
> +meaning it can also be used as a
> +.I dirfd
> +argument
> +to "*at()" system calls.
> +.P
> +.I flags
> +controls the creation of the returned file descriptor.
> +A value for
> +.I flags
> +is constructed by bitwise ORing
> +zero or more of the following constants:
> +.RS
> +.TP
> +.B FSMOUNT_CLOEXEC
> +Set the close-on-exec
> +.RB ( FD_CLOEXEC )
> +flag on the new file descriptor.
> +See the description of the
> +.B O_CLOEXEC
> +flag in
> +.BR open (2)
> +for reasons why this may be useful.
> +.RE
> +.P
> +.I attr_flags
> +specifies mount attributes
> +which will be applied to the created mount object,
> +in the form of
> +.BI \%MOUNT_ATTR_ *
> +flags.
> +The flags are interpreted as though
> +.BR mount_setattr (2)
> +was called with
> +.I attr.attr_set
> +set to the same value as
> +.IR attr_flags .
> +.BI \%MOUNT_ATTR_ *
> +flags which would require
> +specifying additional fields in
> +.BR mount_attr (2type)
> +(such as
> +.BR \%MOUNT_ATTR_IDMAP )
> +are not valid flag values for
> +.IR attr_flags .
> +.P
> +If the
> +.BR fsmount ()
> +operation is successful,
> +the filesystem context
> +associated with the file descriptor
> +.I fsfd
> +is reset
> +and placed into reconfiguration mode,
> +as if it were just returned by
> +.BR fspick (2).
> +You may continue to use
> +.BR fsconfig (2)
> +with the now-reset filesystem context,
> +including issuing the
> +.B \%FSCONFIG_CMD_RECONFIGURE
> +command
> +to reconfigure the filesystem instance.
> +.SH RETURN VALUE
> +On success, a new file descriptor is returned.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EBUSY
> +The filesystem context associated with
> +.I fsfd
> +is not in the right state
> +to be used by
> +.BR fsmount ().
> +.TP
> +.B EINVAL
> +.I flags
> +had an invalid flag set.
> +.TP
> +.B EINVAL
> +.I attr_flags
> +had an invalid
> +.BI MOUNT_ATTR_ *
> +flag set.
> +.TP
> +.B EMFILE
> +The calling process has too many open files to create more.
> +.TP
> +.B ENFILE
> +The system has too many open files to create more.
> +.TP
> +.B ENOSPC
> +The "anonymous" mount namespace
> +necessary to contain the new mount object
> +could not be allocated,
> +as doing so would exceed
> +the configured per-user limit on
> +the number of mount namespaces in the current user namespace.
> +(See also
> +.BR namespaces (7).)
> +.TP
> +.B ENOMEM
> +The kernel could not allocate sufficient memory to complete the operation.
> +.TP
> +.B EPERM
> +The calling process does not have the required
> +.B CAP_SYS_ADMIN
> +capability.
> +.SH STANDARDS
> +Linux.
> +.SH HISTORY
> +Linux 5.2.
> +.\" commit 93766fbd2696c2c4453dd8e1070977e9cd4e6b6d
> +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> +glibc 2.36.
> +.SH EXAMPLES
> +.in +4n
> +.EX
> +int fsfd, mntfd, tmpfd;
> +\&
> +fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC,
> +                MOUNT_ATTR_NODEV | MOUNT_ATTR_NOEXEC);
> +\&
> +/* Create a new file without attaching the mount object */
> +tmpfd = openat(mntfd, "tmpfile", O_CREAT | O_EXCL | O_RDWR, 0600);
> +unlinkat(mntfd, "tmpfile", 0);
> +\&
> +/* Attach the mount object to "/tmp" */
> +move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.SH SEE ALSO
> +.BR fsconfig (2),
> +.BR fsopen (2),
> +.BR fspick (2),
> +.BR mount (2),
> +.BR mount_setattr (2),
> +.BR move_mount (2),
> +.BR open_tree (2),
> +.BR mount_namespaces (7)
> 
> -- 
> 2.51.0
> 
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 5/8] man/man2/move_mount.2: document "new" mount API

[Alejandro Colomar]

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Hi Aleksa,

On Thu, Sep 25, 2025 at 01:31:27AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Thanks!  I've applied this patch, with some minor amendments (see below).
<https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=eb37a3066ccce4f44ab69fae559016a524e4eac>

> ---
>  man/man2/move_mount.2 | 646 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 646 insertions(+)
> 
> diff --git a/man/man2/move_mount.2 b/man/man2/move_mount.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..f954f36c43c444afb167088cc665607dfeb10676
> --- /dev/null
> +++ b/man/man2/move_mount.2
> @@ -0,0 +1,646 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH move_mount 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +move_mount \- move or attach mount object to filesystem
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ,\~ \-lc )
> +.SH SYNOPSIS
> +.nf
> +.BR "#include <fcntl.h>" "          /* Definition of " AT_* " constants */"
> +.B #include <sys/mount.h>
> +.P
> +.BI "int move_mount(int " from_dirfd ", const char *" from_path ,
> +.BI "               int " to_dirfd ", const char *" to_path ,
> +.BI "               unsigned int " flags );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR move_mount ()
> +system call is part of
> +the suite of file-descriptor-based mount facilities in Linux.
> +.P
> +.BR move_mount ()
> +moves the mount object indicated by
> +.I from_dirfd
> +and
> +.I from_path
> +to the path indicated by
> +.I to_dirfd
> +and
> +.IR to_path .
> +The mount object being moved
> +can be an existing mount point in the current mount namespace,
> +or a detached mount object created by
> +.BR fsmount (2)
> +or
> +.BR open_tree (2)
> +with
> +.BR \%OPEN_TREE_CLONE .
> +.P
> +To access the source mount object
> +or the destination mount point,
> +no permissions are required on the object itself,
> +but if either pathname is supplied,
> +execute (search) permission is required
> +on all of the directories specified in
> +.I from_path
> +or
> +.IR to_path .
> +.P
> +The calling process must have the
> +.B \%CAP_SYS_ADMIN
> +capability in order to move or attach a mount object.
> +.P
> +As with "*at()" system calls,
> +.BR move_mount ()
> +uses the
> +.I from_dirfd
> +and
> +.I to_dirfd
> +arguments
> +in conjunction with the
> +.I from_path
> +and
> +.I to_path
> +arguments to determine the source and destination objects to operate on
> +(respectively), as follows:
> +.IP \[bu] 3
> +If the pathname given in
> +.I *_path

In this case, where the non-variable part is already in italics, the
variable part is written in roman, for distinguishing it.  (See
groff_man(7).)


Have a lovely day!
Alex

> +is absolute, then
> +the corresponding
> +.I *_dirfd
> +is ignored.
> +.IP \[bu]
> +If the pathname given in
> +.I *_path
> +is relative and
> +the corresponding
> +.I *_dirfd
> +is the special value
> +.BR \%AT_FDCWD ,
> +then
> +.I *_path
> +is interpreted relative to
> +the current working directory
> +of the calling process (like
> +.BR open (2)).
> +.IP \[bu]
> +If the pathname given in
> +.I *_path
> +is relative,
> +then it is interpreted relative to
> +the directory referred to by
> +the corresponding file descriptor
> +.I *_dirfd
> +(rather than relative to
> +the current working directory
> +of the calling process,
> +as is done by
> +.BR open (2)
> +for a relative pathname).
> +In this case,
> +the corresponding
> +.I *_dirfd
> +must be a directory
> +that was opened for reading
> +.RB ( O_RDONLY )
> +or using the
> +.B O_PATH
> +flag.
> +.IP \[bu]
> +If
> +.I *_path
> +is an empty string,
> +and
> +.I flags
> +contains the appropriate
> +.BI \%MOVE_MOUNT_ * _EMPTY_PATH
> +flag,
> +then the corresponding file descriptor
> +.I *_dirfd
> +is operated on directly.
> +In this case,
> +the corresponding
> +.I *_dirfd
> +may refer to any type of file,
> +not just a directory.
> +.P
> +See
> +.BR openat (2)
> +for an explanation of why the
> +.I *_dirfd
> +arguments are useful.
> +.P
> +.I flags
> +can be used to control aspects of the path lookup
> +for both the source and destination objects,
> +as well as other properties of the mount operation.
> +A value for
> +.I flags
> +is constructed by bitwise ORing
> +zero or more of the following constants:
> +.RS
> +.TP
> +.B MOVE_MOUNT_F_EMPTY_PATH
> +If
> +.I from_path
> +is an empty string, operate on the file referred to by
> +.I from_dirfd
> +(which may have been obtained from
> +.BR open (2),
> +.BR fsmount (2),
> +or
> +.BR open_tree (2)).
> +In this case,
> +.I from_dirfd
> +may refer to any type of file,
> +not just a directory.
> +If
> +.I from_dirfd
> +is
> +.BR \%AT_FDCWD ,
> +.BR move_mount ()
> +will operate on the current working directory
> +of the calling process.
> +.IP
> +This is the most common mechanism
> +used to attach detached mount objects
> +produced by
> +.BR fsmount (2)
> +and
> +.BR open_tree (2)
> +to a mount point.
> +.TP
> +.B MOVE_MOUNT_T_EMPTY_PATH
> +As with
> +.BR \%MOVE_MOUNT_F_EMPTY_PATH ,
> +except operating on
> +.I to_dirfd
> +and
> +.IR to_path .
> +.TP
> +.B MOVE_MOUNT_F_SYMLINKS
> +If
> +.I from_path
> +references a symbolic link,
> +then dereference it.
> +The default behaviour for
> +.BR move_mount ()
> +is to
> +.I not follow
> +symbolic links.
> +.TP
> +.B MOVE_MOUNT_T_SYMLINKS
> +As with
> +.BR \%MOVE_MOUNT_F_SYMLINKS ,
> +except operating on
> +.I to_dirfd
> +and
> +.IR to_path .
> +.TP
> +.B MOVE_MOUNT_F_NO_AUTOMOUNT
> +Do not automount the terminal ("basename") component of
> +.I \%from_path
> +if it is a directory that is an automount point.
> +This allows a mount object
> +that has an automount point at its root
> +to be moved
> +and prevents unintended triggering of an automount point.
> +This flag has no effect
> +if the automount point has already been mounted over.
> +.TP
> +.B MOVE_MOUNT_T_NO_AUTOMOUNT
> +As with
> +.BR \%MOVE_MOUNT_F_NO_AUTOMOUNT ,
> +except operating on
> +.I to_dirfd
> +and
> +.IR to_path .
> +This allows an automount point to be manually mounted over.
> +.TP
> +.BR MOVE_MOUNT_SET_GROUP " (since Linux 5.15)"
> +Add the attached private-propagation mount object indicated by
> +.I to_dirfd
> +and
> +.I to_path
> +into the mount propagation "peer group"
> +of the attached non-private-propagation mount object indicated by
> +.I from_dirfd
> +and
> +.IR from_path .
> +.IP
> +Unlike other
> +.BR move_mount ()
> +operations,
> +this operation does not move or attach any mount objects.
> +Instead, it only updates the metadata
> +of attached mount objects.
> +(Also, take careful note of
> +the argument order\[em]\c
> +the mount object being modified
> +by this operation is the one specified by
> +.I to_dirfd
> +and
> +.IR to_path .)
> +.IP
> +This makes it possible to first create a mount tree
> +consisting only of private mounts
> +and then configure the desired propagation layout afterwards.
> +(See the "SHARED SUBTREES" section of
> +.BR mount_namespaces (7)
> +for more information about mount propagation and peer groups.)
> +.TP
> +.BR MOVE_MOUNT_BENEATH " (since Linux 6.5)"
> +If the path indicated by
> +.I to_dirfd
> +and
> +.I to_path
> +is an existing mount object,
> +rather than attaching or moving the mount object
> +indicated by
> +.I from_dirfd
> +and
> +.I from_path
> +on top of the mount stack,
> +attach or move it beneath the current top mount
> +on the mount stack.
> +.IP
> +After using
> +.BR \%MOVE_MOUNT_BENEATH ,
> +it is possible to
> +.BR umount (2)
> +the top mount
> +in order to reveal the mount object
> +which was attached beneath it earlier.
> +This allows for the seamless (and atomic) replacement
> +of intricate mount trees,
> +which can further be used
> +to "upgrade" a mount tree with a newer version.
> +.IP
> +This operation has several restrictions:
> +.RS
> +.IP \[bu] 3
> +Mount objects cannot be attached beneath the filesystem root,
> +including cases where
> +the filesystem root was configured by
> +.BR chroot (2)
> +or
> +.BR pivot_root (2).
> +To mount beneath the filesystem root,
> +.BR pivot_root (2)
> +must be used.
> +.IP \[bu]
> +The target path indicated by
> +.I to_dirfd
> +and
> +.I to_path
> +must not be a detached mount object,
> +such as those produced by
> +.BR open_tree (2)
> +with
> +.B \%OPEN_TREE_CLONE
> +or
> +.BR fsmount (2).
> +.IP \[bu]
> +The current top mount
> +of the target path's mount stack
> +and its parent mount
> +must be in the calling process's mount namespace.
> +.IP \[bu]
> +The caller must have sufficient privileges
> +to unmount the top mount
> +of the target path's mount stack,
> +to prove they have privileges
> +to reveal the underlying mount.
> +.IP \[bu]
> +Mount propagation events triggered by this
> +.BR move_mount ()
> +operation
> +(as described in
> +.BR mount_namespaces (7))
> +are calculated based on the parent mount
> +of the current top mount
> +of the target path's mount stack.
> +.IP \[bu]
> +The target path's mount
> +cannot be an ancestor in the mount tree of
> +the source mount object.
> +.IP \[bu]
> +The source mount object
> +must not have any overmounts,
> +otherwise it would be possible to create "shadow mounts"
> +(i.e., two mounts mounted on the same parent mount at the same mount point).
> +.IP \[bu]
> +It is not possible to move a mount
> +beneath a top mount
> +if the parent mount
> +of the current top mount
> +propagates to the top mount itself.
> +Otherwise,
> +.B \%MOVE_MOUNT_BENEATH
> +would cause the mount object
> +to be propagated
> +to the top mount
> +from the parent mount,
> +defeating the purpose of using
> +.BR \%MOVE_MOUNT_BENEATH .
> +.IP \[bu]
> +It is not possible to move a mount
> +beneath a top mount
> +if the parent mount
> +of the current top mount
> +propagates to the mount object
> +being mounted beneath.
> +Otherwise, this would cause a similar propagation issue
> +to the previous point,
> +also defeating the purpose of using
> +.BR \%MOVE_MOUNT_BENEATH .
> +.RE
> +.RE
> +.P
> +If
> +.I from_dirfd
> +is a mount object file descriptor and
> +.BR move_mount ()
> +is operating on it directly,
> +.I from_dirfd
> +will remain associated with the mount object after
> +.BR move_mount ()
> +succeeds,
> +so you may repeatedly use
> +.I from_dirfd
> +with
> +.BR move_mount (2)
> +and/or "*at()" system calls
> +as many times as necessary.
> +.SH RETURN VALUE
> +On success,
> +.BR move_mount ()
> +returns 0.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EACCES
> +Search permission is denied
> +for one of the directories
> +in the path prefix of one of
> +.I from_path
> +or
> +.IR to_path .
> +(See also
> +.BR path_resolution (7).)
> +.TP
> +.B EBADF
> +One of
> +.I from_dirfd
> +or
> +.I to_dirfd
> +is not a valid file descriptor.
> +.TP
> +.B EFAULT
> +One of
> +.I from_path
> +or
> +.I to_path
> +is NULL
> +or a pointer to a location
> +outside the calling process's accessible address space.
> +.TP
> +.B EINVAL
> +Invalid flag specified in
> +.IR flags .
> +.TP
> +.B EINVAL
> +The path indicated by
> +.I from_dirfd
> +and
> +.I from_path
> +is not a mount object.
> +.TP
> +.B EINVAL
> +The mount object type
> +of the source mount object and target inode
> +are not compatible
> +(i.e., the source is a file but the target is a directory, or vice-versa).
> +.TP
> +.B EINVAL
> +The source mount object or target path
> +are not in the calling process's mount namespace
> +(or an anonymous mount namespace of the calling process).
> +.TP
> +.B EINVAL
> +The source mount object's parent mount
> +has shared mount propagation,
> +and thus cannot be moved
> +(as described in
> +.BR mount_namespaces (7)).
> +.TP
> +.B EINVAL
> +The source mount has
> +.B MS_UNBINDABLE
> +child mounts
> +but the target path
> +resides on a mount tree with shared mount propagation,
> +which would otherwise cause the unbindable mounts to be propagated
> +(as described in
> +.BR mount_namespaces (7)).
> +.TP
> +.B EINVAL
> +.B \%MOVE_MOUNT_BENEATH
> +was attempted,
> +but one of the listed restrictions was violated.
> +.TP
> +.B ELOOP
> +Too many symbolic links encountered
> +when resolving one of
> +.I from_path
> +or
> +.IR to_path .
> +.TP
> +.B ENAMETOOLONG
> +One of
> +.I from_path
> +or
> +.I to_path
> +is longer than
> +.BR PATH_MAX .
> +.TP
> +.B ENOENT
> +A component of one of
> +.I from_path
> +or
> +.I to_path
> +does not exist.
> +.TP
> +.B ENOENT
> +One of
> +.I from_path
> +or
> +.I to_path
> +is an empty string,
> +but the corresponding
> +.BI MOVE_MOUNT_ * _EMPTY_PATH
> +flag is not specified in
> +.IR flags .
> +.TP
> +.B ENOTDIR
> +A component of the path prefix of one of
> +.I from_path
> +or
> +.I to_path
> +is not a directory,
> +or one of
> +.I from_path
> +or
> +.I to_path
> +is relative
> +and the corresponding
> +.I from_dirfd
> +or
> +.I to_dirfd
> +is a file descriptor referring to a file other than a directory.
> +.TP
> +.B ENOMEM
> +The kernel could not allocate sufficient memory to complete the operation.
> +.TP
> +.B EPERM
> +The calling process does not have the required
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.SH STANDARDS
> +Linux.
> +.SH HISTORY
> +Linux 5.2.
> +.\" commit 2db154b3ea8e14b04fee23e3fdfd5e9d17fbc6ae
> +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> +glibc 2.36.
> +.SH EXAMPLES
> +.BR move_mount ()
> +can be used to move attached mounts like the following:
> +.P
> +.in +4n
> +.EX
> +move_mount(AT_FDCWD, "/a", AT_FDCWD, "/b", 0);
> +.EE
> +.in
> +.P
> +This would move the mount object mounted on
> +.I /a
> +to
> +.IR /b .
> +The above procedure is functionally equivalent to
> +the following mount operation
> +using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount("/a", "/b", NULL, MS_MOVE, NULL);
> +.EE
> +.in
> +.P
> +.BR move_mount ()
> +can also be used in conjunction with file descriptors returned from
> +.BR open_tree (2)
> +or
> +.BR open (2):
> +.P
> +.in +4n
> +.EX
> +int fd = open_tree(AT_FDCWD, "/mnt", 0); /* open("/mnt", O_PATH); */
> +move_mount(fd, "", AT_FDCWD, "/mnt2", MOVE_MOUNT_F_EMPTY_PATH);
> +move_mount(fd, "", AT_FDCWD, "/mnt3", MOVE_MOUNT_F_EMPTY_PATH);
> +move_mount(fd, "", AT_FDCWD, "/mnt4", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +This would move the mount object mounted at
> +.I /mnt
> +to
> +.IR /mnt2 ,
> +then
> +.IR /mnt3 ,
> +and then
> +.IR /mnt4 .
> +.P
> +If the source mount object
> +indicated by
> +.I from_dirfd
> +and
> +.I from_path
> +is a detached mount object,
> +.BR move_mount ()
> +can be used to attach it to a mount point:
> +.P
> +.in +4n
> +.EX
> +int fsfd, mntfd;
> +\&
> +fsfd = fsopen("ext4", FSOPEN_CLOEXEC);
> +fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/sda1", 0);
> +fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
> +fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
> +mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NODEV);
> +move_mount(mntfd, "", AT_FDCWD, "/home", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +This would create a new filesystem configuration context for ext4,
> +configure it,
> +create a detached mount object,
> +and then attach it to
> +.IR /home .
> +The above procedure is functionally equivalent to
> +the following mount operation
> +using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount("/dev/sda1", "/home", "ext4", MS_NODEV, "user_xattr");
> +.EE
> +.in
> +.P
> +The same operation also works with detached bind-mounts created with
> +.BR open_tree (2)
> +with
> +.BR OPEN_TREE_CLONE :
> +.P
> +.in +4n
> +.EX
> +int mntfd = open_tree(AT_FDCWD, "/home/cyphar", OPEN_TREE_CLONE);
> +move_mount(mntfd, "", AT_FDCWD, "/root", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +This would create a new bind-mount of
> +.I /home/cyphar
> +as a detached mount object,
> +and then attach it to
> +.IR /root .
> +The above procedure is functionally equivalent to
> +the following mount operation
> +using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount("/home/cyphar", "/root", NULL, MS_BIND, NULL);
> +.EE
> +.in
> +.SH SEE ALSO
> +.BR fsconfig (2),
> +.BR fsmount (2),
> +.BR fsopen (2),
> +.BR fspick (2),
> +.BR mount (2),
> +.BR mount_setattr (2),
> +.BR open_tree (2),
> +.BR mount_namespaces (7)
> 
> -- 
> 2.51.0
> 
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 7/8] man/man2/open_tree{,_attr}.2: document new open_tree_attr() API

[Askar Safin]

Aleksa Sarai <cyphar@cyphar.com>:
> +mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> +                   &attr, sizeof(attr));

Your whole so-called "open_tree_attr example" doesn't contain any open_tree_attr
calls. :)

I think you meant open_tree_attr here.

> +\&
> +/* Create a new copy with the id-mapping cleared */
> +memset(&attr, 0, sizeof(attr));
> +attr.attr_clr = MOUNT_ATTR_IDMAP;
> +mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> +                   &attr, sizeof(attr));

And here.

Otherwise your whole patchset looks good. Add to whole patchset:
Reviewed-by: Askar Safin <safinaskar@gmail.com>

-- 
Askar Safin

Re: [PATCH v5 7/8] man/man2/open_tree{,_attr}.2: document new open_tree_attr() API

[Alejandro Colomar]

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Hi Askar,

On Wed, Oct 01, 2025 at 03:38:41AM +0300, Askar Safin wrote:
> Aleksa Sarai <cyphar@cyphar.com>:
> > +mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > +                   &attr, sizeof(attr));
> 
> Your whole so-called "open_tree_attr example" doesn't contain any open_tree_attr
> calls. :)
> 
> I think you meant open_tree_attr here.

I'll wait for Aleksa to confirm before applying and amending.

> > +\&
> > +/* Create a new copy with the id-mapping cleared */
> > +memset(&attr, 0, sizeof(attr));
> > +attr.attr_clr = MOUNT_ATTR_IDMAP;
> > +mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > +                   &attr, sizeof(attr));
> 
> And here.
> 
> Otherwise your whole patchset looks good. Add to whole patchset:
> Reviewed-by: Askar Safin <safinaskar@gmail.com>

Thanks!  I'll retro-fit that to the commits I've aplied already too, as
I haven't pushed them to master yet.


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 7/8] man/man2/open_tree{,_attr}.2: document new open_tree_attr() API

[Aleksa Sarai]

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On 2025-10-01, Askar Safin <safinaskar@gmail.com> wrote:
> Aleksa Sarai <cyphar@cyphar.com>:
> > +mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > +                   &attr, sizeof(attr));
> 
> Your whole so-called "open_tree_attr example" doesn't contain any open_tree_attr
> calls. :)
> 
> I think you meant open_tree_attr here.

Oops.

> 
> > +\&
> > +/* Create a new copy with the id-mapping cleared */
> > +memset(&attr, 0, sizeof(attr));
> > +attr.attr_clr = MOUNT_ATTR_IDMAP;
> > +mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > +                   &attr, sizeof(attr));
> 
> And here.

Oops x2.

> Otherwise your whole patchset looks good. Add to whole patchset:
> Reviewed-by: Askar Safin <safinaskar@gmail.com>

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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Re: [PATCH v5 7/8] man/man2/open_tree{,_attr}.2: document new open_tree_attr() API

[Aleksa Sarai]

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On 2025-10-01, Alejandro Colomar <alx@kernel.org> wrote:
> Hi Askar,
> 
> On Wed, Oct 01, 2025 at 03:38:41AM +0300, Askar Safin wrote:
> > Aleksa Sarai <cyphar@cyphar.com>:
> > > +mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > > +                   &attr, sizeof(attr));
> > 
> > Your whole so-called "open_tree_attr example" doesn't contain any open_tree_attr
> > calls. :)
> > 
> > I think you meant open_tree_attr here.
> 
> I'll wait for Aleksa to confirm before applying and amending.

Yeah, Askar is right, they were a copy-paste snafu.

> > > +\&
> > > +/* Create a new copy with the id-mapping cleared */
> > > +memset(&attr, 0, sizeof(attr));
> > > +attr.attr_clr = MOUNT_ATTR_IDMAP;
> > > +mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > > +                   &attr, sizeof(attr));
> > 
> > And here.
> > 
> > Otherwise your whole patchset looks good. Add to whole patchset:
> > Reviewed-by: Askar Safin <safinaskar@gmail.com>
> 
> Thanks!  I'll retro-fit that to the commits I've aplied already too, as
> I haven't pushed them to master yet.
> 
> 
> Have a lovely day!
> Alex
> 
> -- 
> <https://www.alejandro-colomar.es>
> Use port 80 (that is, <...:80/>).



-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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Re: [PATCH v5 6/8] man/man2/open_tree.2: document "new" mount API

[Alejandro Colomar]

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Hi Aleksa,

On Thu, Sep 25, 2025 at 01:31:28AM +1000, Aleksa Sarai wrote:
> This is loosely based on the original documentation written by David
> Howells and later maintained by Christian Brauner, but has been
> rewritten to be more from a user perspective (as well as fixing a few
> critical mistakes).
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---

Patch applied.  Thanks!


Have a lovely night!
Alex

>  man/man2/open_tree.2 | 518 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 518 insertions(+)
> 
> diff --git a/man/man2/open_tree.2 b/man/man2/open_tree.2
> new file mode 100644
> index 0000000000000000000000000000000000000000..6b04a80927a8b6a394cf7ab341b8d6b29d42d304
> --- /dev/null
> +++ b/man/man2/open_tree.2
> @@ -0,0 +1,518 @@
> +.\" Copyright, the authors of the Linux man-pages project
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH open_tree 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +open_tree \- open path or create detached mount object and attach to fd
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ,\~ \-lc )
> +.SH SYNOPSIS
> +.nf
> +.BR "#define _GNU_SOURCE         " "/* See feature_test_macros(7) */"
> +.BR "#include <fcntl.h>" "          /* Definition of " AT_* " constants */"
> +.B #include <sys/mount.h>
> +.P
> +.BI "int open_tree(int " dirfd ", const char *" path ", unsigned int " flags );
> +.fi
> +.SH DESCRIPTION
> +The
> +.BR open_tree ()
> +system call is part of
> +the suite of file-descriptor-based mount facilities in Linux.
> +.IP \[bu] 3
> +If
> +.I flags
> +contains
> +.BR \%OPEN_TREE_CLONE ,
> +.BR open_tree ()
> +creates a detached mount object
> +which consists of a bind-mount of
> +the path specified by the
> +.IR path .
> +A new file descriptor
> +associated with the detached mount object
> +is then returned.
> +The mount object is equivalent to a bind-mount
> +that would be created by
> +.BR mount (2)
> +called with
> +.BR \%MS_BIND ,
> +except that it is tied to a file descriptor
> +and is not mounted onto the filesystem.
> +.IP
> +As with file descriptors returned from
> +.BR fsmount (2),
> +the resultant file descriptor can then be used with
> +.BR move_mount (2),
> +.BR mount_setattr (2),
> +or other such system calls to do further mount operations.
> +.IP
> +This mount object will be unmounted and destroyed
> +when the file descriptor is closed
> +if it was not otherwise attached to a mount point
> +by calling
> +.BR move_mount (2).
> +This implicit unmount operation is lazy\[em]\c
> +akin to calling
> +.BR umount2 (2)
> +with
> +.BR \%MNT_DETACH ;
> +thus,
> +any existing open references to files
> +from the mount object
> +will continue to work,
> +and the mount object will only be completely destroyed
> +once it ceases to be busy.
> +.IP \[bu]
> +If
> +.I flags
> +does not contain
> +.BR \%OPEN_TREE_CLONE ,
> +.BR open_tree ()
> +returns a file descriptor
> +that is exactly equivalent to
> +one produced by
> +.BR openat (2)
> +when called with the same
> +.I dirfd
> +and
> +.IR path .
> +.P
> +In either case, the resultant file descriptor
> +acts the same as one produced by
> +.BR open (2)
> +with
> +.BR O_PATH ,
> +meaning it can also be used as a
> +.I dirfd
> +argument to
> +"*at()" system calls.
> +However,
> +unlike
> +.BR open (2)
> +called with
> +.BR O_PATH ,
> +automounts will
> +by default
> +be triggered by
> +.BR open_tree ()
> +unless
> +.B \%AT_NO_AUTOMOUNT
> +is included in
> +.IR flags .
> +.P
> +As with "*at()" system calls,
> +.BR open_tree ()
> +uses the
> +.I dirfd
> +argument in conjunction with the
> +.I path
> +argument to determine the path to operate on, as follows:
> +.IP \[bu] 3
> +If the pathname given in
> +.I path
> +is absolute, then
> +.I dirfd
> +is ignored.
> +.IP \[bu]
> +If the pathname given in
> +.I path
> +is relative and
> +.I dirfd
> +is the special value
> +.BR \%AT_FDCWD ,
> +then
> +.I path
> +is interpreted relative to
> +the current working directory
> +of the calling process (like
> +.BR open (2)).
> +.IP \[bu]
> +If the pathname given in
> +.I path
> +is relative,
> +then it is interpreted relative to
> +the directory referred to by the file descriptor
> +.I dirfd
> +(rather than relative to
> +the current working directory
> +of the calling process,
> +as is done by
> +.BR open (2)
> +for a relative pathname).
> +In this case,
> +.I dirfd
> +must be a directory
> +that was opened for reading
> +.RB ( \%O_RDONLY )
> +or using the
> +.B O_PATH
> +flag.
> +.IP \[bu]
> +If
> +.I path
> +is an empty string,
> +and
> +.I flags
> +contains
> +.BR \%AT_EMPTY_PATH ,
> +then the file descriptor
> +.I dirfd
> +is operated on directly.
> +In this case,
> +.I dirfd
> +may refer to any type of file,
> +not just a directory.
> +.P
> +See
> +.BR openat (2)
> +for an explanation of why the
> +.I dirfd
> +argument is useful.
> +.P
> +.I flags
> +can be used to control aspects of the path lookup
> +and properties of the returned file descriptor.
> +A value for
> +.I flags
> +is constructed by bitwise ORing
> +zero or more of the following constants:
> +.RS
> +.TP
> +.B \%AT_EMPTY_PATH
> +If
> +.I path
> +is an empty string, operate on the file referred to by
> +.I dirfd
> +(which may have been obtained from
> +.BR open (2),
> +.BR fsmount (2),
> +or from another
> +.BR open_tree ()
> +call).
> +In this case,
> +.I dirfd
> +may refer to any type of file, not just a directory.
> +If
> +.I dirfd
> +is
> +.BR \%AT_FDCWD ,
> +.BR open_tree ()
> +will operate on the current working directory
> +of the calling process.
> +This flag is Linux-specific;
> +define
> +.B \%_GNU_SOURCE
> +to obtain its definition.
> +.TP
> +.B \%AT_NO_AUTOMOUNT
> +Do not automount the terminal ("basename") component of
> +.I path
> +if it is a directory that is an automount point.
> +This allows you to create a handle to the automount point itself,
> +rather than the location it would mount.
> +This flag has no effect if the mount point has already been mounted over.
> +This flag is Linux-specific;
> +define
> +.B \%_GNU_SOURCE
> +to obtain its definition.
> +.TP
> +.B \%AT_SYMLINK_NOFOLLOW
> +If
> +.I path
> +is a symbolic link, do not dereference it;
> +instead,
> +create either a handle to the link itself
> +or a bind-mount of it.
> +The resultant file descriptor is indistinguishable from one produced by
> +.BR openat (2)
> +with
> +.BR \%O_PATH | O_NOFOLLLOW .
> +.TP
> +.B \%OPEN_TREE_CLOEXEC
> +Set the close-on-exec
> +.RB ( FD_CLOEXEC )
> +flag on the new file descriptor.
> +See the description of the
> +.B O_CLOEXEC
> +flag in
> +.BR open (2)
> +for reasons why this may be useful.
> +.TP
> +.B \%OPEN_TREE_CLONE
> +Rather than creating an
> +.BR openat (2)-style
> +.B O_PATH
> +file descriptor,
> +create a bind-mount of
> +.I path
> +(akin to
> +.IR \%mount\~\-\-bind )
> +as a detached mount object.
> +In order to do this operation,
> +the calling process must have the
> +.B \%CAP_SYS_ADMIN
> +capability.
> +.TP
> +.B \%AT_RECURSIVE
> +Create a recursive bind-mount of the path
> +(akin to
> +.IR \%mount\~\-\-rbind )
> +as a detached mount object.
> +This flag is only permitted in conjunction with
> +.BR \%OPEN_TREE_CLONE .
> +.SH RETURN VALUE
> +On success, a new file descriptor is returned.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EACCES
> +Search permission is denied for one of the directories
> +in the path prefix of
> +.IR path .
> +(See also
> +.BR path_resolution (7).)
> +.TP
> +.B EBADF
> +.I path
> +is relative but
> +.I dirfd
> +is neither
> +.B \%AT_FDCWD
> +nor a valid file descriptor.
> +.TP
> +.B EFAULT
> +.I path
> +is NULL
> +or a pointer to a location
> +outside the calling process's accessible address space.
> +.TP
> +.B EINVAL
> +Invalid flag specified in
> +.IR flags .
> +.TP
> +.B ELOOP
> +Too many symbolic links encountered when resolving
> +.IR path .
> +.TP
> +.B EMFILE
> +The calling process has too many open files to create more.
> +.TP
> +.B ENAMETOOLONG
> +.I path
> +is longer than
> +.BR PATH_MAX .
> +.TP
> +.B ENFILE
> +The system has too many open files to create more.
> +.TP
> +.B ENOENT
> +A component of
> +.I path
> +does not exist, or is a dangling symbolic link.
> +.TP
> +.B ENOENT
> +.I path
> +is an empty string, but
> +.B AT_EMPTY_PATH
> +is not specified in
> +.IR flags .
> +.TP
> +.B ENOTDIR
> +A component of the path prefix of
> +.I path
> +is not a directory, or
> +.I path
> +is relative and
> +.I dirfd
> +is a file descriptor referring to a file other than a directory.
> +.TP
> +.B ENOSPC
> +The "anonymous" mount namespace
> +necessary to contain the
> +.B \%OPEN_TREE_CLONE
> +detached bind-mount mount object
> +could not be allocated,
> +as doing so would exceed
> +the configured per-user limit on
> +the number of mount namespaces in the current user namespace.
> +(See also
> +.BR namespaces (7).)
> +.TP
> +.B ENOMEM
> +The kernel could not allocate sufficient memory to complete the operation.
> +.TP
> +.B EPERM
> +.I flags
> +contains
> +.B \%OPEN_TREE_CLONE
> +but the calling process does not have the required
> +.B CAP_SYS_ADMIN
> +capability.
> +.SH STANDARDS
> +Linux.
> +.SH HISTORY
> +Linux 5.2.
> +.\" commit a07b20004793d8926f78d63eb5980559f7813404
> +.\" commit 400913252d09f9cfb8cce33daee43167921fc343
> +glibc 2.36.
> +.SH NOTES
> +.SS Mount propagation
> +The bind-mount mount objects created by
> +.BR open_tree ()
> +with
> +.B \%OPEN_TREE_CLONE
> +are not associated with
> +the mount namespace of the calling process.
> +Instead, each mount object is placed
> +in a newly allocated "anonymous" mount namespace
> +associated with the calling process.
> +.P
> +One of the side-effects of this is that
> +(unlike bind-mounts created with
> +.BR mount (2)),
> +mount propagation
> +(as described in
> +.BR mount_namespaces (7))
> +will not be applied to bind-mounts created by
> +.BR open_tree ()
> +until the bind-mount is attached with
> +.BR move_mount (2),
> +at which point the mount object
> +will be associated with the mount namespace
> +where it was attached
> +and mount propagation will resume.
> +Note that any mount propagation events that occurred
> +before the mount object was attached
> +will
> +.I not
> +be propagated to the mount object,
> +even after it is attached.
> +.SH EXAMPLES
> +The following examples show how
> +.BR open_tree ()
> +can be used in place of more traditional
> +.BR mount (2)
> +calls with
> +.BR MS_BIND .
> +.P
> +.in +4n
> +.EX
> +int srcfd = open_tree(AT_FDCWD, "/var", OPEN_TREE_CLONE);
> +move_mount(srcfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +First,
> +a detached bind-mount mount object of
> +.I /var
> +is created
> +and associated with the file descriptor
> +.IR srcfd .
> +Then, the mount object is attached to
> +.I /mnt
> +using
> +.BR move_mount (2)
> +with
> +.B \%MOVE_MOUNT_F_EMPTY_PATH
> +to request that the detached mount object
> +associated with the file descriptor
> +.I srcfd
> +be moved (and thus attached) to
> +.IR /mnt .
> +.P
> +The above procedure is functionally equivalent to
> +the following mount operation using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount("/var", "/mnt", NULL, MS_BIND, NULL);
> +.EE
> +.in
> +.P
> +.B \%OPEN_TREE_CLONE
> +can be combined with
> +.B \%AT_RECURSIVE
> +to create recursive detached bind-mount mount objects,
> +which in turn can be attached to mount points
> +to create recursive bind-mounts.
> +.P
> +.in +4n
> +.EX
> +int srcfd = open_tree(AT_FDCWD, "/var",
> +                      OPEN_TREE_CLONE | AT_RECURSIVE);
> +move_mount(srcfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +The above procedure is functionally equivalent to
> +the following mount operation using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount("/var", "/mnt", NULL, MS_BIND | MS_REC, NULL);
> +.EE
> +.in
> +.P
> +One of the primary benefits of using
> +.BR open_tree ()
> +and
> +.BR move_mount (2)
> +over the traditional
> +.BR mount (2)
> +is that operating with
> +.IR dirfd -style
> +file descriptors is far easier and more intuitive.
> +.P
> +.in +4n
> +.EX
> +int srcfd = open_tree(100, "", AT_EMPTY_PATH | OPEN_TREE_CLONE);
> +move_mount(srcfd, "", 200, "foo", MOVE_MOUNT_F_EMPTY_PATH);
> +.EE
> +.in
> +.P
> +The above procedure is roughly equivalent to
> +the following mount operation using
> +.BR mount (2):
> +.P
> +.in +4n
> +.EX
> +mount("/proc/self/fd/100",
> +      "/proc/self/fd/200/foo",
> +      NULL, MS_BIND, NULL);
> +.EE
> +.in
> +.P
> +In addition, you can use the file descriptor returned by
> +.BR open_tree ()
> +as the
> +.I dirfd
> +argument to any "*at()" system calls:
> +.P
> +.in +4n
> +.EX
> +int dirfd, fd;
> +\&
> +dirfd = open_tree(AT_FDCWD, "/etc", OPEN_TREE_CLONE);
> +fd = openat(dirfd, "passwd", O_RDONLY);
> +fchmodat(dirfd, "shadow", 0000, 0);
> +close(dirfd);
> +close(fd);
> +/* The bind-mount is now destroyed */
> +.EE
> +.in
> +.SH SEE ALSO
> +.BR fsconfig (2),
> +.BR fsmount (2),
> +.BR fsopen (2),
> +.BR fspick (2),
> +.BR mount (2),
> +.BR mount_setattr (2),
> +.BR move_mount (2),
> +.BR mount_namespaces (7)
> 
> -- 
> 2.51.0
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 7/8] man/man2/open_tree{,_attr}.2: document new open_tree_attr() API

[Alejandro Colomar]

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Hi Aleksa,

On Wed, Oct 01, 2025 at 05:35:45PM +1000, Aleksa Sarai wrote:
> On 2025-10-01, Askar Safin <safinaskar@gmail.com> wrote:
> > Aleksa Sarai <cyphar@cyphar.com>:
> > > +mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > > +                   &attr, sizeof(attr));
> > 
> > Your whole so-called "open_tree_attr example" doesn't contain any open_tree_attr
> > calls. :)
> > 
> > I think you meant open_tree_attr here.
> 
> Oops.
> 
> > 
> > > +\&
> > > +/* Create a new copy with the id-mapping cleared */
> > > +memset(&attr, 0, sizeof(attr));
> > > +attr.attr_clr = MOUNT_ATTR_IDMAP;
> > > +mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > > +                   &attr, sizeof(attr));
> > 
> > And here.
> 
> Oops x2.
> 
> > Otherwise your whole patchset looks good. Add to whole patchset:
> > Reviewed-by: Askar Safin <safinaskar@gmail.com>

I've applied the patch, with the following amendment:

	diff --git i/man/man2/open_tree.2 w/man/man2/open_tree.2
	index 8b48f3b78..f6f2fbecd 100644
	--- i/man/man2/open_tree.2
	+++ w/man/man2/open_tree.2
	@@ -683,14 +683,14 @@ .SS open_tree_attr()
	 .\" Using .attr_clr is not strictly necessary but makes the intent clearer.
	 attr.attr_set = MOUNT_ATTR_IDMAP;
	 attr.userns_fd = nsfd2;
	-mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
	-                   &attr, sizeof(attr));
	+mntfd2 = open_tree_attr(mntfd1, "", OPEN_TREE_CLONE,
	+                        &attr, sizeof(attr));
	 \&
	 /* Create a new copy with the id-mapping cleared */
	 memset(&attr, 0, sizeof(attr));
	 attr.attr_clr = MOUNT_ATTR_IDMAP;
	-mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
	-                   &attr, sizeof(attr));
	+mntfd3 = open_tree_attr(mntfd1, "", OPEN_TREE_CLONE,
	+                        &attr, sizeof(attr));
	 .EE
	 .in
	 .P


(Hopefully I got it right.)


Cheers,
Alex

> 
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> https://www.cyphar.com/



-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 0/8] man2: document "new" mount API

[Alejandro Colomar]

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Hi Aleksa, Askar,

On Thu, Sep 25, 2025 at 01:31:22AM +1000, Aleksa Sarai wrote:
> Back in 2019, the new mount API was merged[1]. David Howells then set
> about writing man pages for these new APIs, and sent some patches back
> in 2020[2].
> 

[...]

> 
> In addition, I have also included a man page for open_tree_attr(2) (as a
> subsection of the new open_tree(2) man page), which was merged in Linux
> 6.15.
> 
> [1]: https://lore.kernel.org/all/20190507204921.GL23075@ZenIV.linux.org.uk/
> [2]: https://lore.kernel.org/linux-man/159680892602.29015.6551860260436544999.stgit@warthog.procyon.org.uk/
> [3]: https://github.com/brauner/man-pages-md
> 
> Co-authored-by: David Howells <dhowells@redhat.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Co-authored-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

The full patch set has been merged now.  I've done a merge commit where
I've pasted this cover letter, and amended it so that Aleksa is the
author of the merge commit.  I've also included Askar's Reviewed-by tag
in the merge commit itself.

I'll have it in a separate branch for a few days, in case I need to fix
anything.  You can check it here:

<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?h=fs>

I editorialized the titles, but other than that, I didn't do much.
I think I mentioned most of the changes in replies to each patch.

Thanks a lot for your contributions!


Have a lovely night!
Alex

> ---
> Changes in v5:
> - `sed -i s|file descriptor based|file-descriptor-based|`.
>   [Alejandro Colomar]
> - fsconfig(2): use bullets instead of ordered list for workflow
>   description. [Alejandro Colomar]
> - mount_setattr(2): fix minor wording nit in new attribute-parameter
>   subsection.
> - fsopen(2): remove brackets around "message" for message retrieval
>   interface description. [Alejandro Colomar]
> - {move_mount,fspick}(2): fix remaining incorrect no-automount text.
>   [Askar Safin]
> - {fsmount,open_tree}(2): `sed -i s|MOUNT_DETACH|MNT_DETACH|g`.
>   [Askar Safin]
> - mount_setattr(2): fix copy-paste snafu in attribute-parameter
>   subsection. [Askar Safin]
> - *: clean `make -R build-catman-troff`. [Alejandro Colomar]
> - *: switch to \[em]\c where appropriate.
> - open_tree(2): clean up MNT_DETACH-on-close description and make it
>   slightly more prominent. [Alejandro Colomar]
> - open_tree(2): mention the distinction from open(O_PATH) with regards
>   to automounts. Askar suggested it be put in the section about
>   ~OPEN_TREE_CLONE, but the change in behaviour also applies to
>   OPEN_TREE_CLONE and it looked awkward to include it in the
>   dentry_open() case because O_PATH only gets mentioned in the following
>   paragraph (where I've put the text now). [Askar Safin]
> - {move_mount,open_tree{,_attr}}(2): fix column-width-related "make -R
>   check" failures.
> - *: fix remaining "make -R lint" failures.
> - open_tree_attr(2): add example using MOUNT_ATTR_IDMAP.
> - v4: <https://lore.kernel.org/r/20250919-new-mount-api-v4-0-1261201ab562@cyphar.com>
> 
> Changes in v4:
> - `sed -i s|\\% |\\%|g`.
> - Remove unneeded quotes in SYNOPSIS. [Alejandro Colomar]
> - open_tree(2): fix leftover confusing usages of "attach" when referring
>   to file descriptors being associated with mount objects.
> - open_tree(2): rename "Anonymous mount namespaces" NOTES subsection to
>   the far more informative "Mount propagation" and clean up the wording
>   a little.
> - open_tree_attr(2): add a code comment about
>   <https://lore.kernel.org/all/20250808-open_tree_attr-bugfix-idmap-v1-0-0ec7bc05646c@cyphar.com/>
> - {fsconfig,open_tree_attr}(2): use _Nullable.
> - {fsmount,open_tree}(2): mention the the unmount-on-close behaviour is
>   actually lazy (a-la MNT_DETACH).
> - {fsconfig,mount_setattr}(2): improve "mount attributes and filesystem
>   parameters" wording to make it clearer that superblock and mount flags
>   are sibling properties, not the same thing.
> - open_tree(2): mention that any mount propagation events while the
>   mount object is detached are completely lost -- i.e., they don't get
>   replayed once you attach the mount somewhere.
> - fsconfig(2): fix minor grammatical / missing joining word issues.
> - fsconfig(2): fix final leftover `.IR A " and " B` cases.
> - fsconfig(2): explain that failed fsconfig(FSCONFIG_CMD_*) operations
>   render the filesystem context invalid.
> - fsconfig(2): rework the description of superblock reuse, as the
>   previous text was very wrong. (Though there has been discussion about
>   changing this behaviour...)
> - fsconfig(2): remove misleading wording in FSCONFIG_CMD_CREATE_EXCL
>   about how we are requesting a new filesystem instance -- in theory
>   filesystems could take this request into account but in practice none
>   do (and it seems unlikely any ever will).
> - fsconfig(2): mention that key, value, and aux must be 0 or NULL for
>   FSCONFIG_CMD_RECONF.
> - fsmount(2): fix usage of "filesystem instance" in relation to
>   fsmount() and open_tree() comparison. [Askar Safin]
> - move_mount(2): "as attached" -> "as a detached" [Askar Safin]
> - fspick(2): add note about filesystem parameter list being copied
>   rather than reset with FSCONFIG_CMD_RECONFIGURE. [Askar Safin]
> - v3: <https://lore.kernel.org/r/20250809-new-mount-api-v3-0-f61405c80f34@cyphar.com>
> 
> Changes in v3:
> - `sed -i s|Co-developed-by|Co-authored-by|g`. [Alejandro Colomar]
>   - Add Signed-off-by for co-authors. [Christian Brauner]
> - `sed -i s|needs-mount|awaiting-mount|g`, to match the kernel parlance.
> - Fix VERSIONS/HISTORY mixup in mount_attr(2type) that was copied from
>   open_how(2type). [Alejandro Colomar]
> - Fix incorrect .BR usage in SYNOPSIS.
> - Some more semantic newlines fixes. [Alejandro Colomar]
> - Minor fixes suggested by Alejandro. [Alejandro Colomar]
> - open_tree_attr(2): heavily reword everything to be better formatted
>   and more explicit about its behaviour.
> - open_tree(2): write proper explanatory paragraphs for the EXAMPLES.
> - mount_setattr(2): fix stray doublequote in SYNOPSIS. [Askar Safin]
> - fsopen(2): rework structure of the DESCRIPTION introduction.
> - fsopen(2): explicitly say that read(2) errors in the message retrieval
>   interface are actual errors, not return 0. [Askar Safin]
> - fsopen(2): add BUGS section to describe the unfortunate -ENODATA
>   message dropping behaviour that should be fixed by
>   <https://lore.kernel.org/r/20250807-fscontext-log-cleanups-v3-0-8d91d6242dc3@cyphar.com/>.
> - fsconfig(2): add a NOTES subsection about generic filesystem
>   parameters.
> - fsconfig(2): add comment about the weirdness surrounding
>   FSCONFIG_SET_PATH.
> - {fspick,open_tree}(2): Correct AT_NO_AUTOMOUNT description (copied
>   from David, who probably copied it from statx(2)) -- AT_NO_AUTOMOUNT
>   applies to all path components, not just the final one. [Christian
>   Brauner]
> - statx(2): fix AT_NO_AUTOMOUNT documentation.
> - open_tree(2): swap open(2) reference for openat(2) when saying that
>   the result is identical. [Askar Safin]
> - fsmount(2): fix DESCRIPTION introduction, and rework attr_flags
>   description to better reference mount_setattr(2).
> - {fsopen,fspick,fsmount,open_tree}(2): don't use "attach" when talking
>   about the file descriptors we return that reference in-kernel objects,
>   to avoid confusing readers with mount object attachment status.
> - fsconfig(2): remove pidns argument example, as it was kind of unclear
>   and referenced kernel features not yet merged.
> - fsconfig(2): remove rambling FSCONFIG_SET_PATH_EMPTY text (which
>   mostly describes an academic issue that doesn't apply to any existing
>   filesystem), and instead add a CAVEATS section which touches on the
>   weird type behaviour of fsconfig(2).
> - v2: <https://lore.kernel.org/r/20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
> 
> Changes in v2:
> - `make -R lint-man`. [Alejandro Colomar]
> - `sed -i s|Glibc|glibc|g`. [Alejandro Colomar]
> - `sed -i s|pathname|path|g` [Alejandro Colomar]
> - Clean up macro usage, example code, and synopsis. [Alejandro Colomar]
> - Try to use semantic newlines. [Alejandro Colomar]
> - Make sure the usage of "filesystem context", "filesystem instance",
>   and "mount object" are consistent. [Askar Safin]
> - Avoid referring to these syscalls without an "at" suffix as "*at()
>   syscalls". [Askar Safin]
> - Use \% to avoid hyphenation of constants. [Askar Safin, G. Branden Robinson]
> - Add a new subsection to mount_setattr(2) to describe the distinction
>   between mount attributes and filesystem parameters.
> - (Under protest) double-space-after-period formatted commit messages.
> - v1: <https://lore.kernel.org/r/20250806-new-mount-api-v1-0-8678f56c6ee0@cyphar.com>
> 
> ---
> Aleksa Sarai (8):
>       man/man2/fsopen.2: document "new" mount API
>       man/man2/fspick.2: document "new" mount API
>       man/man2/fsconfig.2: document "new" mount API
>       man/man2/fsmount.2: document "new" mount API
>       man/man2/move_mount.2: document "new" mount API
>       man/man2/open_tree.2: document "new" mount API
>       man/man2/open_tree{,_attr}.2: document new open_tree_attr() API
>       man/man2/{fsconfig,mount_setattr}.2: add note about attribute-parameter distinction
> 
>  man/man2/fsconfig.2       | 741 ++++++++++++++++++++++++++++++++++++++++++++++
>  man/man2/fsmount.2        | 231 +++++++++++++++
>  man/man2/fsopen.2         | 385 ++++++++++++++++++++++++
>  man/man2/fspick.2         | 343 +++++++++++++++++++++
>  man/man2/mount_setattr.2  |  39 +++
>  man/man2/move_mount.2     | 646 ++++++++++++++++++++++++++++++++++++++++
>  man/man2/open_tree.2      | 709 ++++++++++++++++++++++++++++++++++++++++++++
>  man/man2/open_tree_attr.2 |   1 +
>  8 files changed, 3095 insertions(+)
> ---
> base-commit: f17990c243eafc1891ff692f90b6ce42e6449be8
> change-id: 20250802-new-mount-api-436db984f432
> 
> 
> Kind regards,
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> https://www.cyphar.com/
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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Re: [PATCH v5 7/8] man/man2/open_tree{,_attr}.2: document new open_tree_attr() API

[Aleksa Sarai]

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On 2025-10-01, Alejandro Colomar <alx@kernel.org> wrote:
> Hi Aleksa,
> 
> On Wed, Oct 01, 2025 at 05:35:45PM +1000, Aleksa Sarai wrote:
> > On 2025-10-01, Askar Safin <safinaskar@gmail.com> wrote:
> > > Aleksa Sarai <cyphar@cyphar.com>:
> > > > +mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > > > +                   &attr, sizeof(attr));
> > > 
> > > Your whole so-called "open_tree_attr example" doesn't contain any open_tree_attr
> > > calls. :)
> > > 
> > > I think you meant open_tree_attr here.
> > 
> > Oops.
> > 
> > > 
> > > > +\&
> > > > +/* Create a new copy with the id-mapping cleared */
> > > > +memset(&attr, 0, sizeof(attr));
> > > > +attr.attr_clr = MOUNT_ATTR_IDMAP;
> > > > +mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> > > > +                   &attr, sizeof(attr));
> > > 
> > > And here.
> > 
> > Oops x2.
> > 
> > > Otherwise your whole patchset looks good. Add to whole patchset:
> > > Reviewed-by: Askar Safin <safinaskar@gmail.com>
> 
> I've applied the patch, with the following amendment:
> 
> 	diff --git i/man/man2/open_tree.2 w/man/man2/open_tree.2
> 	index 8b48f3b78..f6f2fbecd 100644
> 	--- i/man/man2/open_tree.2
> 	+++ w/man/man2/open_tree.2
> 	@@ -683,14 +683,14 @@ .SS open_tree_attr()
> 	 .\" Using .attr_clr is not strictly necessary but makes the intent clearer.
> 	 attr.attr_set = MOUNT_ATTR_IDMAP;
> 	 attr.userns_fd = nsfd2;
> 	-mntfd2 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> 	-                   &attr, sizeof(attr));
> 	+mntfd2 = open_tree_attr(mntfd1, "", OPEN_TREE_CLONE,
> 	+                        &attr, sizeof(attr));
> 	 \&
> 	 /* Create a new copy with the id-mapping cleared */
> 	 memset(&attr, 0, sizeof(attr));
> 	 attr.attr_clr = MOUNT_ATTR_IDMAP;
> 	-mntfd3 = open_tree(mntfd1, "", OPEN_TREE_CLONE,
> 	-                   &attr, sizeof(attr));
> 	+mntfd3 = open_tree_attr(mntfd1, "", OPEN_TREE_CLONE,
> 	+                        &attr, sizeof(attr));
> 	 .EE
> 	 .in
> 	 .P
> 
> 
> (Hopefully I got it right.)

That looks correct -- thanks!

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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Re: [PATCH v5 0/8] man2: document "new" mount API

[Askar Safin]

Alejandro Colomar <alx@kernel.org>:
> The full patch set has been merged now.  I've done a merge commit where

Alejandro, I still don't see manpages for "new" mount API here:
https://man7.org/linux/man-pages/dir_section_2.html

Please, publish.

-- 
Askar Safin

Re: [PATCH v5 0/8] man2: document "new" mount API

[Alejandro Colomar]

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Hi Askar,

On Sun, Oct 26, 2025 at 03:27:42PM +0300, Askar Safin wrote:
> Alejandro Colomar <alx@kernel.org>:
> > The full patch set has been merged now.  I've done a merge commit where
> 
> Alejandro, I still don't see manpages for "new" mount API here:
> https://man7.org/linux/man-pages/dir_section_2.html

<man7.org> is not official.  It's Michael Kerrisk's (previous
maintainer) website.  He usually publishes new pages shortly-ish after
each new release, and I haven't issued a new release yet.

I have plans to release soon-ish, but have internet issues at home (the
cable in the street is broken, so I'm connecting on cell internet from
the laptop).  Hopefully, I'll be able to release this month.


Have a lovely day!
Alex

> 
> Please, publish.
> 
> -- 
> Askar Safin
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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