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

Alejandro Colomar
2025-09-25T10:13:33Z

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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
>
>

--
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