[PATCH 00/38] VFS: Introduce filesystem context [ver #10]

David Howells
2018-07-27T17:31:31Z

Hi Al,

[!] NOTE: This is a preview of the patches; Apparmor is currently broken and
needs fixing.

Here are a set of patches to create a filesystem context prior to setting
up a new mount, populating it with the parsed options/binary data, creating
the superblock and then effecting the mount. This is also used for remount
since much of the parsing stuff is common in many filesystems.

This allows namespaces and other information to be conveyed through the
mount procedure.

This also allows Miklós Szeredi's idea of doing:

fd = fsopen("nfs");
fsconfig(fd, fsconfig_set_string, "option", "val", 0);
fsconfig(fd, fsconfig_cmd_create, NULL, NULL, 0);
mfd = fsmount(fd, MS_NODEV);
move_mount(mfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);

that he presented at LSF-2017 to be implemented (see the relevant patches
in the series).

I didn't use netlink as that would make the core kernel depend on
CONFIG_NET and CONFIG_NETLINK and would introduce network namespacing
issues.

I've implemented filesystem context handling for procfs, nfs, mqueue,
cpuset, kernfs, sysfs, cgroup and afs filesystems.

Unconverted filesystems are handled by a legacy filesystem wrapper.

====================
WHY DO WE WANT THIS?
====================

Firstly, there's a bunch of problems with the mount(2) syscall:

(1) It's actually six or seven different interfaces rolled into one and weird
combinations of flags make it do different things beyond the original
specification of the syscall.

(2) It produces a particularly large and diverse set of errors, which have to
be mapped back to a small error code. Yes, there's dmesg - if you have
it configured - but you can't necessarily see that if you're doing a
mount inside of a container.

(3) It copies a PAGE_SIZE block of data for each of the type, device name and
options.

(4) The size of the buffers is PAGE_SIZE - and this is arch dependent.

(5) You can't mount into another mount namespace. I could, for example,
build a container without having to be in that container's namespace if I
can do it from outside.

(6) It's not really geared for the specification of multiple sources, but
some filesystems really want that - overlayfs, for example.

and some problems in the internal kernel api:

(1) There's no defined way to supply namespace configuration for the
superblock - so, for instance, I can't say that I want to create a
superblock in a particular network namespace (on automount, say).

NFS hacks around this by creating multiple shadow file_system_types with
different ->mount() ops.

(2) When calling mount internally, unless you have NFS-like hacks, you have
to generate or otherwise provide text config data which then gets parsed,
when some of the time you could bypass the parsing stage entirely.

(3) The amount of data in the data buffer is not known, but the data buffer
might be on a kernel stack somewhere, leading to the possibility of
tripping the stack underrun guard.

and other issues too:

(1) Superblock remount in some filesystems applies options on an as-parsed
basis, so if there's a parse failure, a partial alteration with no
rollback is effected.

(2) Under some circumstances, the mount data may get copied multiple times so
that it can have multiple parsers applied to it or because it has to be
parsed multiple times - for instance, once to get the preliminary info
required to access the on-disk superblock and then again to update the
superblock record in the kernel.

I want to be able to add support for a bunch of things:

(1) UID, GID and Project ID mapping/translation. I want to be able to
install a translation table of some sort on the superblock to translate
source identifiers (which may be foreign numeric UIDs/GIDs, text names,
GUIDs) into system identifiers. This needs to be done before the
superblock is published[*].

Note that this may, for example, involve using the context and the
superblock held therein to issue an RPC to a server to look up
translations.

[*] By "published" I mean made available through mount so that other
userspace processes can access it by path.

Maybe specifying a translation range element with something like:

fsconfig(fd, fsconfig_translate_uid, "<srcuid> <nsuid> <count>", 0, 0);

The translation information also needs to propagate over an automount in
some circumstances.

(2) Namespace configuration. I want to be able to tell the superblock
creation process what namespaces should be applied when it created (in
particular the userns and netns) for containerisation purposes, e.g.:

fsconfig(fd, fsconfig_set_namespace, "user", 0, userns_fd);
fsconfig(fd, fsconfig_set_namespace, "net", 0, netns_fd);

(3) Namespace propagation. I want to have a properly defined mechanism for
propagating namespace configuration over automounts within the kernel.
This will be particularly useful for network filesystems.

(4) Pre-mount attribute query. A chunk of the changes is actually the
fsinfo() syscall to query attributes of the filesystem beyond what's
available in statx() and statfs(). This will allow a created superblock
to be queried before it is published.

(5) Upcall for configuration. I would like to be able to query configuration
that's stored in userspace when an automount is made. For instance, to
look up network parameters for NFS or to find a cache selector for
fscache.

The internal fs_context could be passed to the upcall process or the
kernel could read a config file directly if named appropriately for the
superblock, perhaps:

[/etc/fscontext.d/afs/example.com/cell.cfg]
realm = EXAMPLE.COM
translation = uid,3000,4000,100
fscache = tag=fred

(6) Event notifications. I want to be able to install a watch on a
superblock before it is published to catch things like quota events and
EIO.

(7) Large and binary parameters. There might be at some point a need to pass
large/binary objects like Microsoft PACs around. If I understand PACs
correctly, you can obtain these from the Kerberos server and then pass
them to the file server when you connect.

Having it possible to pass large or binary objects as individual fsconfig
calls make parsing these trivial. OTOH, some or all of this can
potentially be handled with the use of the keyrings interface - as the afs
filesystem does for passing kerberos tokens around; it's just that that
seems overkill for a parameter you may only need once.

===================
SIGNIFICANT CHANGES
===================

ver #10:

(*) Renamed "option" to "parameter" in a number of places.

(*) Replaced the use of write() to drive the configuration with an fsconfig()
syscall. This also allows at-style paths and fds to be presented as typed
object.

(*) Routed the key=value parameter concept all the way through from the
fsconfig() system call to the LSM and filesystem.

(*) Added a parameter-description concept and helper functions to help
interpret a parameter and possibly convert the value.

(*) Made it possible to query the parameter description using the fsinfo()
syscall. Added a test-fs-query sample to dump the parameters used by a
filesystem.

ver #9:

(*) Dropped the fd cookie stuff and the FMODE_*/O_* split stuff.

(*) Al added an open_tree() system call to allow a mount tree to be picked
referenced or cloned into an O_PATH-style fd. This can then be used
with sys_move_mount(). Dropped the O_CLONE_MOUNT and O_NON_RECURSIVE
open() flags.

(*) Brought error logging back in, though only in the fs_context and not
in the task_struct.

(*) Separated MS_REMOUNT|MS_BIND handling from MS_REMOUNT handling.

(*) Used anon_inodes for the fd returned by fsopen() and fspick(). This
requires making it unconditional.

(*) Fixed lots of bugs. Especial thanks to Al and Eric Biggers for
finding them and providing patches.

(*) Wrote manual pages, which I'll post separately.

ver #8:

(*) Changed the way fsmount() mounts into the namespace according to some
of Al's ideas.

(*) Put better typing on the fd cookie obtained from __fdget() & co..

(*) Stored the fd cookie in struct nameidata rather than the dfd number.

(*) Changed sys_fsmount() to return an O_PATH-style fd rather than
actually mounting into the mount namespace.

(*) Separated internal FMODE_* handling from O_* handling to free up
certain O_* flag numbers.

(*) Added two new open flags (O_CLONE_MOUNT and O_NON_RECURSIVE) for use
with open(O_PATH) to copy a mount or mount-subtree to an O_PATH fd.

(*) Added a new syscall, sys_move_mount(), to move a mount from an
dfd+path source to a dfd+path destination.

(*) Added a file->f_mode flag (FMODE_NEED_UNMOUNT) that indicates that the
vfsmount attached to file->f_path needs 'unmounting' if set.

(*) Made sys_move_mount() clear FMODE_NEED_UNMOUNT if successful.

[!] This doesn't work quite right.

(*) Added a new syscall, fsinfo(), to query information about a
filesystem. The idea being that this will, in future, work with the
fd from fsopen() too and permit querying of the parameters and
metadata before fsmount() is called.

ver #7:

(*) Undo an incorrect MS_* -> SB_* conversion.

(*) Pass the mount data buffer size to all the mount-related functions that
take the data pointer. This fixes a problem where someone (say SELinux)
tries to copy the mount data, assuming it to be a page in size, and
overruns the buffer - thereby incurring an oops by hitting a guard page.

(*) Made the AFS filesystem use them as an example. This is a much easier to
deal with than with NFS or Ext4 as there are very few mount options.

ver #6:

(*) Dropped the supplementary error string facility for the moment.

(*) Dropped the NFS patches for the moment.

(*) Dropped the reserved file descriptor argument from fsopen() and
replaced it with three reserved pointers that must be NULL.

ver #5:

(*) Renamed sb_config -> fs_context and adjusted variable names.

(*) Differentiated the flags in sb->s_flags (now named SB_*) from those
passed to mount(2) (named MS_*).

(*) Renamed __vfs_new_fs_context() to vfs_new_fs_context() and made the
caller always provide a struct file_system_type pointer and the
parameters required.

(*) Got rid of vfs_submount_fc() in favour of passing
FS_CONTEXT_FOR_SUBMOUNT to vfs_new_fs_context(). The purpose is now
used more.

(*) Call ->validate() on the remount path.

(*) Got rid of the inode locking in sys_fsmount().

(*) Call security_sb_mountpoint() in the mount(2) path.

ver #4:

(*) Split the sb_config patch up somewhat.

(*) Made the supplementary error string facility something attached to the
task_struct rather than the sb_config so that error messages can be
obtained from NFS doing a mount-root-and-pathwalk inside the
nfs_get_tree() operation.

Further, made this managed and read by prctl rather than through the
mount fd so that it's more generally available.

ver #3:

(*) Rebased on 4.12-rc1.

(*) Split the NFS patch up somewhat.

ver #2:

(*) Removed the ->fill_super() from sb_config_operations and passed it in
directly to functions that want to call it. NFS now calls
nfs_fill_super() directly rather than jumping through a pointer to it
since there's only the one option at the moment.

(*) Removed ->mnt_ns and ->sb from sb_config and moved ->pid_ns into
proc_sb_config.

(*) Renamed create_super -> get_tree.

(*) Renamed struct mount_context to struct sb_config and amended various
variable names.

(*) sys_fsmount() acquired AT_* flags and MS_* flags (for MNT_* flags)
arguments.

ver #1:

(*) Split the sb_config stuff out into its own header.

(*) Support non-context aware filesystems through a special set of
sb_config operations.

(*) Stored the created superblock and root dentry into the sb_config after
creation rather than directly into a vfsmount. This allows some
arguments to be removed to various NFS functions.

(*) Added an explicit superblock-creation step. This allows a created
superblock to then be mounted multiple times.

(*) Added a flag to say that the sb_config is degraded and cannot have
another go at having a superblock creation whilst getting rid of the
one that says it's already mounted.

Possible further developments:

(*) Implement sb reconfiguration (for now it returns ENOANO).

(*) Implement mount context support in more filesystems, ext4 being next
on my list.

(*) Move the walk-from-root stuff that nfs has to generic code so that you
can do something akin to:

mount /dev/sda1:/foo/bar /mnt

See nfs_follow_remote_path() and mount_subtree(). This is slightly
tricky in NFS as we have to prevent referral loops.

(*) Work out how to get at the error message incurred by submounts
encountered during nfs_follow_remote_path().

Should the error message be moved to task_struct and made more
general, perhaps retrieved with a prctl() function?

(*) Clean up/consolidate the security functions. Possibly add a
validation hook to be called at the same time as the mount context
validate op.

The patches can be found here also:

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git

on branch:

mount-api

David
---
Al Viro (2):
vfs: syscall: Add open_tree(2) to reference or clone a mount
teach move_mount(2) to work with OPEN_TREE_CLONE

David Howells (36):
vfs: syscall: Add move_mount(2) to move mounts around
vfs: Suppress MS_* flag defs within the kernel unless explicitly enabled
vfs: Introduce the basic header for the new mount API's filesystem context
vfs: Introduce logging functions
vfs: Add configuration parser helpers
vfs: Add LSM hooks for the new mount API
selinux: Implement the new mount API LSM hooks
smack: Implement filesystem context security hooks
apparmor: Implement security hooks for the new mount API
vfs: Pass key and value into LSM and FS and provide a helper parser
tomoyo: Implement security hooks for the new mount API
vfs: Separate changing mount flags full remount
vfs: Implement a filesystem superblock creation/configuration context
vfs: Remove unused code after filesystem context changes
procfs: Move proc_fill_super() to fs/proc/root.c
proc: Add fs_context support to procfs
ipc: Convert mqueue fs to fs_context
cpuset: Use fs_context
kernfs, sysfs, cgroup, intel_rdt: Support fs_context
hugetlbfs: Convert to fs_context
vfs: Remove kern_mount_data()
vfs: Provide documentation for new mount API
Make anon_inodes unconditional
vfs: syscall: Add fsopen() to prepare for superblock creation
vfs: Implement logging through fs_context
vfs: Add some logging to the core users of the fs_context log
vfs: syscall: Add fsconfig() for configuring and managing a context
vfs: syscall: Add fsmount() to create a mount for a superblock
vfs: syscall: Add fspick() to select a superblock for reconfiguration
afs: Add fs_context support
afs: Use fs_context to pass parameters over automount
vfs: syscall: Add fsinfo() to query filesystem information
afs: Add fsinfo support
vfs: Add a sample program for the new mount API
vfs: Allow fsinfo() to query what's in an fs_context
vfs: Allow fsinfo() to be used to query an fs parameter description

Documentation/filesystems/mount_api.txt | 706 ++++++++++++++++++++++++
arch/arc/kernel/setup.c | 1
arch/arm/kernel/atags_parse.c | 1
arch/sh/kernel/setup.c | 1
arch/sparc/kernel/setup_32.c | 1
arch/sparc/kernel/setup_64.c | 1
arch/x86/entry/syscalls/syscall_32.tbl | 7
arch/x86/entry/syscalls/syscall_64.tbl | 7
arch/x86/kernel/cpu/intel_rdt.h | 15 +
arch/x86/kernel/cpu/intel_rdt_rdtgroup.c | 184 ++++--
arch/x86/kernel/setup.c | 1
drivers/base/devtmpfs.c | 1
fs/Kconfig | 7
fs/Makefile | 5
fs/afs/internal.h | 9
fs/afs/mntpt.c | 148 +++--
fs/afs/super.c | 605 ++++++++++++++-------
fs/afs/volume.c | 4
fs/f2fs/super.c | 2
fs/file_table.c | 9
fs/filesystems.c | 4
fs/fs_context.c | 778 +++++++++++++++++++++++++++
fs/fs_parser.c | 476 ++++++++++++++++
fs/fsopen.c | 490 +++++++++++++++++
fs/hugetlbfs/inode.c | 392 ++++++++------
fs/internal.h | 13
fs/kernfs/mount.c | 89 +--
fs/libfs.c | 19 +
fs/namei.c | 4
fs/namespace.c | 866 +++++++++++++++++++++++-------
fs/pnode.c | 1
fs/proc/inode.c | 51 --
fs/proc/internal.h | 6
fs/proc/root.c | 245 ++++++--
fs/statfs.c | 574 ++++++++++++++++++++
fs/super.c | 368 ++++++++++---
fs/sysfs/mount.c | 67 ++
include/linux/cgroup.h | 3
include/linux/fs.h | 25 +
include/linux/fs_context.h | 207 +++++++
include/linux/fs_parser.h | 116 ++++
include/linux/fsinfo.h | 40 +
include/linux/kernfs.h | 39 +
include/linux/lsm_hooks.h | 70 ++
include/linux/module.h | 6
include/linux/mount.h | 5
include/linux/security.h | 61 ++
include/linux/syscalls.h | 13
include/uapi/linux/fcntl.h | 2
include/uapi/linux/fs.h | 82 +--
include/uapi/linux/fsinfo.h | 301 ++++++++++
include/uapi/linux/mount.h | 75 +++
init/Kconfig | 10
init/do_mounts.c | 1
init/do_mounts_initrd.c | 1
ipc/mqueue.c | 121 +++-
kernel/cgroup/cgroup-internal.h | 50 +-
kernel/cgroup/cgroup-v1.c | 347 +++++++-----
kernel/cgroup/cgroup.c | 256 ++++++---
kernel/cgroup/cpuset.c | 68 ++
samples/Kconfig | 7
samples/Makefile | 2
samples/mount_api/Makefile | 7
samples/mount_api/test-fsmount.c | 118 ++++
samples/statx/Makefile | 7
samples/statx/test-fs-query.c | 137 +++++
samples/statx/test-fsinfo.c | 539 +++++++++++++++++++
security/apparmor/include/mount.h | 11
security/apparmor/lsm.c | 106 ++++
security/apparmor/mount.c | 47 ++
security/security.c | 51 ++
security/selinux/hooks.c | 311 ++++++++++-
security/smack/smack.h | 11
security/smack/smack_lsm.c | 370 ++++++++++++-
security/tomoyo/common.h | 3
security/tomoyo/mount.c | 46 ++
security/tomoyo/tomoyo.c | 15 +
77 files changed, 8375 insertions(+), 1470 deletions(-)
create mode 100644 Documentation/filesystems/mount_api.txt
create mode 100644 fs/fs_context.c
create mode 100644 fs/fs_parser.c
create mode 100644 fs/fsopen.c
create mode 100644 include/linux/fs_context.h
create mode 100644 include/linux/fs_parser.h
create mode 100644 include/linux/fsinfo.h
create mode 100644 include/uapi/linux/fsinfo.h
create mode 100644 include/uapi/linux/mount.h
create mode 100644 samples/mount_api/Makefile
create mode 100644 samples/mount_api/test-fsmount.c
create mode 100644 samples/statx/test-fs-query.c
create mode 100644 samples/statx/test-fsinfo.c