Please consider pulling these changes from the signed vfs-7.4-rc1.idmap tag.
Thanks!
Christian
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Merge tag 'vfs-7.4-rc1.idmap' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs
Pull vfs idmap constification from Christian Brauner:
"This makes struct mnt_idmap const throughout the tree.
A mount's idmapping is immutable. The only thing modified after
alloc_mnt_idmap() is the reference count and that is hidden behind
mnt_idmap_get() and mnt_idmap_put(). Everything else only reads from
it. That is the same model struct cred uses and the idmapping is
sensitive enough to deserve it.
The conversion goes bottom up through the vfs{g,u}id and fs{g,u}id
helpers, the privilege checks, acl, xattr and quota and then every
inode operation that takes an idmap, so ->permission(), ->getattr(),
->setattr(), ->create(), ->mkdir(), ->mknod(), ->symlink(),
->rename(), ->tmpfile(), ->get_acl(), ->set_acl() and ->fileattr_set()
with their implementations in every filesystem, plus the LSM hooks.
struct vfsmount, struct mount_kattr and struct kstatmount store a
const pointer and nop_mnt_idmap and invalid_mnt_idmap live in rodata,
so anything that tries to modify them faults instead of silently
corrupting the initial idmapping"
* tag 'vfs-7.4-rc1.idmap' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (27 commits)
fs: make nop_mnt_idmap and invalid_mnt_idmap const
fs: port mnt_idmap() and file_mnt_idmap() to const mnt_idmap
fs: port vfs_*() helpers to const mnt_idmap
fs: port ->setattr() to pass const mnt_idmap
fs: port ->getattr() to pass const mnt_idmap
fs: port ->create() to pass const mnt_idmap
fs: port ->symlink() to pass const mnt_idmap
fs: port ->mkdir() to pass const mnt_idmap
fs: port ->rename() to pass const mnt_idmap
fs: port ->mknod() to pass const mnt_idmap
fs: port ->tmpfile() to pass const mnt_idmap
fs: port ->get_acl() to pass const mnt_idmap
fs: port ->set_acl() to pass const mnt_idmap
fs: port ->fileattr_set() to pass const mnt_idmap
fs: port xattr to const mnt_idmap
fs: port ->permission() to pass const mnt_idmap
fs: port acl to const mnt_idmap
fs: port inode_init_owner() to const mnt_idmap
fs: port inode_owner_or_capable() to const mnt_idmap
fs: port privilege checking helpers to const mnt_idmap
...
Add nullfs_new_file() to allocate an empty immutable regular file on a
nullfs instance as a dentry of its own. It is never hashed under the
root and so can't be found by lookup. Reads return nothing, changes are
refused, file locks, leases and delegations are refused as.
Link: https://patch.msgid.link/20261002-work-mount-cover-v1-1-232a8f52b43c@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
iterate_dir() takes the directory's i_rwsem shared and holds it across
->iterate_shared(). For a directory that never has an entry and is
never removed the lock keeps nothing still, it only orders every reader
and every writer of that inode behind each other.
For the directory of a nullfs instance that matters. The instance of
the initial mount namespace is the root of every empty mount namespace
and the private instance is the root of every kernel thread, so one
inode is shared across users who have nothing else in common. And a
reader can hold the lock for as long as it likes: back the getdents()
buffer with a mapping of a file on a FUSE mount of your own, let the
copy of "." and ".." fault and let the server wait. Queue an exclusive
taker behind it, a mkdir() in that directory goes through start_dirop()
before the read-only mount is reported, and from then on every lookup
that misses the dcache in that directory, every create and every mount
on it waits until the server answers. One user of an empty mount
namespace stalls all the others.
Add FOP_IMMUTABLE for the file operations of a directory that never
changes and is never removed and let iterate_dir() skip the lock for
it. The flag never changes for a file, ->f_pos is protected by
f_pos_lock since directories are FMODE_ATOMIC_POS, IS_DEADDIR can't be
set on such a directory and neither touch_atime() nor fsnotify take
i_rwsem. Set it on the nullfs directory. The placeholder directories of
libfs never have an entry either but their owners remove them, so they
keep the lock.
Fixes: 9d4e752a24 ("namespace: allow creating empty mount namespaces")
Cc: stable@vger.kernel.org # v7.1+
Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-19-dd44b89d44ce@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Refuse flock() and POSIX locks on nullfs. Its one inode is the root of
every kernel thread and the following patches make it the directory
that stands in for an unmounted mount, so a lock taken through one such
directory would block the locks of every other holder and F_GETLK would
tell them the pid of the holder. Give the directory file operations of
its own: what libfs gives an empty directory plus ->lock and ->flock
that fail with ENOLCK, the way a filesystem without lock support does.
Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-17-dd44b89d44ce@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Don't let nullfs be watched. fanotify refuses mount and filesystem marks
on SB_NOUSER superblocks but inode marks of inotify, fanotify and
dnotify go through. The one inode of knullfs is the root of every
kernel thread and the following patches make it reachable from
userspace as the directory that stands in for an unmounted mount. A
watch placed through one such directory would report the opens through
all the others, across users.
Add FS_DISALLOW_NOTIFY next to FS_DISALLOW_NOTIFY_PERM, refuse a mark on
any object of such a filesystem in fsnotify_add_mark_list() where every
backend ends up and set it for nullfs. There's nothing to watch on a
permanently empty and immutable filesystem.
Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-16-dd44b89d44ce@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Please consider pulling these changes from the signed vfs-7.3-rc1.misc tag.
Thanks!
Christian
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Merge tag 'vfs-7.3-rc1.misc' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs
Pull misc vfs updates from Christian Brauner:
"Bigger cleanups:
- The lockref dead-count handling is tidied up.
The open-coded check for a count below zero as the dead marker
relies on information the caller should not have.
- make put_mnt_ns() leave mounts connected. Destroying a mount
namespace disconnected its mounts from their mount points. So a
file descriptor still open on the parent of a mount point could be
used to peek under it.
Locked mounts were already kept connected to prevent exactly that.
But a mount is only locked when its tree is copied across a user
namespace boundary. So a mount namespace set up by a privileged
component had no locked mounts and its mounts were disconnected.
Passing UMOUNT_CONNECTED keeps every mount connected and prevents
that bug.
- vfs_prepare_mode() passes S_IFDIR for directories. I meant to fix
that ago but didn't get to it. So now someone finally did it.
This kills the exception where the mode could be 0 when a directory
was created whereas every other creation operation passed it
explicitly already.
- move long delayed work for ufs, jffs2, hfsplus, hfs and affs from
the per-cpu system_long_wq to the new unbound system_dfl_long_wq.
None of that work relies on per-cpu state and the work item is
enqueued with queue_delayed_work() whose timer is global anyway. So
it may as well benefit from scheduler task placement.
Smaller fixes and cleanups:
- unlock_buffer() and journal_end_buffer_io_sync() use
clear_and_wake_up_bit()
- the pipe page pools are unified into a single per-pipe pool and the
extra wake_up(rd_wait) is limited to EPOLLET consumers
- eventpoll now computes its timer slack lazily in ep_poll()
- shrink_dcache_for_umount() keeps making progress on busy roots
- excess xarray nodes are freed in clear_inode()
- romfs detects hard link cycles
- the user path of nested backing files is fixed
- pidfd holds exec_update_lock around the namespace ioctl
- non-memcg-aware nr_cached_objects is skipped during memcg slab
shrink
- iomap_write_iter() always returns status
- mangle_path() is renamed to seq_mangle_path()
- inode timestamp accessors are annotated
- new regression test for pipe->poll_usage.
- a few documentation, kernel-doc and selftest fixes"
* tag 'vfs-7.3-rc1.misc' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (67 commits)
selftests/namespaces: Fix racy pipe handshake in timens and pidns_separate
selftests/epoll: add a regression test for pipe->poll_usage
pipe: only enable the extra wake_up(rd_wait) for EPOLLET consumers
pidfd: hold exec_update_lock around namespace ioctl
fs: fix user path of nested backing files
fs: remove stale inode_insert5() kernel-doc parameter
fs: fix switch/case indentation in sysfs() syscall
fs: document semantics of kstat::{uid,gid} fields
dcache: keep shrink_dcache_for_umount() making progress on busy roots
seq_file: rename mangle_path to seq_mangle_path
nstree: add/fix struct ns_id_req kernel-doc member fields
dcache: use lockref routines for dead count checks
lockref: tidy up dead count handling
initramfs: fix typo in reserve_initrd_mem comment
fs/pipe: unify the page pools into a single per-pipe pool
fs: annotate inode timestamp accessors
eventpoll: compute timer slack lazily in ep_poll()
selftests/filesystems: add mntns cleanup test
put_mnt_ns(): leave mounts connected
affs: Move long delayed work on system_dfl_long_wq
...
Allow multiple instances of nullfs to be created. Right now we're only
going to use it for kernel-internal purposes but ultimately we can allow
userspace to use it too to e.g., safely overmount stuff.
Link: https://patch.msgid.link/20260601-work-kthread-nullfs-v4-22-77ee053060e0@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
The nullfs_fs_type is declared in mount.h but when declared
in nullfs.c there is a warning as mount.h is not being
included.
Add include of "mount.h" to remove the following sparse warning:
fs/nullfs.c:66:25: warning: symbol 'nullfs_fs_type' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://patch.msgid.link/20260617104721.900914-1-ben.dooks@codethink.co.uk
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Currently pivot_root() doesn't work on the real rootfs because it
cannot be unmounted. Userspace has to do a recursive removal of the
initramfs contents manually before continuing the boot.
Really all we want from the real rootfs is to serve as the parent mount
for anything that is actually useful such as the tmpfs or ramfs for
initramfs unpacking or the rootfs itself. There's no need for the real
rootfs to actually be anything meaningful or useful. Add a immutable
rootfs called "nullfs" that can be selected via the "nullfs_rootfs"
kernel command line option.
The kernel will mount a tmpfs/ramfs on top of it, unpack the initramfs
and fire up userspace which mounts the rootfs and can then just do:
chdir(rootfs);
pivot_root(".", ".");
umount2(".", MNT_DETACH);
and be done with it. (Ofc, userspace can also choose to retain the
initramfs contents by using something like pivot_root(".", "/initramfs")
without unmounting it.)
Technically this also means that the rootfs mount in unprivileged
namespaces doesn't need to become MNT_LOCKED anymore as it's guaranteed
that the immutable rootfs remains permanently empty so there cannot be
anything revealed by unmounting the covering mount.
In the future this will also allow us to create completely empty mount
namespaces without risking to leak anything.
systemd already handles this all correctly as it tries to pivot_root()
first and falls back to MS_MOVE only when that fails.
This goes back to various discussion in previous years and a LPC 2024
presentation about this very topic.
Link: https://patch.msgid.link/20260112-work-immutable-rootfs-v2-3-88dd1c34a204@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
