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fs: notice when init abandons fs sharing

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PID 1 may choose to stop sharing fs_struct state with us. Either via
unshare(CLONE_FS) or unshare(CLONE_NEWNS). Of course, PID 1 could have
chosen to create arbitrary process trees that all share fs_struct state
via CLONE_FS. This is a strong statement: We only care about PID 1 aka
the thread-group leader so subthread's fs_struct state doesn't matter.

PID 1 unsharing fs_struct state is a bug. PID 1 relies on various
kthreads to be able to perform work based on its fs_struct state.
Breaking that contract sucks for both sides. So just don't bother with
extra work for this. No sane init system should ever do this.

Link: https://patch.msgid.link/20260601-work-kthread-nullfs-v4-2-77ee053060e0@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-06-01 15:56:35 +02:00
parent 21bcea3ef2
commit d114eb8577
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1 changed files with 25 additions and 0 deletions
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25
fs/fs_struct.c
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@ -147,6 +147,30 @@ int unshare_fs_struct(void)
}
EXPORT_SYMBOL_GPL(unshare_fs_struct);
/*
* PID 1 may choose to stop sharing fs_struct state with us.
* Either via unshare(CLONE_FS) or unshare(CLONE_NEWNS). Of
* course, PID 1 could have chosen to create arbitrary process
* trees that all share fs_struct state via CLONE_FS. This is a
* strong statement: We only care about PID 1 aka the thread-group
* leader so subthread's fs_struct state doesn't matter.
*
* PID 1 unsharing fs_struct state is a bug. PID 1 relies on
* various kthreads to be able to perform work based on its
* fs_struct state. Breaking that contract sucks for both sides.
* So just don't bother with extra work for this. No sane init
* system should ever do this.
*/
static inline void validate_fs_switch(struct fs_struct *old_fs)
{
if (likely(current->pid != 1))
return;
/* @old_fs may be dangling but for comparison it's fine */
if (old_fs != &init_fs)
return;
pr_warn("VFS: Pid 1 stopped sharing filesystem state\n");
}
struct fs_struct *switch_fs_struct(struct fs_struct *new_fs)
{
struct fs_struct *fs;
@ -162,6 +186,7 @@ struct fs_struct *switch_fs_struct(struct fs_struct *new_fs)
read_sequnlock_excl(&fs->seq);
}
validate_fs_switch(fs);
return new_fs;
}