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new predicate: mount_is_ancestor()

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mount_is_ancestor(p1, p2) returns true iff there is a possibly
empty ancestry chain from p1 to p2.

Convert the open-coded checks.  Unlike those open-coded variants
it does not depend upon p1 not being root...

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2025-06-08 23:10:33 -04:00
parent 592238c03e
commit 9cb79ed60e
1 changed files with 15 additions and 6 deletions
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21
fs/namespace.c
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@ -3483,6 +3483,17 @@ static inline bool path_overmounted(const struct path *path)
return unlikely(!no_child);
}
/*
* Check if there is a possibly empty chain of descent from p1 to p2.
* Locks: namespace_sem (shared) or mount_lock (read_seqlock_excl).
*/
static bool mount_is_ancestor(const struct mount *p1, const struct mount *p2)
{
while (p2 != p1 && mnt_has_parent(p2))
p2 = p2->mnt_parent;
return p2 == p1;
}
/**
* can_move_mount_beneath - check that we can mount beneath the top mount
* @from: mount to mount beneath
@ -3534,9 +3545,8 @@ static int can_move_mount_beneath(const struct path *from,
if (parent_mnt_to == current->nsproxy->mnt_ns->root)
return -EINVAL;
for (struct mount *p = mnt_from; mnt_has_parent(p); p = p->mnt_parent)
if (p == mnt_to)
return -EINVAL;
if (mount_is_ancestor(mnt_to, mnt_from))
return -EINVAL;
/*
* If the parent mount propagates to the child mount this would
@ -3705,9 +3715,8 @@ static int do_move_mount(struct path *old_path,
err = -ELOOP;
if (!check_for_nsfs_mounts(old))
goto out;
for (; mnt_has_parent(p); p = p->mnt_parent)
if (p == old)
goto out;
if (mount_is_ancestor(old, p))
goto out;
err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp, flags);
if (err)