Holding namespace_sem is enough to make sure that result remains valid.
It is *not* enough to avoid false negatives from __lookup_mnt(). Mounts
can be unhashed outside of namespace_sem (stuck children getting detached
on final mntput() of lazy-umounted mount) and having an unrelated mount
removed from the hash chain while we traverse it may end up with false
negative from __lookup_mnt(). We need to sample and recheck the seqlock
component of mount_lock...
Bug predates the introduction of path_overmount() - it had come from
the code in finish_automount() that got abstracted into that helper.
Fixes: 26df6034fdb2 ("fix automount/automount race properly")
Fixes: 6ac392815628 ("fs: allow to mount beneath top mount")
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
fs/namespace.c | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index a33553bc12d0..1722deadfb88 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3478,18 +3478,25 @@ static int do_set_group(struct path *from_path, struct path *to_path)
* Check if path is overmounted, i.e., if there's a mount on top of
* @path->mnt with @path->dentry as mountpoint.
*
- * Context: This function expects namespace_lock() to be held.
+ * Context: namespace_sem must be held at least shared.
+ * MUST NOT be called under lock_mount_hash() (there one should just
+ * call __lookup_mnt() and check if it returns NULL).
* Return: If path is overmounted true is returned, false if not.
*/
static inline bool path_overmounted(const struct path *path)
{
+ unsigned seq = read_seqbegin(&mount_lock);
+ bool no_child;
+
rcu_read_lock();
- if (unlikely(__lookup_mnt(path->mnt, path->dentry))) {
- rcu_read_unlock();
- return true;
- }
+ no_child = !__lookup_mnt(path->mnt, path->dentry);
rcu_read_unlock();
- return false;
+ if (need_seqretry(&mount_lock, seq)) {
+ read_seqlock_excl(&mount_lock);
+ no_child = !__lookup_mnt(path->mnt, path->dentry);
+ read_sequnlock_excl(&mount_lock);
+ }
+ return unlikely(!no_child);
}
/**
--
2.39.5
