namespace: fix proc mount iteration

The m->index isn't updated when m->show() overflows and retains its
value before the current mount causing a restart to start at the same
value. If that happens in short order to due a quickly expanding mount
table this would cause the same mount to be shown again and again.

Ensure that *pos always equals the mount id of the mount that was
returned by start/next. On restart after overflow mnt_find_id_at(*pos)
finds the exact mount. This should avoid duplicates, avoid skips and
should handle concurrent modification just fine.

Cc: <stable@vger.kernel.org>
Fixed: 2eea9ce ("mounts: keep list of mounts in an rbtree")
Link: https://patch.msgid.link/20260129-geleckt-treuhand-4bb940acacd9@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/namespace.c

@@ -1531,23 +1531,33 @@ static struct mount *mnt_find_id_at_reverse(struct mnt_namespace *ns, u64 mnt_id
 static void *m_start(struct seq_file *m, loff_t *pos)
 {
 	struct proc_mounts *p = m->private;
+	struct mount *mnt;
 
 	down_read(&namespace_sem);
 
-	return mnt_find_id_at(p->ns, *pos);
+	mnt = mnt_find_id_at(p->ns, *pos);
+	if (mnt)
+		*pos = mnt->mnt_id_unique;
+	return mnt;
 }
 
 static void *m_next(struct seq_file *m, void *v, loff_t *pos)
 {
-	struct mount *next = NULL, *mnt = v;
+	struct mount *mnt = v;
 	struct rb_node *node = rb_next(&mnt->mnt_node);
 
-	++*pos;
 	if (node) {
-		next = node_to_mount(node);
+		struct mount *next = node_to_mount(node);
 		*pos = next->mnt_id_unique;
+		return next;
 	}
-	return next;
+
+	/*
+	 * No more mounts. Set pos past current mount's ID so that if
+	 * iteration restarts, mnt_find_id_at() returns NULL.
+	 */
+	*pos = mnt->mnt_id_unique + 1;
+	return NULL;
 }
 
 static void m_stop(struct seq_file *m, void *v)