From: Al Viro <viro@zeniv.linux.org.uk>
Date: Wed, 20 Aug 2025 07:23:12 +0000 (-0400)
Subject: new helper: topmost_overmount()
X-Git-Tag: v6.18-rc1~128^2~31
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=25423edc787842d17520b3f9df4d0a58a6a663b1;p=thirdparty%2Flinux.git

new helper: topmost_overmount()

Returns the final (topmost) mount in the chain of overmounts
starting at given mount.  Same locking rules as for any mount
tree traversal - either the spinlock side of mount_lock, or
rcu + sample the seqcount side of mount_lock before the call
and recheck afterwards.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---

diff --git a/fs/mount.h b/fs/mount.h
index ed8c83ba836a7..04d0eadc4c106 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -235,4 +235,11 @@ static inline void mnt_notify_add(struct mount *m)
 }
 #endif
 
+static inline struct mount *topmost_overmount(struct mount *m)
+{
+	while (m->overmount)
+		m = m->overmount;
+	return m;
+}
+
 struct mnt_namespace *mnt_ns_from_dentry(struct dentry *dentry);
diff --git a/fs/namespace.c b/fs/namespace.c
index b48bfb46b351d..b6983adaa73b7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2696,10 +2696,9 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 				 child->mnt_mountpoint);
 		commit_tree(child);
 		if (q) {
+			struct mount *r = topmost_overmount(child);
 			struct mountpoint *mp = root.mp;
-			struct mount *r = child;
-			while (unlikely(r->overmount))
-				r = r->overmount;
+
 			if (unlikely(shorter) && child != source_mnt)
 				mp = shorter;
 			mnt_change_mountpoint(r, mp, q);
@@ -6168,9 +6167,7 @@ bool current_chrooted(void)
 
 	guard(mount_locked_reader)();
 
-	root = current->nsproxy->mnt_ns->root;
-	while (unlikely(root->overmount))
-		root = root->overmount;
+	root = topmost_overmount(current->nsproxy->mnt_ns->root);
 
 	return fs_root.mnt != &root->mnt || !path_mounted(&fs_root);
 }