Patch "fs: introduce lock_rename_child() helper" has been added to the 6.1-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    fs: introduce lock_rename_child() helper

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     fs-introduce-lock_rename_child-helper.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 1713573bb43a2ac28f0643e58afa5730feba14b0
Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date:   Sun Dec 31 16:12:36 2023 +0900

    fs: introduce lock_rename_child() helper
    
    [ Upstream commit 9bc37e04823b5280dd0f22b6680fc23fe81ca325 ]
    
    Pass the dentry of a source file and the dentry of a destination directory
    to lock parent inodes for rename. As soon as this function returns,
    ->d_parent of the source file dentry is stable and inodes are properly
    locked for calling vfs-rename. This helper is needed for ksmbd server.
    rename request of SMB protocol has to rename an opened file, no matter
    which directory it's in.
    
    Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
    Signed-off-by: Namjae Jeon <linkinjeon@xxxxxxxxxx>
    Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/namei.c b/fs/namei.c
index 5e1c2ab2ae709..6daaf84567195 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2980,20 +2980,10 @@ static inline int may_create(struct user_namespace *mnt_userns,
 	return inode_permission(mnt_userns, dir, MAY_WRITE | MAY_EXEC);
 }
 
-/*
- * p1 and p2 should be directories on the same fs.
- */
-struct dentry *lock_rename(struct dentry *p1, struct dentry *p2)
+static struct dentry *lock_two_directories(struct dentry *p1, struct dentry *p2)
 {
 	struct dentry *p;
 
-	if (p1 == p2) {
-		inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
-		return NULL;
-	}
-
-	mutex_lock(&p1->d_sb->s_vfs_rename_mutex);
-
 	p = d_ancestor(p2, p1);
 	if (p) {
 		inode_lock_nested(p2->d_inode, I_MUTEX_PARENT);
@@ -3012,8 +3002,64 @@ struct dentry *lock_rename(struct dentry *p1, struct dentry *p2)
 			I_MUTEX_PARENT, I_MUTEX_PARENT2);
 	return NULL;
 }
+
+/*
+ * p1 and p2 should be directories on the same fs.
+ */
+struct dentry *lock_rename(struct dentry *p1, struct dentry *p2)
+{
+	if (p1 == p2) {
+		inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
+		return NULL;
+	}
+
+	mutex_lock(&p1->d_sb->s_vfs_rename_mutex);
+	return lock_two_directories(p1, p2);
+}
 EXPORT_SYMBOL(lock_rename);
 
+/*
+ * c1 and p2 should be on the same fs.
+ */
+struct dentry *lock_rename_child(struct dentry *c1, struct dentry *p2)
+{
+	if (READ_ONCE(c1->d_parent) == p2) {
+		/*
+		 * hopefully won't need to touch ->s_vfs_rename_mutex at all.
+		 */
+		inode_lock_nested(p2->d_inode, I_MUTEX_PARENT);
+		/*
+		 * now that p2 is locked, nobody can move in or out of it,
+		 * so the test below is safe.
+		 */
+		if (likely(c1->d_parent == p2))
+			return NULL;
+
+		/*
+		 * c1 got moved out of p2 while we'd been taking locks;
+		 * unlock and fall back to slow case.
+		 */
+		inode_unlock(p2->d_inode);
+	}
+
+	mutex_lock(&c1->d_sb->s_vfs_rename_mutex);
+	/*
+	 * nobody can move out of any directories on this fs.
+	 */
+	if (likely(c1->d_parent != p2))
+		return lock_two_directories(c1->d_parent, p2);
+
+	/*
+	 * c1 got moved into p2 while we were taking locks;
+	 * we need p2 locked and ->s_vfs_rename_mutex unlocked,
+	 * for consistency with lock_rename().
+	 */
+	inode_lock_nested(p2->d_inode, I_MUTEX_PARENT);
+	mutex_unlock(&c1->d_sb->s_vfs_rename_mutex);
+	return NULL;
+}
+EXPORT_SYMBOL(lock_rename_child);
+
 void unlock_rename(struct dentry *p1, struct dentry *p2)
 {
 	inode_unlock(p1->d_inode);
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 00fee52df8423..2b66021c740dd 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -81,6 +81,7 @@ extern int follow_down(struct path *);
 extern int follow_up(struct path *);
 
 extern struct dentry *lock_rename(struct dentry *, struct dentry *);
+extern struct dentry *lock_rename_child(struct dentry *, struct dentry *);
 extern void unlock_rename(struct dentry *, struct dentry *);
 
 extern int __must_check nd_jump_link(const struct path *path);