VFS/nfsd/cachefiles/ovl: introduce start_removing() and end_removing()

start_removing() is similar to start_creating() but will only return a
positive dentry with the expectation that it will be removed.  This is
used by nfsd, cachefiles, and overlayfs.  They are changed to also use
end_removing() to terminate the action begun by start_removing().  This
is a simple alias for end_dirop().

Apart from changes to the error paths, as we no longer need to unlock on
a lookup error, an effect on callers is that they don't need to test if
the found dentry is positive or negative - they can be sure it is
positive.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-6-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 14 insertions, 18 deletions

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index 0a136eb434da..c7f0c6ab9b88 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -260,6 +260,7 @@ static int cachefiles_unlink(struct cachefiles_cache *cache,
  * - File backed objects are unlinked
  * - Directory backed objects are stuffed into the graveyard for userspace to
  *   delete
+ * On entry dir must be locked.  It will be unlocked on exit.
  */
 int cachefiles_bury_object(struct cachefiles_cache *cache,
 			   struct cachefiles_object *object,
@@ -274,28 +275,30 @@ int cachefiles_bury_object(struct cachefiles_cache *cache,
 
 	_enter(",'%pd','%pd'", dir, rep);
 
+	/* end_removing() will dput() @rep but we need to keep
+	 * a ref, so take one now.  This also stops the dentry
+	 * being negated when unlinked which we need.
+	 */
+	dget(rep);
+
 	if (rep->d_parent != dir) {
-		inode_unlock(d_inode(dir));
+		end_removing(rep);
 		_leave(" = -ESTALE");
 		return -ESTALE;
 	}
 
 	/* non-directories can just be unlinked */
 	if (!d_is_dir(rep)) {
-		dget(rep); /* Stop the dentry being negated if it's only pinned
-			    * by a file struct.
-			    */
 		ret = cachefiles_unlink(cache, object, dir, rep, why);
-		dput(rep);
+		end_removing(rep);
 
-		inode_unlock(d_inode(dir));
 		_leave(" = %d", ret);
 		return ret;
 	}
 
 	/* directories have to be moved to the graveyard */
 	_debug("move stale object to graveyard");
-	inode_unlock(d_inode(dir));
+	end_removing(rep);
 
 try_again:
 	/* first step is to make up a grave dentry in the graveyard */
@@ -749,26 +752,20 @@ static struct dentry *cachefiles_lookup_for_cull(struct cachefiles_cache *cache,
 	struct dentry *victim;
 	int ret = -ENOENT;
 
-	inode_lock_nested(d_inode(dir), I_MUTEX_PARENT);
+	victim = start_removing(&nop_mnt_idmap, dir, &QSTR(filename));
 
-	victim = lookup_one(&nop_mnt_idmap, &QSTR(filename), dir);
 	if (IS_ERR(victim))
 		goto lookup_error;
-	if (d_is_negative(victim))
-		goto lookup_put;
 	if (d_inode(victim)->i_flags & S_KERNEL_FILE)
 		goto lookup_busy;
 	return victim;
 
 lookup_busy:
 	ret = -EBUSY;
-lookup_put:
-	inode_unlock(d_inode(dir));
-	dput(victim);
+	end_removing(victim);
 	return ERR_PTR(ret);
 
 lookup_error:
-	inode_unlock(d_inode(dir));
 	ret = PTR_ERR(victim);
 	if (ret == -ENOENT)
 		return ERR_PTR(-ESTALE); /* Probably got retired by the netfs */
@@ -816,18 +813,17 @@ int cachefiles_cull(struct cachefiles_cache *cache, struct dentry *dir,
 
 	ret = cachefiles_bury_object(cache, NULL, dir, victim,
 				     FSCACHE_OBJECT_WAS_CULLED);
+	dput(victim);
 	if (ret < 0)
 		goto error;
 
 	fscache_count_culled();
-	dput(victim);
 	_leave(" = 0");
 	return 0;
 
 error_unlock:
-	inode_unlock(d_inode(dir));
+	end_removing(victim);
 error:
-	dput(victim);
 	if (ret == -ENOENT)
 		return -ESTALE; /* Probably got retired by the netfs */