VFS: change kern_path_locked() and user_path_locked_at() to never return negative dentry

No callers of kern_path_locked() or user_path_locked_at() want a
negative dentry.  So change them to return -ENOENT instead.  This
simplifies callers.

This results in a subtle change to bcachefs in that an ioctl will now
return -ENOENT in preference to -EXDEV.  I believe this restores the
behaviour to what it was prior to
 Commit bbe6a7c ("bch2_ioctl_subvolume_destroy(): fix locking")

Signed-off-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20250217003020.3170652-2-neilb@suse.de
Acked-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: neilbrown

Documentation/filesystems/porting.rst

@@ -1157,3 +1157,11 @@ in normal case it points into the pathname being looked up.
 NOTE: if you need something like full path from the root of filesystem,
 you are still on your own - this assists with simple cases, but it's not
 magic.
+
+---
+
+** recommended**
+
+kern_path_locked() and user_path_locked() no longer return a negative
+dentry so this doesn't need to be checked.  If the name cannot be found,
+ERR_PTR(-ENOENT) is returned.

drivers/base/devtmpfs.c

@@ -245,15 +245,12 @@ static int dev_rmdir(const char *name)
 	dentry = kern_path_locked(name, &parent);
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
-	if (d_really_is_positive(dentry)) {
-		if (d_inode(dentry)->i_private == &thread)
-			err = vfs_rmdir(&nop_mnt_idmap, d_inode(parent.dentry),
-					dentry);
-		else
-			err = -EPERM;
-	} else {
-		err = -ENOENT;
-	}
+	if (d_inode(dentry)->i_private == &thread)
+		err = vfs_rmdir(&nop_mnt_idmap, d_inode(parent.dentry),
+				dentry);
+	else
+		err = -EPERM;
+
 	dput(dentry);
 	inode_unlock(d_inode(parent.dentry));
 	path_put(&parent);
@@ -310,39 +307,37 @@ static int handle_remove(const char *nodename, struct device *dev)
 {
 	struct path parent;
 	struct dentry *dentry;
+	struct kstat stat;
+	struct path p;
 	int deleted = 0;
 	int err;
 
 	dentry = kern_path_locked(nodename, &parent);
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 
-	if (d_really_is_positive(dentry)) {
-		struct kstat stat;
-		struct path p = {.mnt = parent.mnt, .dentry = dentry};
-		err = vfs_getattr(&p, &stat, STATX_TYPE | STATX_MODE,
-				  AT_STATX_SYNC_AS_STAT);
-		if (!err && dev_mynode(dev, d_inode(dentry), &stat)) {
-			struct iattr newattrs;
-			/*
-			 * before unlinking this node, reset permissions
-			 * of possible references like hardlinks
-			 */
-			newattrs.ia_uid = GLOBAL_ROOT_UID;
-			newattrs.ia_gid = GLOBAL_ROOT_GID;
-			newattrs.ia_mode = stat.mode & ~0777;
-			newattrs.ia_valid =
-				ATTR_UID|ATTR_GID|ATTR_MODE;
-			inode_lock(d_inode(dentry));
-			notify_change(&nop_mnt_idmap, dentry, &newattrs, NULL);
-			inode_unlock(d_inode(dentry));
-			err = vfs_unlink(&nop_mnt_idmap, d_inode(parent.dentry),
-					 dentry, NULL);
-			if (!err || err == -ENOENT)
-				deleted = 1;
-		}
-	} else {
-		err = -ENOENT;
+	p.mnt = parent.mnt;
+	p.dentry = dentry;
+	err = vfs_getattr(&p, &stat, STATX_TYPE | STATX_MODE,
+			  AT_STATX_SYNC_AS_STAT);
+	if (!err && dev_mynode(dev, d_inode(dentry), &stat)) {
+		struct iattr newattrs;
+		/*
+		 * before unlinking this node, reset permissions
+		 * of possible references like hardlinks
+		 */
+		newattrs.ia_uid = GLOBAL_ROOT_UID;
+		newattrs.ia_gid = GLOBAL_ROOT_GID;
+		newattrs.ia_mode = stat.mode & ~0777;
+		newattrs.ia_valid =
+			ATTR_UID|ATTR_GID|ATTR_MODE;
+		inode_lock(d_inode(dentry));
+		notify_change(&nop_mnt_idmap, dentry, &newattrs, NULL);
+		inode_unlock(d_inode(dentry));
+		err = vfs_unlink(&nop_mnt_idmap, d_inode(parent.dentry),
+				 dentry, NULL);
+		if (!err || err == -ENOENT)
+			deleted = 1;
 	}
 	dput(dentry);
 	inode_unlock(d_inode(parent.dentry));

fs/bcachefs/fs-ioctl.c

@@ -511,10 +511,6 @@ static long bch2_ioctl_subvolume_destroy(struct bch_fs *c, struct file *filp,
 		ret = -EXDEV;
 		goto err;
 	}
-	if (!d_is_positive(victim)) {
-		ret = -ENOENT;
-		goto err;
-	}
 	ret = __bch2_unlink(dir, victim, true);
 	if (!ret) {
 		fsnotify_rmdir(dir, victim);

fs/namei.c

@@ -2741,6 +2741,10 @@ static struct dentry *__kern_path_locked(int dfd, struct filename *name, struct
 	}
 	inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
 	d = lookup_one_qstr_excl(&last, path->dentry, 0);
+	if (!IS_ERR(d) && d_is_negative(d)) {
+		dput(d);
+		d = ERR_PTR(-ENOENT);
+	}
 	if (IS_ERR(d)) {
 		inode_unlock(path->dentry->d_inode);
 		path_put(path);

kernel/audit_watch.c

@@ -350,11 +350,10 @@ static int audit_get_nd(struct audit_watch *watch, struct path *parent)
 	struct dentry *d = kern_path_locked(watch->path, parent);
 	if (IS_ERR(d))
 		return PTR_ERR(d);
-	if (d_is_positive(d)) {
-		/* update watch filter fields */
-		watch->dev = d->d_sb->s_dev;
-		watch->ino = d_backing_inode(d)->i_ino;
-	}
+	/* update watch filter fields */
+	watch->dev = d->d_sb->s_dev;
+	watch->ino = d_backing_inode(d)->i_ino;
+
 	inode_unlock(d_backing_inode(parent->dentry));
 	dput(d);
 	return 0;
@@ -419,10 +418,11 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
 	/* caller expects mutex locked */
 	mutex_lock(&audit_filter_mutex);
 
-	if (ret) {
+	if (ret && ret != -ENOENT) {
 		audit_put_watch(watch);
 		return ret;
 	}
+	ret = 0;
 
 	/* either find an old parent or attach a new one */
 	parent = audit_find_parent(d_backing_inode(parent_path.dentry));