VFS: change vfs_mkdir() to unlock on failure.

vfs_mkdir() already drops the reference to the dentry on failure but it
leaves the parent locked.
This complicates end_creating() which needs to unlock the parent even
though the dentry is no longer available.

If we change vfs_mkdir() to unlock on failure as well as releasing the
dentry, we can remove the "parent" arg from end_creating() and simplify
the rules for calling it.

Note that cachefiles_get_directory() can choose to substitute an error
instead of actually calling vfs_mkdir(), for fault injection.  In that
case it needs to call end_creating(), just as vfs_mkdir() now does on
error.

ovl_create_real() will now unlock on error.  So the conditional
end_creating() after the call is removed, and end_creating() is called
internally on error.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-15-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 9 insertions, 19 deletions

diff --git a/include/linux/namei.h b/include/linux/namei.h
index 208aed1d6728..0ef73d739a31 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -105,34 +105,24 @@ struct dentry *start_creating_dentry(struct dentry *parent,
 struct dentry *start_removing_dentry(struct dentry *parent,
 				     struct dentry *child);
 
-/**
- * end_creating - finish action started with start_creating
- * @child:  dentry returned by start_creating() or vfs_mkdir()
- * @parent: dentry given to start_creating(),
- *
- * Unlock and release the child.
+/* end_creating - finish action started with start_creating
+ * @child: dentry returned by start_creating() or vfs_mkdir()
  *
- * Unlike end_dirop() this can only be called if start_creating() succeeded.
- * It handles @child being and error as vfs_mkdir() might have converted the
- * dentry to an error - in that case the parent still needs to be unlocked.
+ * Unlock and release the child. This can be called after
+ * start_creating() whether that function succeeded or not,
+ * but it is not needed on failure.
  *
  * If vfs_mkdir() was called then the value returned from that function
  * should be given for @child rather than the original dentry, as vfs_mkdir()
- * may have provided a new dentry.  Even if vfs_mkdir() returns an error
- * it must be given to end_creating().
+ * may have provided a new dentry.
+ *
  *
  * If vfs_mkdir() was not called, then @child will be a valid dentry and
  * @parent will be ignored.
  */
-static inline void end_creating(struct dentry *child, struct dentry *parent)
+static inline void end_creating(struct dentry *child)
 {
-	if (IS_ERR(child))
-		/* The parent is still locked despite the error from
-		 * vfs_mkdir() - must unlock it.
-		 */
-		inode_unlock(parent->d_inode);
-	else
-		end_dirop(child);
+	end_dirop(child);
 }
 
 /**