eventfs: Do not allow NULL parent to eventfs_start_creating()

The eventfs directory is dynamically created via the meta data supplied by
the existing trace events. All files and directories in eventfs has a
parent. Do not allow NULL to be passed into eventfs_start_creating() as
the parent because that should never happen. Warn if it does.

Link: https://lkml.kernel.org/r/20231121231112.693841807@goodmis.org

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/inode.c

@@ -509,20 +509,15 @@ struct dentry *eventfs_start_creating(const char *name, struct dentry *parent)
 	struct dentry *dentry;
 	int error;
 
+	/* Must always have a parent. */
+	if (WARN_ON_ONCE(!parent))
+		return ERR_PTR(-EINVAL);
+
 	error = simple_pin_fs(&trace_fs_type, &tracefs_mount,
 			      &tracefs_mount_count);
 	if (error)
 		return ERR_PTR(error);
 
-	/*
-	 * If the parent is not specified, we create it in the root.
-	 * We need the root dentry to do this, which is in the super
-	 * block. A pointer to that is in the struct vfsmount that we
-	 * have around.
-	 */
-	if (!parent)
-		parent = tracefs_mount->mnt_root;
-
 	if (unlikely(IS_DEADDIR(parent->d_inode)))
 		dentry = ERR_PTR(-ENOENT);
 	else