[PATCH] check for null vfsmount in dentry_open()

Make sure no-one calls dentry_open with a NULL vfsmount argument and crap
out with a stacktrace otherwise. A NULL file->f_vfsmnt has always been
problematic, but with the per-mount r/o tracking we can't accept anymore
at all.

[AV] the last place that passed NULL had been eliminated by the previous
patch (reiserfs xattr stuff)

Acked-by: Al Viro <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/open.c

···
 	int error;
 	struct file *f;
 
+	/*
+	 * We must always pass in a valid mount pointer.   Historically
+	 * callers got away with not passing it, but we must enforce this at
+	 * the earliest possible point now to avoid strange problems deep in the
+	 * filesystem stack.
+	 */
+	if (!mnt) {
+		printk(KERN_WARNING "%s called with NULL vfsmount\n", __func__);
+		dump_stack();
+		return ERR_PTR(-EINVAL);
+	}
+
 	error = -ENFILE;
 	f = get_empty_filp();
 	if (f == NULL) {