From: Dave Hansen <haveblue@us.ibm.com>

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.

Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/open.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff -puN fs/open.c~check-for-null-vfsmount-in-dentry_open fs/open.c
--- a/fs/open.c~check-for-null-vfsmount-in-dentry_open
+++ a/fs/open.c
@@ -903,6 +903,18 @@ struct file *dentry_open(struct dentry *
 	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) {
_