cifs: guard against hardlinking directories

When we made serverino the default, we trusted that the field sent by the
server in the "uniqueid" field was actually unique. It turns out that it
isn't reliably so.

Samba, in particular, will just put the st_ino in the uniqueid field when
unix extensions are enabled. When a share spans multiple filesystems, it's
quite possible that there will be collisions. This is a server bug, but
when the inodes in question are a directory (as is often the case) and
there is a collision with the root inode of the mount, the result is a
kernel panic on umount.

Fix this by checking explicitly for directory inodes with the same
uniqueid. If that is the case, then we can assume that using server inode
numbers will be a problem and that they should be disabled.

Fixes Samba bugzilla 7407

Signed-off-by: Jeff Layton <jlayton@redhat.com>
CC: Stable <stable@kernel.org>
Reviewed-and-Tested-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsglob.h

@@ -502,6 +502,7 @@ struct dfs_info3_param {
 #define CIFS_FATTR_DFS_REFERRAL		0x1
 #define CIFS_FATTR_DELETE_PENDING	0x2
 #define CIFS_FATTR_NEED_REVAL		0x4
+#define CIFS_FATTR_INO_COLLISION	0x8
 
 struct cifs_fattr {
 	u32		cf_flags;

fs/cifs/inode.c

@@ -715,6 +715,16 @@ cifs_find_inode(struct inode *inode, void *opaque)
 	if (CIFS_I(inode)->uniqueid != fattr->cf_uniqueid)
 		return 0;
 
+	/*
+	 * uh oh -- it's a directory. We can't use it since hardlinked dirs are
+	 * verboten. Disable serverino and return it as if it were found, the
+	 * caller can discard it, generate a uniqueid and retry the find
+	 */
+	if (S_ISDIR(inode->i_mode) && !list_empty(&inode->i_dentry)) {
+		fattr->cf_flags |= CIFS_FATTR_INO_COLLISION;
+		cifs_autodisable_serverino(CIFS_SB(inode->i_sb));
+	}
+
 	return 1;
 }
 
@@ -734,15 +744,22 @@ cifs_iget(struct super_block *sb, struct cifs_fattr *fattr)
 	unsigned long hash;
 	struct inode *inode;
 
+retry_iget5_locked:
 	cFYI(1, ("looking for uniqueid=%llu", fattr->cf_uniqueid));
 
 	/* hash down to 32-bits on 32-bit arch */
 	hash = cifs_uniqueid_to_ino_t(fattr->cf_uniqueid);
 
 	inode = iget5_locked(sb, hash, cifs_find_inode, cifs_init_inode, fattr);
-
-	/* we have fattrs in hand, update the inode */
 	if (inode) {
+		/* was there a problematic inode number collision? */
+		if (fattr->cf_flags & CIFS_FATTR_INO_COLLISION) {
+			iput(inode);
+			fattr->cf_uniqueid = iunique(sb, ROOT_I);
+			fattr->cf_flags &= ~CIFS_FATTR_INO_COLLISION;
+			goto retry_iget5_locked;
+		}
+
 		cifs_fattr_to_inode(inode, fattr);
 		if (sb->s_flags & MS_NOATIME)
 			inode->i_flags |= S_NOATIME | S_NOCMTIME;