xfs: don't crash the vfs on a garbage inline symlink

The VFS routine that calls ->get_link blindly copies whatever's returned
into the user's buffer.  If we return a NULL pointer, the vfs will
crash on the null pointer.  Therefore, return -EFSCORRUPTED instead of
blowing up the kernel.

[dgc: clean up with hch's suggestions]

Reported-by: wen.xu@gatech.edu
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_iops.c

@@ -471,8 +471,18 @@ xfs_vn_get_link_inline(
 	struct inode		*inode,
 	struct delayed_call	*done)
 {
+	char			*link;
+
 	ASSERT(XFS_I(inode)->i_df.if_flags & XFS_IFINLINE);
-	return XFS_I(inode)->i_df.if_u1.if_data;
+
+	/*
+	 * The VFS crashes on a NULL pointer, so return -EFSCORRUPTED if
+	 * if_data is junk.
+	 */
+	link = XFS_I(inode)->i_df.if_u1.if_data;
+	if (!link)
+		return ERR_PTR(-EFSCORRUPTED);
+	return link;
 }
 
 STATIC int