xfs: zero proper structure size for geometry calls

Commit 493f3358cb added this call to
xfs_fs_geometry() in order to avoid passing kernel stack data back
to user space:

+       memset(geo, 0, sizeof(*geo));

Unfortunately, one of the callers of that function passes the
address of a smaller data type, cast to fit the type that
xfs_fs_geometry() requires.  As a result, this can happen:

Kernel panic - not syncing: stack-protector: Kernel stack is corrupted
in: f87aca93

Pid: 262, comm: xfs_fsr Not tainted 2.6.38-rc6-493f3358cb2+ #1
Call Trace:

[<c12991ac>] ? panic+0x50/0x150
[<c102ed71>] ? __stack_chk_fail+0x10/0x18
[<f87aca93>] ? xfs_ioc_fsgeometry_v1+0x56/0x5d [xfs]

Fix this by fixing that one caller to pass the right type and then
copy out the subset it is interested in.

Note: This patch is an alternative to one originally proposed by
Eric Sandeen.

Reported-by: Jeffrey Hundstad <jeffrey.hundstad@mnsu.edu>
Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Tested-by: Jeffrey Hundstad <jeffrey.hundstad@mnsu.edu>

fs/xfs/linux-2.6/xfs_ioctl.c

@@ -695,14 +695,19 @@ xfs_ioc_fsgeometry_v1(
 	xfs_mount_t		*mp,
 	void			__user *arg)
 {
-	xfs_fsop_geom_v1_t	fsgeo;
+	xfs_fsop_geom_t         fsgeo;
 	int			error;
 
-	error = xfs_fs_geometry(mp, (xfs_fsop_geom_t *)&fsgeo, 3);
+	error = xfs_fs_geometry(mp, &fsgeo, 3);
 	if (error)
 		return -error;
 
-	if (copy_to_user(arg, &fsgeo, sizeof(fsgeo)))
+	/*
+	 * Caller should have passed an argument of type
+	 * xfs_fsop_geom_v1_t.  This is a proper subset of the
+	 * xfs_fsop_geom_t that xfs_fs_geometry() fills in.
+	 */
+	if (copy_to_user(arg, &fsgeo, sizeof(xfs_fsop_geom_v1_t)))
 		return -XFS_ERROR(EFAULT);
 	return 0;
 }