xfs: fix up xfs_swap_extent_forks inline extent handling

There have been several reports over the years of NULL pointer
dereferences in xfs_trans_log_inode during xfs_fsr processes,
when the process is doing an fput and tearing down extents
on the temporary inode, something like:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
PID: 29439  TASK: ffff880550584fa0  CPU: 6   COMMAND: "xfs_fsr"
    [exception RIP: xfs_trans_log_inode+0x10]
 raspberrypi#9 [ffff8800a57bbbe0] xfs_bunmapi at ffffffffa037398e [xfs]
raspberrypi#10 [ffff8800a57bbce8] xfs_itruncate_extents at ffffffffa0391b29 [xfs]
raspberrypi#11 [ffff8800a57bbd88] xfs_inactive_truncate at ffffffffa0391d0c [xfs]
raspberrypi#12 [ffff8800a57bbdb8] xfs_inactive at ffffffffa0392508 [xfs]
raspberrypi#13 [ffff8800a57bbdd8] xfs_fs_evict_inode at ffffffffa035907e [xfs]
raspberrypi#14 [ffff8800a57bbe00] evict at ffffffff811e1b67
raspberrypi#15 [ffff8800a57bbe28] iput at ffffffff811e23a5
raspberrypi#16 [ffff8800a57bbe58] dentry_kill at ffffffff811dcfc8
raspberrypi#17 [ffff8800a57bbe88] dput at ffffffff811dd06c
raspberrypi#18 [ffff8800a57bbea8] __fput at ffffffff811c823b
raspberrypi#19 [ffff8800a57bbef0] ____fput at ffffffff811c846e
raspberrypi#20 [ffff8800a57bbf00] task_work_run at ffffffff81093b27
raspberrypi#21 [ffff8800a57bbf30] do_notify_resume at ffffffff81013b0c
raspberrypi#22 [ffff8800a57bbf50] int_signal at ffffffff8161405d

As it turns out, this is because the i_itemp pointer, along
with the d_ops pointer, has been overwritten with zeros
when we tear down the extents during truncate.  When the in-core
inode fork on the temporary inode used by xfs_fsr was originally
set up during the extent swap, we mistakenly looked at di_nextents
to determine whether all extents fit inline, but this misses extents
generated by speculative preallocation; we should be using if_bytes
instead.

This mistake corrupts the in-memory inode, and code in
xfs_iext_remove_inline eventually gets bad inputs, causing
it to memmove and memset incorrect ranges; this became apparent
because the two values in ifp->if_u2.if_inline_ext[1] contained
what should have been in d_ops and i_itemp; they were memmoved due
to incorrect array indexing and then the original locations
were zeroed with memset, again due to an array overrun.

Fix this by properly using i_df.if_bytes to determine the number
of extents, not di_nextents.

Thanks to dchinner for looking at this with me and spotting the
root cause.

Cc: stable@vger.kernel.org
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_bmap_util.c

@@ -1792,6 +1792,7 @@ xfs_swap_extent_forks(
 	struct xfs_ifork	tempifp, *ifp, *tifp;
 	int			aforkblks = 0;
 	int			taforkblks = 0;
+	xfs_extnum_t		nextents;
 	__uint64_t		tmp;
 	int			error;
 
@@ -1881,7 +1882,8 @@ xfs_swap_extent_forks(
 		 * pointer.  Otherwise it's already NULL or
 		 * pointing to the extent.
 		 */
-		if (ip->i_d.di_nextents <= XFS_INLINE_EXTS) {
+		nextents = ip->i_df.if_bytes / (uint)sizeof(xfs_bmbt_rec_t);
+		if (nextents <= XFS_INLINE_EXTS) {
 			ifp->if_u1.if_extents =
 				ifp->if_u2.if_inline_ext;
 		}
@@ -1900,7 +1902,8 @@ xfs_swap_extent_forks(
 		 * pointer.  Otherwise it's already NULL or
 		 * pointing to the extent.
 		 */
-		if (tip->i_d.di_nextents <= XFS_INLINE_EXTS) {
+		nextents = tip->i_df.if_bytes / (uint)sizeof(xfs_bmbt_rec_t);
+		if (nextents <= XFS_INLINE_EXTS) {
 			tifp->if_u1.if_extents =
 				tifp->if_u2.if_inline_ext;
 		}