[PATCH v2] xfs: fix forkoff miscalculation related to XFS_LITINO(mp)

[Gao Xiang <hsiangkao@redhat.com>]

Currently, commit e9e2eae89ddb dropped a (int) decoration from
XFS_LITINO(mp), and since sizeof() expression is also involved,
the result of XFS_LITINO(mp) is simply as the size_t type
(commonly unsigned long).

Considering the expression in xfs_attr_shortform_bytesfit():
  offset = (XFS_LITINO(mp) - bytes) >> 3;
let "bytes" be (int)340, and
    "XFS_LITINO(mp)" be (unsigned long)336.

on 64-bit platform, the expression is
  offset = ((unsigned long)336 - (int)340) >> 3 =
           (int)(0xfffffffffffffffcUL >> 3) = -1

but on 32-bit platform, the expression is
  offset = ((unsigned long)336 - (int)340) >> 3 =
           (int)(0xfffffffcUL >> 3) = 0x1fffffff
instead.

so offset becomes a large positive number on 32-bit platform, and
cause xfs_attr_shortform_bytesfit() returns maxforkoff rather than 0.

Therefore, one result is
  "ASSERT(new_size <= XFS_IFORK_SIZE(ip, whichfork));"

assertion failure in xfs_idata_realloc(), which was also the root
cause of the original bugreport from Dennis, see:
   https://bugzilla.redhat.com/show_bug.cgi?id=1894177

And it can also be manually triggered with the following commands:
  $ touch a;
  $ setfattr -n user.0 -v "`seq 0 80`" a;
  $ setfattr -n user.1 -v "`seq 0 80`" a

on 32-bit platform.

Fix the case in xfs_attr_shortform_bytesfit() by bailing out
"XFS_LITINO(mp) < bytes" in advance suggested by Eric and a misleading
comment together with this bugfix suggested by Darrick. It seems the
other users of XFS_LITINO(mp) are not impacted.

Reported-by: Dennis Gilmore <dgilmore@redhat.com>
Fixes: e9e2eae89ddb ("xfs: only check the superblock version for dinode size calculation")
Cc: <stable@vger.kernel.org> # 5.7+
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
---
changes since v1:
 - fix 2 typos ">> 8" to ">> 3" mentioned by Eric;
 - directly bail out "XFS_LITINO(mp) < bytes" suggested
   by Eric and Darrick;
 - fix a misleading comment together suggested by Darrick;
 - since (int) decorator doesn't need to be added, so update
   the patch subject as well.

 fs/xfs/libxfs/xfs_attr_leaf.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
index bb128db220ac..c8d91034850b 100644
--- a/fs/xfs/libxfs/xfs_attr_leaf.c
+++ b/fs/xfs/libxfs/xfs_attr_leaf.c
@@ -515,7 +515,7 @@ xfs_attr_copy_value(
  *========================================================================*/
 
 /*
- * Query whether the requested number of additional bytes of extended
+ * Query whether the total requested number of attr fork bytes of extended
  * attribute space will be able to fit inline.
  *
  * Returns zero if not, else the di_forkoff fork offset to be used in the
@@ -535,6 +535,10 @@ xfs_attr_shortform_bytesfit(
 	int			maxforkoff;
 	int			offset;
 
+	/* there is no chance we can fit */
+	if (bytes > XFS_LITINO(mp))
+		return 0;
+
 	/* rounded down */
 	offset = (XFS_LITINO(mp) - bytes) >> 3;
 
-- 
2.18.4