[PATCH 67/82] ocfs2: Refactor intentional wrap-around test

[Kees Cook]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Mark Fasheh <mark@xxxxxxxxxx>
Cc: Joel Becker <jlbec@xxxxxxxxxxxx>
Cc: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
Cc: ocfs2-devel@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/ocfs2/resize.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ocfs2/resize.c b/fs/ocfs2/resize.c
index d65d43c61857..5cc83e1d54a7 100644
--- a/fs/ocfs2/resize.c
+++ b/fs/ocfs2/resize.c
@@ -423,7 +423,7 @@ static int ocfs2_verify_group_and_input(struct inode *inode,
 	else if (next_free != cl_count && next_free != input->chain)
 		mlog(ML_ERROR,
 		     "the add group should be in chain %u\n", next_free);
-	else if (total_clusters + input->clusters < total_clusters)
+	else if (add_would_overflow(total_clusters, input->clusters))
 		mlog(ML_ERROR, "add group's clusters overflow.\n");
 	else if (input->clusters > cl_cpg)
 		mlog(ML_ERROR, "the cluster exceeds the maximum of a group\n");
-- 
2.34.1