[PATCH 76/82] udf: 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: Jan Kara <jack@xxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/udf/balloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/udf/balloc.c b/fs/udf/balloc.c
index ab3ffc355949..5c88300c3de7 100644
--- a/fs/udf/balloc.c
+++ b/fs/udf/balloc.c
@@ -139,7 +139,7 @@ static void udf_bitmap_free_blocks(struct super_block *sb,
 
 	mutex_lock(&sbi->s_alloc_mutex);
 	partmap = &sbi->s_partmaps[bloc->partitionReferenceNum];
-	if (bloc->logicalBlockNum + count < count ||
+	if (add_would_overflow(count, bloc->logicalBlockNum) ||
 	    (bloc->logicalBlockNum + count) > partmap->s_partition_len) {
 		udf_debug("%u < %d || %u + %u > %u\n",
 			  bloc->logicalBlockNum, 0,
@@ -390,7 +390,7 @@ static void udf_table_free_blocks(struct super_block *sb,
 
 	mutex_lock(&sbi->s_alloc_mutex);
 	partmap = &sbi->s_partmaps[bloc->partitionReferenceNum];
-	if (bloc->logicalBlockNum + count < count ||
+	if (add_would_overflow(count, bloc->logicalBlockNum) ||
 	    (bloc->logicalBlockNum + count) > partmap->s_partition_len) {
 		udf_debug("%u < %d || %u + %u > %u\n",
 			  bloc->logicalBlockNum, 0,
-- 
2.34.1