[44/82] btrfs: Refactor intentional wrap-around test

Commit Message

Kees Cook Jan. 23, 2024, 12:27 a.m. UTC

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: Chris Mason <[e-mail]>
Cc: Josef Bacik <[e-mail]>
Cc: David Sterba <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
---
 fs/btrfs/ordered-data.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

David Sterba Jan. 23, 2024, 6 p.m. UTC | #1

On Mon, Jan 22, 2024 at 04:27:19PM -0800, Kees Cook wrote:
> 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: Chris Mason <[e-mail]>
> Cc: Josef Bacik <[e-mail]>
> Cc: David Sterba <[e-mail]>
> Cc: [e-mail]
> Signed-off-by: Kees Cook <[e-mail]>

Acked-by: David Sterba <[e-mail]>

Patch

diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index 59850dc17b22..2e0865693cee 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -813,7 +813,7 @@  int btrfs_wait_ordered_range(struct inode *inode, u64 start, u64 len)
 	u64 orig_end;
 	struct btrfs_ordered_extent *ordered;
 
-	if (start + len < start) {
+	if (add_would_overflow(start, len)) {
 		orig_end = OFFSET_MAX;
 	} else {
 		orig_end = start + len - 1;