[PATCH 50/82] drm/vc4: 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: Maxime Ripard <mripard at kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: David Airlie <airlied at gmail.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: dri-devel at lists.freedesktop.org
Signed-off-by: Kees Cook <keescook at chromium.org>
---
 drivers/gpu/drm/vc4/vc4_validate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vc4/vc4_validate.c b/drivers/gpu/drm/vc4/vc4_validate.c
index 9affba9c58b3..677d9975f888 100644
--- a/drivers/gpu/drm/vc4/vc4_validate.c
+++ b/drivers/gpu/drm/vc4/vc4_validate.c
@@ -206,7 +206,7 @@ vc4_check_tex_size(struct vc4_exec_info *exec, struct drm_gem_dma_object *fbo,
 	stride = aligned_width * cpp;
 	size = stride * aligned_height;
 
-	if (size + offset < size ||
+	if (add_would_overflow(size, offset) ||
 	    size + offset > fbo->base.size) {
 		DRM_DEBUG("Overflow in %dx%d (%dx%d) fbo size (%d + %d > %zd)\n",
 			  width, height,
-- 
2.34.1