[PATCH 20/30] drm/i915/gem: Make an alignment check more sensible

[Jason Ekstrand]

What we really want to check is that size of the engines array, i.e.
args->size - sizeof(*user) is divisible by the element size, i.e.
sizeof(*user->engines) because that's what's required for computing the
array length right below the check.  However, we're currently not doing
this and instead doing a compile-time check that sizeof(*user) is
divisible by sizeof(*user->engines) and avoiding the subtraction.  As
far as I can tell, the only reason for the more confusing pair of checks
is to avoid a single subtraction of a constant.

The other thing the BUILD_BUG_ON might be trying to implicitly check is
that offsetof(user->engines) == sizeof(*user) and we don't have any
weird padding throwing us off.  However, that's not the check it's doing
and it's not even a reliable way to do that check.

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/gem/i915_gem_context.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 8e7c0e3f070ed..c9bae1a1726e1 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1764,9 +1764,8 @@ set_engines(struct i915_gem_context *ctx,
 		goto replace;
 	}
 
-	BUILD_BUG_ON(!IS_ALIGNED(sizeof(*user), sizeof(*user->engines)));
 	if (args->size < sizeof(*user) ||
-	    !IS_ALIGNED(args->size, sizeof(*user->engines))) {
+	    !IS_ALIGNED(args->size -  sizeof(*user), sizeof(*user->engines))) {
 		drm_dbg(&i915->drm, "Invalid size for engine array: %d\n",
 			args->size);
 		return -EINVAL;
-- 
2.31.1