On Thu, May 27, 2021 at 11:26:41AM -0500, Jason Ekstrand wrote:
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@jlekstrand.net
Yeah a non-dense compiler should be able to figure this out, plus set_engines isn't a hotpath.
Reviewed-by: Daniel Vetter daniel.vetter@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 12a148ba421b6..cf7c281977a3e 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c @@ -1758,9 +1758,8 @@ 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))) {-- 2.31.1