From: Jason Ekstrand Date: Thu, 8 Jul 2021 15:48:25 +0000 (-0500) Subject: drm/i915/gem: Make an alignment check more sensible X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=def25b7bc342d7eb6643408300127404608d3df2;p=linux.git drm/i915/gem: Make an alignment check more sensible 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 Reviewed-by: Daniel Vetter Signed-off-by: Daniel Vetter Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-21-jason@jlekstrand.net --- diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c index 3c59d1e4080c4..f135fbc97c5a7 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c @@ -1723,9 +1723,8 @@ 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;