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@xxxxxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
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 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;
--
2.31.1
