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drm/i915/gem: Make an alignment check more sensible

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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>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-21-jason@jlekstrand.net
This commit is contained in:
Jason Ekstrand 2021-07-08 10:48:25 -05:00 committed by Daniel Vetter
parent bc2ceb7a08
commit def25b7bc3
1 changed files with 1 additions and 2 deletions
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3
drivers/gpu/drm/i915/gem/i915_gem_context.c
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@ -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;