Re: [Intel-gfx] [PATCH 1/3] drm/i915/gt: Do not allow setting ring size for legacy ring submission

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 21/06/2021 13:08, Tvrtko Ursulin wrote:
> 
> I had some questions on the trybot mailing list, let me copy&paste..
> 
> On 21/06/2021 12:41, Maarten Lankhorst wrote:
>> It doesn't work for legacy ring submission, and is in the best case
>> ignored.
> 
> Looks rejected instead of ignored:
> 
> static int set_ringsize(struct i915_gem_context *ctx,
>              struct drm_i915_gem_context_param *args)
> {
>      if (!HAS_LOGICAL_RING_CONTEXTS(ctx->i915))
>          return -ENODEV;
>>
>> In the worst case we end up freeing engine->legacy.ring for all other
>> active engines, resulting in a use-after-free.
> 
> Worst case is cloning because ring_context_alloc is not taking a 
> reference to engine->legacy.ring, or something else?

No can't be that, it was my incomplete analysis last week. Since 
ring_context_destroy does not actually free the legacy ring I don't see 
any use after free paths.

Regards,

Tvrtko

>>
>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>> Fixes: 88be76cdafc7 ("drm/i915: Allow userspace to specify ringsize on 
>> construction")
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> ---
>>   drivers/gpu/drm/i915/gt/intel_context_param.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/gt/intel_context_param.c 
>> b/drivers/gpu/drm/i915/gt/intel_context_param.c
>> index 65dcd090245d..412c36d1b1dd 100644
>> --- a/drivers/gpu/drm/i915/gt/intel_context_param.c
>> +++ b/drivers/gpu/drm/i915/gt/intel_context_param.c
>> @@ -12,6 +12,9 @@ int intel_context_set_ring_size(struct intel_context 
>> *ce, long sz)
>>   {
>>       int err;
>> +    if (ce->engine->gt->submission_method == INTEL_SUBMISSION_RING)
>> +        return 0;
>> +
>>       if (intel_context_lock_pinned(ce))
>>           return -EINTR;
>>
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