[Intel-gfx] [bug report] drm/i915: Small compaction of the engine init code

[Tvrtko Ursulin]

On 15/12/2016 20:54, Chris Wilson wrote:
> On Thu, Dec 15, 2016 at 11:44:13PM +0300, Dan Carpenter wrote:
>> Hello Tvrtko Ursulin,
>>
>> The patch a19d6ff29a82: "drm/i915: Small compaction of the engine
>> init code" from Jun 23, 2016, leads to the following static checker
>> warning:
>>
>> 	drivers/gpu/drm/i915/intel_lrc.c:1973 logical_render_ring_init()
>> 	warn: passing freed memory 'engine'
>>
>> drivers/gpu/drm/i915/intel_lrc.c
>>   1970
>>   1971          ret = logical_ring_init(engine);
>>   1972          if (ret) {
>>   1973                  lrc_destroy_wa_ctx_obj(engine);
>>
>> The problem is that logical_ring_init() frees "engine" on the error
>> path so this is a use after free.
>
> And calls lrc_destroy_wa_ctx_obj() in the process. So we can
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 067394b0a769..1c1bad8ae7b0 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -1940,12 +1940,7 @@ int logical_render_ring_init(struct intel_engine_cs *engine)
>                           ret);
>         }
>
> -       ret = logical_ring_init(engine);
> -       if (ret) {
> -               lrc_destroy_wa_ctx_obj(engine);
> -       }
> -
> -       return ret;
> +       return logical_ring_init(engine);
>  }
>
>  int logical_xcs_ring_init(struct intel_engine_cs *engine)
>

I've marked this as TODO for later.

Interesting that it was detected only now. Even more so, the referenced 
commit just moved the code around.

I think that the real issue was introduced around "drm/i915/gen8: 
Re-order init pipe_control in lrc mode".

Regards,

Tvrtko