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

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

On 16/12/2016 08:02, Tvrtko Ursulin wrote:
>
> 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".

Actually it was probably the dynamic engine allocation patch.

Regards,

Tvrtko
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