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

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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

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