linux-kernel - Re: [Linaro-mm-sig] Re: [PATCH v2] drm/gem: Fix GEM handle release errors

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Message-ID: <71e47fe6-440b-e9ea-cd66-8362c41428ca@amd.com>
Date:   Tue, 9 Aug 2022 11:08:56 +0200
From:   Christian König <christian.koenig@....com>
To:     Chen Jeffy <jeffy.chen@...k-chips.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Andy Yan <andy.yan@...k-chips.com>,
        Jianqun Xu <jay.xu@...k-chips.com>,
        Maxime Ripard <mripard@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        linux-media@...r.kernel.org, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [Linaro-mm-sig] Re: [PATCH v2] drm/gem: Fix GEM handle release
 errors

Hi Jeffy,

Am 09.08.22 um 09:55 schrieb Christian König:
> [SNIP]
>>>
>>>>
>>>>
>>>> So we are allowing GEM object to have multiple handles, and GEM 
>>>> object could have at most one dma-buf, doesn't that means that 
>>>> dma-buf could map to multiple handles?
>>>
>>> No, at least not for the same GEM file private. That's the reason 
>>> why the rb is indexed by the dma_buf object and not the handle.
>>>
>>> In other words the rb is so that you have exactly one handle for 
>>> each dma_buf in each file private.
>>
>> I don't think so, because if user get multiple handles for the same 
>> GEM obj and use drm_gem_prime_handle_to_fd() for those handles
>
> Mhm, that works? This is illegal and should have been prevented somehow.

At least I see the problem now. I'm just not sure how to fix it.

Your v2 patch indeed prevents leakage of the drm_prime_member for the 
additional handles, but those shouldn't have been added in the first place.

The issue is that with this we make it unpredictable which handle is 
returned. E.g. if we have handle 2,5,7 it can be that because of 
re-balancing the tree sometimes 2 and sometimes 5 is returned.

That's not really a good idea and breaks a couple of assumptions as far 
as I know.

Ideas?

Thanks,
Christian.