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Date:   Wed, 29 Jun 2022 11:22:02 +0300
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Subject: Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to
 drm_gem_mmap_obj()
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To:     =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= 
        <thomas_os@shipmail.org>
Cc:     intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org,
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        linux-tegra@vger.kernel.org, Dmitry Osipenko <digetx@gmail.com>,
        kernel@collabora.com, linux-media@vger.kernel.org,
        David Airlie <airlied@linux.ie>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
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        Emil Velikov <emil.l.velikov@gmail.com>,
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        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        Thierry Reding <thierry.reding@gmail.com>,
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        Marek Szyprowski <m.szyprowski@samsung.com>,
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        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
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References: <20220526235040.678984-1-dmitry.osipenko@collabora.com>
 <20220526235040.678984-3-dmitry.osipenko@collabora.com>
 <b8271f0c-d6a3-4194-1959-e112859756a3@shipmail.org>
From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
In-Reply-To: <b8271f0c-d6a3-4194-1959-e112859756a3@shipmail.org>
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On 6/29/22 09:40, Thomas Hellström (Intel) wrote:
> 
> On 5/27/22 01:50, Dmitry Osipenko wrote:
>> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
>> handle imported dma-bufs properly, which results in mapping of something
>> else than the imported dma-buf. For example, on NVIDIA Tegra we get a
>> hard
>> lockup when userspace writes to the memory mapping of a dma-buf that was
>> imported into Tegra's DRM GEM.
>>
>> To fix this bug, move mapping of imported dma-bufs to drm_gem_mmap_obj().
>> Now mmaping of imported dma-bufs works properly for all DRM drivers.
> Same comment about Fixes: as in patch 1,
>>
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>> ---
>>   drivers/gpu/drm/drm_gem.c              | 3 +++
>>   drivers/gpu/drm/drm_gem_shmem_helper.c | 9 ---------
>>   drivers/gpu/drm/tegra/gem.c            | 4 ++++
>>   3 files changed, 7 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
>> index 86d670c71286..7c0b025508e4 100644
>> --- a/drivers/gpu/drm/drm_gem.c
>> +++ b/drivers/gpu/drm/drm_gem.c
>> @@ -1038,6 +1038,9 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj,
>> unsigned long obj_size,
>>       if (obj_size < vma->vm_end - vma->vm_start)
>>           return -EINVAL;
>>   +    if (obj->import_attach)
>> +        return dma_buf_mmap(obj->dma_buf, vma, 0);
> 
> If we start enabling mmaping of imported dma-bufs on a majority of
> drivers in this way, how do we ensure that user-space is not blindly
> using the object mmap without calling the needed DMA_BUF_IOCTL_SYNC
> which is needed before and after cpu access of mmap'ed dma-bufs?
> 
> I was under the impression (admittedly without looking) that the few
> drivers that actually called into dma_buf_mmap() had some private
> user-mode driver code in place that ensured this happened.

Since it's a userspace who does the mapping, then it should be a
responsibility of userspace to do all the necessary syncing. I'm not
sure whether anyone in userspace really needs to map imported dma-bufs
in practice. Nevertheless, this use-case is broken and should be fixed
by either allowing to do the mapping or prohibiting it.

-- 
Best regards,
Dmitry