Re: [PATCH] drm/prime: Ensure mmap offset is initialized

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 30.05.22 um 15:47 schrieb Rob Clark:
> On Mon, May 30, 2022 at 12:26 AM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>>
>> Hi
>>
>> Am 29.05.22 um 18:29 schrieb Rob Clark:
>>> From: Rob Clark <robdclark@chromium.org>
>>>
>>> If a GEM object is allocated, and then exported as a dma-buf fd which is
>>> mmap'd before or without the GEM buffer being directly mmap'd, the
>>> vma_node could be unitialized.  This leads to a situation where the CPU
>>> mapping is not correctly torn down in drm_vma_node_unmap().
>>
>> Which drivers are affected by this problem?
>>
>> I checked several drivers and most appear to be initializing the offset
>> during object construction, such as GEM SHMEM. [1] TTM-based drivers
>> also seem unaffected. [2]
>>
>>   From a quick grep, only etnaviv, msm and omapdrm appear to be affected?
>> They only seem to run drm_gem_create_mmap_offset() from their
>> ioctl-handling code.
>>
>> If so, I'd say it's preferable to fix these drivers and put a
>> drm_WARN_ONCE() into drm_gem_prime_mmap().
> 
> That is good if fewer drivers are affected, however I disagree with
> your proposal.  At least for freedreno userspace, a lot of bo's never
> get mmap'd (either directly of via dmabuf), so we should not be
> allocating a mmap offset unnecessarily.

I see.

I the reason I'm arguing against the current patch is that the fix 
appears like a workaround and 6 months from now, few will remember why 
it's there. Especially since most drivers initialize the offset 
correctly. (Not too long ago, I refactored the handling of these mmap 
calls throughout DRM drivers and it was confusing at times.)

So here's another suggestion:  I further looked at the 3 drivers that I 
mentioned. etnaviv and msm can easily wrap the call to 
drm_gem_prime_mmap() and init the offset first. [1][2]  omapdrm doesn't 
actually use drm_gem_prime_mmap(). The offset can instead be initialized 
at the top of the driver's dmabuf mmap function. [3]

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/etnaviv/etnaviv_drv.c#L480
[2] 
https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/msm/msm_drv.c#L961
[3] 
https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/omapdrm/omap_gem_dmabuf.c#L66

> 
> BR,
> -R
> 
>> Best regards
>> Thomas
>>
>> [1]
>> https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/drm_gem_shmem_helper.c#L85
>> [2]
>> https://elixir.bootlin.com/linux/v5.18/source/drivers/gpu/drm/ttm/ttm_bo.c#L1002
>>
>>>
>>> Fixes: e5516553999f ("drm: call drm_gem_object_funcs.mmap with fake offset")
>>> Signed-off-by: Rob Clark <robdclark@chromium.org>
>>> ---
>>> Note, it's possible the issue existed in some related form prior to the
>>> commit tagged with Fixes.
>>>
>>>    drivers/gpu/drm/drm_prime.c | 5 +++++
>>>    1 file changed, 5 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>>> index e3f09f18110c..849eea154dfc 100644
>>> --- a/drivers/gpu/drm/drm_prime.c
>>> +++ b/drivers/gpu/drm/drm_prime.c
>>> @@ -716,6 +716,11 @@ int drm_gem_prime_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>>>        struct file *fil;
>>>        int ret;
>>>
>>> +     /* Ensure that the vma_node is initialized: */
>>> +     ret = drm_gem_create_mmap_offset(obj);
>>> +     if (ret)
>>> +             return ret;
>>> +
>>>        /* Add the fake offset */
>>>        vma->vm_pgoff += drm_vma_node_start(&obj->vma_node);
>>>
>>
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Maxfeldstr. 5, 90409 Nürnberg, Germany
>> (HRB 36809, AG Nürnberg)
>> Geschäftsführer: Ivo Totev

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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