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

[Rob Clark <robdclark@gmail.com>]

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.

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)
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