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

[Rob Clark]

On Mon, May 30, 2022 at 7:16 AM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>
> Hi
>
> Am 30.05.22 um 15:47 schrieb Rob Clark:
> > On Mon, May 30, 2022 at 12:26 AM Thomas Zimmermann <tzimmermann at suse.de> wrote:
> >>
> >> Hi
> >>
> >> Am 29.05.22 um 18:29 schrieb Rob Clark:
> >>> From: Rob Clark <robdclark at 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.)

I dispute the "correctly" part.. and that this is a workaround ;-)

But I can send a v2 with the addition of a comment explaining the
reason, so git-blame archeology isn't required to understand the
reasoning

BR,
-R

> 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 at 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