linux-kernel - Re: [PATCH] drm/amdgpu: fix an underflow on non-4KB-page systems

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Date:   Tue, 30 Mar 2021 03:40:04 +0800
From:   Xi Ruoyao <xry111@...gyan1223.wang>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>
Cc:     David Airlie <airlied@...ux.ie>,
        Felix Kuehling <Felix.Kuehling@....com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Dan Horák <dan@...ny.cz>,
        amd-gfx@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>,
        stable@...r.kernel.org
Subject: Re: [PATCH] drm/amdgpu: fix an underflow on non-4KB-page systems

On 2021-03-29 21:36 +0200, Christian König wrote:
> Am 29.03.21 um 21:27 schrieb Xi Ruoyao:
> > Hi Christian,
> > 
> > I don't think there is any constraint implemented to ensure `num_entries %
> > AMDGPU_GPU_PAGES_IN_CPU_PAGE == 0`.  For example, in `amdgpu_vm_bo_map()`:
> > 
> >          /* validate the parameters */
> >          if (saddr & AMDGPU_GPU_PAGE_MASK || offset & AMDGPU_GPU_PAGE_MASK
> > ||
> >              size == 0 || size & AMDGPU_GPU_PAGE_MASK)
> >                  return -EINVAL;
> > 
> > /* snip */
> > 
> >          saddr /= AMDGPU_GPU_PAGE_SIZE;
> >          eaddr /= AMDGPU_GPU_PAGE_SIZE;
> > 
> > /* snip */
> > 
> >          mapping->start = saddr;
> >          mapping->last = eaddr;
> > 
> > 
> > If we really want to ensure (mapping->last - mapping->start + 1) %
> > AMDGPU_GPU_PAGES_IN_CPU_PAGE == 0, then we should replace
> > "AMDGPU_GPU_PAGE_MASK"
> > in "validate the parameters" with "PAGE_MASK".
> 
> Yeah, good point.
> 
> > I tried it and it broke userspace: Xorg startup fails with EINVAL with this
> > change.
> 
> Well in theory it is possible that we always fill the GPUVM on a 4k 
> basis while the native page size of the CPU is larger. Let me double 
> check the code.
> 
> BTW: What code base are you based on? The code your post here is quite 
> outdated.

Linus' tree.

I'll go to sleep now (it's 03:39 here :( ), when I wake up I can try to fetch
drm-next or something.
-- 
Xi Ruoyao <xry111@...gyan1223.wang>
School of Aerospace Science and Technology, Xidian University