Re: [PATCH v2] drm/amdgpu: fix fdinfo race with process exit

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

Sorry I'm on vacation and can't reply immediately.

This is the wrong approach. The fdinfo should have grabbed a reference 
to the fd it prints the info for.

So we should never race here. Can you double check how this happens?

Thanks,
Christian.

Am 03.08.21 um 16:06 schrieb philip yang:
> ping?
>
> On 2021-07-29 10:13 p.m., Philip Yang wrote:
> > Get process vm root BO ref in case process is exiting and root BO is
> > freed, to avoid NULL pointer dereference backtrace:
> >
> > BUG: unable to handle kernel NULL pointer dereference at
> > 0000000000000000
> > Call Trace:
> > amdgpu_show_fdinfo+0xfe/0x2a0 [amdgpu]
> > seq_show+0x12c/0x180
> > seq_read+0x153/0x410
> > vfs_read+0x91/0x140[ 3427.206183]  ksys_read+0x4f/0xb0
> > do_syscall_64+0x5b/0x1a0
> > entry_SYSCALL_64_after_hwframe+0x65/0xca
> >
> > v2: rebase to staging
> >
> > Signed-off-by: Philip Yang<Philip.Yang@xxxxxxx>
> > ---
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_fdinfo.c | 11 +++++++++--
> >   1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fdinfo.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_fdinfo.c
> > index d94c5419ec25..5a6857c44bb6 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fdinfo.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fdinfo.c
> > @@ -59,6 +59,7 @@ void amdgpu_show_fdinfo(struct seq_file *m, struct file *f)
> >   	uint64_t vram_mem = 0, gtt_mem = 0, cpu_mem = 0;
> >   	struct drm_file *file = f->private_data;
> >   	struct amdgpu_device *adev = drm_to_adev(file->minor->dev);
> > +	struct amdgpu_bo *root;
> >   	int ret;
> >   
> >   	ret = amdgpu_file_to_fpriv(f, &fpriv);
> > @@ -69,13 +70,19 @@ void amdgpu_show_fdinfo(struct seq_file *m, struct file *f)
> >   	dev = PCI_SLOT(adev->pdev->devfn);
> >   	fn = PCI_FUNC(adev->pdev->devfn);
> >   
> > -	ret = amdgpu_bo_reserve(fpriv->vm.root.bo, false);
> > +	root = amdgpu_bo_ref(fpriv->vm.root.bo);
> > +	if (!root)
> > +		return;
> > +
> > +	ret = amdgpu_bo_reserve(root, false);
> >   	if (ret) {
> >   		DRM_ERROR("Fail to reserve bo\n");
> >   		return;
> >   	}
> >   	amdgpu_vm_get_memory(&fpriv->vm, &vram_mem, &gtt_mem, &cpu_mem);
> > -	amdgpu_bo_unreserve(fpriv->vm.root.bo);
> > +	amdgpu_bo_unreserve(root);
> > +	amdgpu_bo_unref(&root);
> > +
> >   	seq_printf(m, "pdev:\t%04x:%02x:%02x.%d\npasid:\t%u\n", domain, bus,
> >   			dev, fn, fpriv->vm.pasid);
> >   	seq_printf(m, "vram mem:\t%llu kB\n", vram_mem/1024UL);