Re: [PATCH] drm/i915/ppgtt: Prevent NULL deref in reset ioctl

Daniel Vetter
2013-12-20T14:10:27Z

On Thu, Dec 19, 2013 at 10:55:56PM -0800, Ben Widawsky wrote:
> On Fri, Dec 20, 2013 at 07:05:10AM +0100, Daniel Vetter wrote:
> > On Fri, Dec 20, 2013 at 12:22 AM, Ben Widawsky
> > <benjamin.widawsky@intel.com> wrote:
> > > ctx = i915_gem_context_get(file->driver_priv, args->ctx_id);
> > > - if (IS_ERR(ctx)) {
> > > + if (IS_ERR_OR_NULL(ctx)) {
> >
> > We now have half the callers check for IS_ERR and the others not.
> > Afaics i915_gem_context_get can only return NULL or a real context
> > though. Also from a quite read the expected error for a lookup failure
> > is ENOENT (and there seems to be a testcase for this).
> > -Daniel
>
>
> To your first point:
> I think checking null is always the right thing currently, but for
> future proofing, IS_ERR_OR_NULL is really the right thing. After his
> patch, I believe only i915_gem_context_destroy_ioctl is still incorrect.

Using IS_ERR_OR_NULL on a return value which can never contain an
encoded errno value is imo confusing, more so when it's inconsitently
applied.

> To the second:
> This is only based on my memory, so feel free to change whatever you
> need. When I retuned -ENXIO, the test failed.
>
> It should be:
> return ctx ? PTR_ERR(ctx) : -ENOENT;

On a cursory read that's been the semantics before your patch. And there
seems to be a testcase in gem_reset_stat for this, have you run all
subtests?

> I had
> return ctx ? PTR_ERR(ctx) : -ENXIO;
> which made the subtest fail. However, as we've noted, this itself was
> not correct. Try the return above.

Thanks, Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch