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

[Ben Widawsky <ben@bwidawsk.net>]

On Fri, Dec 20, 2013 at 03:11:09PM +0100, Daniel Vetter wrote:
> 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?

As I mentioned on IRC, but I think you missed it - a later subtest hits
a really bad BUG (pre-ppgtt). It was all pass up to that subtest.

> 
> > 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

-- 
Ben Widawsky, Intel Open Source Technology Center