Re: [PATCH] enhance -Warray-bounds to detect out-of-bounds offsets (PR 82455)

[Richard Biener <rguenther at suse dot de>]

On Mon, 30 Oct 2017, Jeff Law wrote:

> On 10/30/2017 05:29 PM, Martin Sebor wrote:
> > On 10/30/2017 03:48 PM, Jeff Law wrote:
> >> On 10/30/2017 09:19 AM, Martin Sebor wrote:
> >>> On 10/30/2017 05:45 AM, Richard Biener wrote:
> >>>> On Sun, 29 Oct 2017, Martin Sebor wrote:
> >>>>
> >>>>> In my work on -Wrestrict, to issue meaningful warnings, I found
> >>>>> it important to detect both out of bounds array indices as well
> >>>>> as offsets in calls to restrict-qualified functions like strcpy.
> >>>>> GCC already detects some of these cases but my tests for
> >>>>> the enhanced warning exposed a few gaps.
> >>>>>
> >>>>> The attached patch enhances -Warray-bounds to detect more instances
> >>>>> out-of-bounds indices and offsets to member arrays and non-array
> >>>>> members.  For example, it detects the out-of-bounds offset in the
> >>>>> call to strcpy below.
> >>>>>
> >>>>> The patch is meant to be applied on top posted here but not yet
> >>>>> committed:
> >>>>>     https://gcc.gnu.org/ml/gcc-patches/2017-10/msg01304.html
> >>>>>
> >>>>> Richard, since this also touches tree-vrp.c I look for your comments.
> >>>>
> >>>> You fail to tell what you are changing and why - I have to reverse
> >>>> engineer this from the patch which a) isn't easy in this case, b) feels
> >>>> like a waste of time.  Esp. since the patch does many things.
> >>>>
> >>>> My first question is why do you add a warning from forwprop?  It
> >>>> _feels_ like you're trying to warn about arbitrary out-of-bound
> >>>> addresses at the point they are folded to MEM_REFs.  And it looks
> >>>> like you're warning about pointer arithmetic like &p->a + 6.
> >>>> That doesn't look correct to me.  Pointer arithmetic in GIMPLE
> >>>> is not restricted to operate within fields that are appearantly
> >>>> accessed here - the only restriction is with respect to the
> >>>> whole underlying pointed-to-object.
> >>>>
> >>>> By doing the warning from forwprop you'll run into all such cases
> >>>> introduced by GCC itself during quite late optimization passes.
> >>>
> >>> I haven't run into any such cases.  What would a more appropriate
> >>> place to detect out-of-bounds offsets?  I'm having a hard time
> >>> distinguishing what is appropriate and what isn't.  For instance,
> >>> if it's okay to detect some out of bounds offsets/indices in vrp
> >>> why is it wrong to do a better job of it in forwpropI think part of
> >>> the problem is there isn't a well defined place to do
> >> this kind of warning.  I suspect it's currently in VRP simply because
> >> that is where we had range information in the past.  It's still the
> >> location with the most accurate range information.
> >>
> >> In a world where we have an embedded context sensitive range analysis
> >> engine, we should *really* look at pulling the out of bounds array
> >> warnings out of any optimization pass an have a distinct pass to deal
> >> with them.
> >>
> >> I guess in the immediate term the question I would ask Martin is what is
> >> it about forwprop that makes it interesting?  Is it because of the
> >> lowering issues we touched on last week?  If so I wonder if we could
> >> recreate an array form from a MEM_REF for the purposes of optimization.
> >> Or if we could just handle MEM_REFs better within the existing warning.
> > 
> > I put it in forwprop only because that was the last stage where
> > there's still enough context before the POINTER_PLUS_EXPR is
> > folded into MEM_REF to tell an offset from the beginning of
> > a subobject from the one from the beginning of the bigger object
> > of which the subobject is a member.  I certainly don't mind moving
> > it somewhere else more appropriate if this isn't ideal, just as
> > long it doesn't cripple the detection (e.g., as long as we still
> > have range information).
> Understood.

Well, it's a long-standing issue with how we do these kind of
warnings, likewise for _b_o_s which also "can't stand" component-refs
to be folded into the MEM_REF offset.

I've said in the past that _b_o_s relying on component-refs to stay
and for them to be constrained the same way they are in C is bogus.
We've added an early _b_o_s pass to mitigate that "issue" somewhat.

Now you're trying to "solve" the same issue as _b_o_s -- in the
end it looks like the warning could well reside in that pass
rather than in forwprop.

Richard.

> 
> [ ... ]
> 
> > 
> > I of course don't want to break anything.  I didn't see any fallout
> > in my testing and I normally test all the front ends, including Ada,
> > but let me check to make sure I tested it this time (I had made some
> > temporary changes to my build script and may have disabled it.)  Let
> > me double check it after I get back from my trip.
> No worries.  Hopefully by the time you're back I'll have something
> publishable on the ripping apart tree-vrp front and we can prototype the
> effectiveness of doing this kind of stuff outside tree-vrp.c
> 
> We should also revisit Aldy's work from last year which started the
> whole effort around fixing how we deal with out out of bounds index
> testing.   He had a version which ran outside tree-vrp.c but used the
> same basic structure and queried range data for the index.  I've got a
> copy here we can poke at.
> 
> jeff
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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