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

[Jeff Law <law at redhat dot com>]

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.


[ ... ]

> 
> 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