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

[Martin Sebor <msebor@gmail.com>]

On 10/30/2017 01:53 PM, Richard Biener wrote:
> On October 30, 2017 4:19:25 PM GMT+01:00, Martin Sebor <msebor@gmail.com> 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 forwprop?
>>
>>>
>>> You're trying to re-do __builtin_object_size even when that wasn't
>>> used.
>>
>> That's not the quite my intent, although it is close.
>>
>>>
>>> So it looks like you're on the wrong track.  Yes,
>>>
>>>    strcpy (p->a + 6, "y");
>>>
>>> _may_ be "invalid" C (I'm not even sure about that!) but it
>>> is certainly not invalid GIMPLE.
>>
>> Adding (or subtracting) an integer to/from a pointer to an array
>> is defined in both C and C++ only if the result points to an element
>> of the array or just past the last element of the array.  Otherwise
>> it's undefined. (A non-array object is considered an array of one
>> for this purpose.)
> 
> On GIMPLE this is indistinguishable from (short *) (p->a) + 3.

Sure, they're both the same:

   pa_3 = &p_2(D)->a;
   _1 = pa_3 + 6;

and that's fine because the implementation of the warning sees and
uses the byte offset from the beginning of a, so I don't understand
the problem you are pointing out.  Can you clarify what you mean?

Martin

> 
> GIMPLE is neither C nor C++.
> 
> Richard.
> 
>>
>> Martin
>>
>>>
>>> Richard.
>>>
>>>> Jeff, this is the enhancement you were interested in when we spoke
>>>> last week.
>>>>
>>>> Thanks
>>>> Martin
>>>>
>>>> $ cat a.c && gcc -O2 -S -Wall a.c
>>>>     struct A { char a[4]; void (*pf)(void); };
>>>>
>>>>     void f (struct A *p)
>>>>     {
>>>>       p->a[5] = 'x';            // existing -Warray-bounds
>>>>
>>>>       strcpy (p->a + 6, "y");   // enhanced -Warray-bounds
>>>>     }
>>>>
>>>>     a.c: In function ‘f’:
>>>>     a.c:7:3: warning: offset 6 is out of bounds of ‘char[4]’
>> [-Warray-bounds]
>>>>      strcpy (p->a + 6, "y");
>>>>      ^~~~~~~~~~~~~~~~~~~~~~
>>>>     a.c:1:17: note: member declared here
>>>>      struct A { char a[4]; void (*pf)(void); };
>>>>                      ^
>>>>     a.c:5:7: warning: array subscript 5 is above array bounds of
>> ‘char[4]’
>>>> [-Warray-bounds]
>>>>        p->a[5] = 'x';
>>>>        ~~~~^~~
>