Re: [PATCH] Fix UBSAN errors in dse.c (PR rtl-optimization/82044).

["Martin Liška" <mliska@suse.cz>]

PING^1

On 10/19/2017 01:36 PM, Martin Liška wrote:
> On 09/20/2017 10:15 AM, Jakub Jelinek wrote:
>> On Wed, Sep 20, 2017 at 09:50:32AM +0200, Martin Liška wrote:
>>> Hello.
>>>
>>> Following patch handles UBSAN (overflow) in dce.c.
>>
>> dse.c ;)
>>
>>> --- a/gcc/dse.c
>>> +++ b/gcc/dse.c
>>> @@ -929,7 +929,9 @@ set_usage_bits (group_info *group, HOST_WIDE_INT offset, HOST_WIDE_INT width,
>>>  {
>>>    HOST_WIDE_INT i;
>>>    bool expr_escapes = can_escape (expr);
>>> -  if (offset > -MAX_OFFSET && offset + width < MAX_OFFSET)
>>> +  if (offset > -MAX_OFFSET
>>> +      && offset < MAX_OFFSET
>>> +      && offset + width < MAX_OFFSET)
>>
>> This can still overflow if width is close to HOST_WIDE_INT_MAX.
>> Anyway, I don't remember this code too much, but wonder if either offset or
>> width or their sum is outside of the -MAX_OFFSET, MAX_OFFSET range if we
>> still don't want to record usage bits at least in the intersection of
>> -MAX_OFFSET, MAX_OFFSET and offset, offset + width (the latter performed
>> with infinite precision; though, if record_store is changed as suggested
>> below, offset + width shouldn't overflow).
>>
>>>      for (i=offset; i<offset+width; i++)
>>>        {
>>>  	bitmap store1;
>>> @@ -1536,7 +1538,11 @@ record_store (rtx body, bb_info_t bb_info)
>>>      }
>>>    store_info->group_id = group_id;
>>>    store_info->begin = offset;
>>> -  store_info->end = offset + width;
>>> +  if (offset > HOST_WIDE_INT_MAX - width)
>>> +    store_info->end = HOST_WIDE_INT_MAX;
>>> +  else
>>> +    store_info->end = offset + width;
>>
>> If offset + width overflows, I think we risk wrong-code by doing this, plus
>> there are 3 other offset + width computations earlier in record_store
>> before we reach this.  I think instead we should treat such cases as wild
>> stores early, i.e.:
>>    if (!canon_address (mem, &group_id, &offset, &base))
>>      {
>>        clear_rhs_from_active_local_stores ();
>>        return 0;
>>      }
>>  
>>    if (GET_MODE (mem) == BLKmode)
>>      width = MEM_SIZE (mem);
>>    else
>>      width = GET_MODE_SIZE (GET_MODE (mem));
>>
>> +  if (offset > HOST_WIDE_INT_MAX - width)
>> +    {
>> +      clear_rhs_from_active_local_stores ();
>> +      return 0;
>> +    }
>>
>> or so.
>>
>>> +
>>>    store_info->is_set = GET_CODE (body) == SET;
>>>    store_info->rhs = rhs;
>>>    store_info->const_rhs = const_rhs;
>>> @@ -1976,6 +1982,14 @@ check_mem_read_rtx (rtx *loc, bb_info_t bb_info)
>>>        return;
>>>      }
>>>  
>>> +  if (offset > MAX_OFFSET)
>>> +    {
>>> +      if (dump_file && (dump_flags & TDF_DETAILS))
>>> +	fprintf (dump_file, " reaches MAX_OFFSET.\n");
>>> +      add_wild_read (bb_info);
>>> +      return;
>>> +    }
>>> +
> 
> Hi.
> 
> The later one works for me. I'm going to regtest that.
> 
> Ready after it survives regression tests?
> 
> Thanks,
> Martin
> 
>>
>> Is offset > MAX_OFFSET really problematic (and not just the width != -1 &&
>> offset + width overflowing case)?
>>
>>>    if (GET_MODE (mem) == BLKmode)
>>>      width = -1;
>>>    else
>>>
>>
>>
>> 	Jakub
>>
>