[cfe-commits] PATCH: Enhance array bounds checking

[Kaelyn Uhrain]

Just realized my patch missed the new unit test file. d'oh!

On Sat, Jul 23, 2011 at 11:35 AM, Ted Kremenek <kremenek at apple.com> wrote:

> This all sounds good to me.  I think the patch can go in as is.
>
> On Jul 21, 2011, at 4:04 PM, Kaelyn Uhrain wrote:
>
> Hi Ted,
>
> I've attached a new version of my patch that moves the pointer arithmetic
> portion of the -Warray-bounds improvements into a separate flag,
> -Warray-bounds-pointer-arithmetic. Since the remaining noise in the pointer
> arithmetic check is more a case of undefined behavior that does the right
> thing with most (all?) compilers than a case of being true false positives,
> the plan is to have -Warray-bounds-pointer-arithmetic initially be disabled
> by default and in later patches to enhance the warning by e.g. providing a
> fixit note to add parens to exprs like "ptr + sizeof("foo") - 1" so they
> become "ptr + (sizeof("foo")) - 1".
>
> Cheers,
> Kaelyn
>
> On Thu, Jul 21, 2011 at 1:18 PM, Ted Kremenek <kremenek at apple.com> wrote:
>
>> Hi Kaelyn,
>>
>> While I think there will be some contention here, I think this is still
>> too high of a false positive rate for this warning to be on by default in
>> the compiler.  Users simply aren't going to tolerate it.  I think it is
>> reasonable for a codebase to adopt a strict policy for this warning, but I
>> don't think a 17% false positive rate (or possibly higher) is acceptable for
>> a default warning for all users of Clang.
>>
>> Unless you believe you think there are additional heuristics to drop the
>> false positive rate down below 5-10%, I think I'm fine with proceeding as
>> putting this in as an opt-in warning, and then refine from there.
>>
>> On Jul 20, 2011, at 11:39 AM, Kaelyn Uhrain wrote:
>>
>> I've attached an updated version of my patch that better handles cases
>> where pointer arithmetic is done after casting a constant-size array to a
>> pointer for a smaller base type (e.g. casting an int array to char*). Of the
>> pointer arithmetic warnings, about 24% could be considered false positives;
>> however, the actual number of false positives is quite small and 2/3 of them
>> stem from the use of a single macro--if you count those as a single warning
>> & false positive, the rate drops to 17%. Of the false positives most are
>> from semi-questionable pointer arithmetic where a constant greater than the
>> length of the array/pointer is being added to the pointer and some int > 1
>> being subtracted from it, e.g.:
>>
>> void foo(int n) {
>>   char x[5];
>>   if (n > 0) bar(x + 6 - n);
>> }
>>
>> Cheers,
>> Kaelyn
>>
>> On Tue, Jul 19, 2011 at 11:44 AM, Kaelyn Uhrain <rikka at google.com> wrote:
>>
>>> I'm still looking at the pointer arithmetic warnings to determine which
>>> are false positives, but I realized I screwed up my previous stats a bit as
>>> I forgot to account for duplicated/repeated warnings (e.g. the same header
>>> included in multiple compilation units and so generating the same warnings
>>> multiple times). For unique warnings, the stats are:
>>>
>>> - 16.5% increase in warnings from before my patch (originally reported a
>>> 24% increase)
>>> - 47% of those new warnings being about pointer arithmetic
>>> - 6.7% of all of the warnings emitted with my patch applied are
>>> concerning pointer arithmetic. (originally reported 8.3%)
>>> - The new pointer arithmetic warnings represent a 7.7% increase in
>>> warnings from before my patch, not 10%.
>>>
>>> I'll send another email once I have a feel for how noisy the pointer
>>> arithmetic warnings are.
>>>
>>> Cheers,
>>>  Kaelyn
>>>
>>>
>>> On Mon, Jul 18, 2011 at 5:54 PM, Kaelyn Uhrain <rikka at google.com> wrote:
>>>
>>>> Ted,
>>>>
>>>> You're welcome. I'll try to figure out what fraction of the pointer
>>>> arithmetic warnings are false positives (requires a bit of manual digging on
>>>> my part to determine if the code is indeed buggy or if it is valid /
>>>> intended). For the overall 24% increase in warnings, keep in mind that over
>>>> half of that is the existing bounds checking now being applied to cases
>>>> where it wasn't before, i.e.:
>>>>
>>>> char *foo[5];
>>>> foo[77];  // -Warray-bounds already found
>>>> &foo[77];  // -Warray-bounds currently misses
>>>> *foo[77];  // -Warray-bounds currently misses
>>>>
>>>> The function that did the bounds checking would never catch the last two
>>>> cases because it would see a UnaryOperator (in the above cases for the '&'
>>>> and the '*') and skip the expression instead of looking inside the
>>>> UnaryOperator expression for the array subscripting.
>>>>
>>>> The new pointer arithmetic bounds checking only represents a 10%
>>>> increase in warnings--and IMHO that is the only part where the number of
>>>> false positives introduced might be an issue.
>>>>
>>>> Cheers,
>>>> Kaelyn
>>>>
>>>>
>>>> On Mon, Jul 18, 2011 at 5:36 PM, Ted Kremenek <kremenek at apple.com>wrote:
>>>>
>>>>> Hi Kaelyn,
>>>>>
>>>>> Thanks for the statistics.  What would be good to know is what fraction
>>>>> of these are false positives (i.e., are these all real bugs)?  A small
>>>>> random sample might be helpful.  A 24% increase in warnings is fairly
>>>>> substantial, and we don't want to do that unless there is a real benefit.
>>>>>
>>>>> Ted
>>>>>
>>>>> On Jul 18, 2011, at 5:18 PM, Kaelyn Uhrain wrote:
>>>>>
>>>>> Ted,
>>>>>
>>>>> On Thu, Jul 14, 2011 at 5:04 PM, Ted Kremenek <kremenek at apple.com>wrote:
>>>>>
>>>>>>
>>>>>> The only other issue: should this be controlled under a separate
>>>>>> warning flag, at least initially so we can experiment with this new warning
>>>>>> and see how noisy it is?  E.g. "-Warray-bounds-pointer-arithmetic".
>>>>>>
>>>>>
>>>>> I've tested the -Warray-bounds changes against the Google codebase and
>>>>> my patch increases the number of warnings from -Warray-bounds by 24%. Of the
>>>>> new warnings, 57.33% are for array indexes that most likely weren't picked
>>>>> up before because of unary operators like & or * (11.1% of all the warnings
>>>>> now emitted), and the remaining 42.67% are from out-of-bounds pointer
>>>>> arithmetic (8.3% of all the warnings from -Warray-bounds).
>>>>>
>>>>> Cheers,
>>>>> Kaelyn
>>>>>
>>>>>
>>>>>
>>>>
>>>
>> <array-bounds-enhancement3.diff>
>>
>>
>>
> <array-bounds-enhancement4.diff>
>
>
>
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