[cfe-commits] PATCH: Enhance array bounds checking

[Ted Kremenek]

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>

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