On Mon, Jun 13, 2016 at 7:17 PM, Evgenii Stepanov <euge...@google.com> wrote:
> On Mon, Jun 13, 2016 at 4:12 PM, Aaron Ballman <aaron.ball...@gmail.com>
> wrote:
>> On Mon, Jun 13, 2016 at 6:30 PM, Evgeniy Stepanov <euge...@google.com> wrote:
>>> eugenis added a subscriber: eugenis.
>>> eugenis added a comment.
>>>
>>> In http://reviews.llvm.org/D20561#446031, @aaron.ballman wrote:
>>>
>>>> In http://reviews.llvm.org/D20561#445824, @rogfer01 wrote:
>>>>
>>>> > I think I wasn't clear with the purpose of the fix-it: there are a few
>>>> > cases where getting the address of an unaligned pointer is safe (i.e.
>>>> > false positives).
>>>> >
>>>> > For instance, when I checked Firefox and Chromium there are cases where
>>>> > getting the address of an unaligned pointer is fine. For the particular
>>>> > case of these two browsers, they both use a library (usrsctp) that
>>>> > represents protocol data as packed structs. That library passes
>>>> > addresses of packed fields to `memcpy` and `memmove` which is OK.
>>>>
>>>>
>>>> I think this is a false-positive that should be fixed.
>>>
>>>
>>> This patch was committed without fixing the false positive case, why?
>>
>> Can you give some more code context, like perhaps a test case we can
>> add to the suite? I believe the current behavior should be essentially
>> false-positive-free because it only triggers the diagnostic when the
>> alignments actually differ, so if there are other false-positives, I
>> agree that we should determine a disposition for them.
>
> This is actually the same library the comment above is talking about:
> https://bugs.chromium.org/p/chromium/issues/detail?id=619640
The first example (gettimeofday()) looks to be a true-positive for
sure. The remainder are tricky. They're not going to trigger UB in
practice because the void * that memcpy() takes will get promptly
typecast into a character pointer to avoid UB with the copy, and
that's 1-byte aligned. There's a part of me that thinks it may be
reasonable to assume that casting to void * always means "trust me,
it'll be fine" because the pointer needs to be cast back out of a void
* to be useful, and so the diagnostic should be skipped in that case.
However, it's also just as likely to see code like (esp in C):
void func(void *ptr) {
int *blah = ptr;
*blah = 12;
}
void foo(void) {
struct some_packed_struct s;
func(&s.something);
}
which has drifted into UB.
Roger, is there a way to whitelist memcpy (memmove, memset, et al) for
this check?
>>> Could this warning be excluded from -Wall?
>>
>> Would you like the warning to be excluded from -Wall even if the
>> false-positive is fixed?
>
> No, the warning seems useful if the false positive is fixed.
Ok, good to know.
~Aaron
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