[PATCH v1] x86: memcmp-avx2-movbe.S and memcmp-evex-movbe.S fix overflow bug.

[Siddhesh Poyarekar]

On 6/9/21 12:02 PM, Noah Goldstein wrote:
> Wait are you saying that the return value overflowing causes UB or
> that the act of passing a maxlen where s + maxlen * sizeof(wchar_t) is
> outside range object is UB?

Not specifically the return value, but more broadly, causing wcsnlen to 
invoke undefined behaviour, which could include the former.

> If the former then why is the follow okay:
> 
> Previous example with a string whose length is 1000
> but because wcslen is passed maxlen where maxlen * sizeof(wchar_t)
> overflows and leads to a result less than 1000 the implementation of
> wcslen in wcsnlen-avx2.S will return a length less than 1000.
> 
> 
> If the latter then:
> 
> For test-wcsnlen which redirects to test-strnlen
> If the UB is when is s+maxlen*sizeof(wchar_t) is outside object bound
> then s + SIZE_MAX + sizeof(wchar_t) surely is.
> 
> Although even then test-strnlen s + SIZE_MAX will also overflow if s non 
> null.

That is a good catch; I did not notice that.  That should be SIZE_MAX / 
sizeof (CHAR).

Thanks,
Siddhesh