[PATCH] Avoid signed overflow in num_get::_M_extract_int (PR libstdc++/67214)

[Jonathan Wakely]

On 22/05/17 11:08 +0100, Jonathan Wakely wrote:
>On 20/05/17 15:10 +0800, Xi Ruoyao wrote:
>>On 2017-05-19 15:38 +0100, Jonathan Wakely wrote:
>>>On 18/05/17 19:10 +0800, Xi Ruoyao wrote:
>>>> This UB has been hiding so long...
>>>
>>>Indeed! Thanks for the patch.
>>>
>>>> 2017-03-11  Xi Ruoyao  <ryxi@stu.xidian.edu.cn>
>>>>
>>>> 	PR libstdc++/67214
>>>> 	* include/bits/locale_facets.tcc (_M_extract_int):
>>>> 	Â Â Add explicit conversion to avoid signed overflow.
>>>> ---
>>>> Â libstdc++-v3/include/bits/locale_facets.tcc | 3 ++-
>>>> Â 1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/libstdc++-v3/include/bits/locale_facets.tcc b/libstdc++-v3/include/bits/locale_facets.tcc
>>>> index 351190c..5f85d15 100644
>>>> --- a/libstdc++-v3/include/bits/locale_facets.tcc
>>>> +++ b/libstdc++-v3/include/bits/locale_facets.tcc
>>>> @@ -470,7 +470,8 @@ _GLIBCXX_BEGIN_NAMESPACE_LDBL
>>>> Â 	bool __testoverflow = false;
>>>> Â 	const __unsigned_type __max =
>>>> Â 	Â Â (__negative && __gnu_cxx::__numeric_traits<_ValueT>::__is_signed)
>>>> -	Â Â ? -__gnu_cxx::__numeric_traits<_ValueT>::__min
>>>> +	Â Â ? -static_cast<__unsigned_type>(__gnu_cxx::
>>>> +	Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â __numeric_traits<_ValueT>::__min)
>>>
>>>Do we need to keep the negation, or can we just cast to
>>>__unsigned_type?
>>
>>For 2's complement we can just cast to __unsigned_type. Â But for
>>clarity and other strange architectures I think we should keep
>>the negation.
>
>https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html
>"GCC only supports two's complement integer types"
>
>I doubt that's ever going to change, but keeping the negation also
>doesn't do any harm. I'll test and commit this, thanks.
>
>

Here's what I committed, which also adds a typedef for the
__numeric_traits type, to make everything more readable.

Tested powerpc64le-linux. Committed to trunk.


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