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Re: glibc 2.29: Reminder, only ~3 weeks to freeze!


Hi Florian,

Le 15/12/2018 à 16:23, Florian Weimer a écrit :
> * Romain Naour:
> 
>> Hi Carlos, Joseph,
>>
>> Le 14/12/2018 à 22:14, Joseph Myers a écrit :
>>> On Fri, 14 Dec 2018, Romain Naour wrote:
>>>
>>>> Actually there is a regression when the toolchain is built with glibc 2.29 and
>>>> gcc 4.9 (when USE_ATOMIC_COMPILER_BUILTINS is not defined):
>>>
>>> What platform is this on?
>>
>> It's on x86 platform.
>>
>> There is another issue x86_64 with GCC 4.9:
>>
>> blob_repeat.c:121:3: error: implicit declaration of function
>> '__builtin_mul_overflow' [-Werror=implicit-function-declaration]
>>    if (__builtin_mul_overflow (page_size >> common_zeros, element_size,
>>    ^
> 
> We should fix that because it's been backported to the release
> branches.  Please try the patch below.

Thanks for the patch, it fixes this issue.

Tested-by: Romain Naour <romain.naour@gmail.com>

Best regards,
Romain

> 
> Thanks,
> Florian
> 
> -----
> Subject: support: Do not require overflow builtin in support/blob_repeat.c
> 
> It is only available in GCC 5 and later.
> 
> 2018-12-15  Florian Weimer  <fweimer@redhat.com>
> 
> 	* support/blob_repeat.c (check_mul_overflow_size_t): New function.
> 	(minimum_stride_size): Use it.
> 	(support_blob_repeat_allocate): Likewise.
> 
> diff --git a/support/blob_repeat.c b/support/blob_repeat.c
> index 16c1e448b9..718846d81d 100644
> --- a/support/blob_repeat.c
> +++ b/support/blob_repeat.c
> @@ -34,6 +34,26 @@
>     optimization because mappings carry a lot of overhead.  */
>  static const size_t maximum_small_size = 4 * 1024 * 1024;
>  
> +/* Set *RESULT to LEFT * RIGHT.  Return true if the multiplication
> +   overflowed.  See <malloc/malloc-internal.h>.  */
> +static inline bool
> +check_mul_overflow_size_t (size_t left, size_t right, size_t *result)
> +{
> +#if __GNUC__ >= 5
> +  return __builtin_mul_overflow (left, right, result);
> +#else
> +  /* size_t is unsigned so the behavior on overflow is defined.  */
> +  *result = left * right;
> +  size_t half_size_t = ((size_t) 1) << (8 * sizeof (size_t) / 2);
> +  if (__glibc_unlikely ((left | right) >= half_size_t))
> +    {
> +      if (__glibc_unlikely (right != 0 && *result / right != left))
> +        return true;
> +    }
> +  return false;
> +#endif
> +}
> +
>  /* Internal helper for fill.  */
>  static void
>  fill0 (char *target, const char *element, size_t element_size,
> @@ -118,8 +138,8 @@ minimum_stride_size (size_t page_size, size_t element_size)
>       common multiple, it appears only once.  Therefore, shift one
>       factor.  */
>    size_t multiple;
> -  if (__builtin_mul_overflow (page_size >> common_zeros, element_size,
> -                              &multiple))
> +  if (check_mul_overflow_size_t (page_size >> common_zeros, element_size,
> +                                 &multiple))
>      return 0;
>    return multiple;
>  }
> @@ -255,7 +275,7 @@ support_blob_repeat_allocate (const void *element, size_t element_size,
>                                size_t count)
>  {
>    size_t total_size;
> -  if (__builtin_mul_overflow (element_size, count, &total_size))
> +  if (check_mul_overflow_size_t (element_size, count, &total_size))
>      {
>        errno = EOVERFLOW;
>        return (struct support_blob_repeat) { 0 };
> 


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