On 12/14/2015 10:58 AM, H. Peter Anvin wrote:
> On 12/11/15 09:52, Daniel Cashman wrote:
>> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
>> index 844b06d..647fecf 100644
>> --- a/arch/x86/mm/mmap.c
>> +++ b/arch/x86/mm/mmap.c
>> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
>> {
>> unsigned long rnd;
>>
>> - /*
>> - * 8 bits of randomness in 32bit mmaps, 20 address space bits
>> - * 28 bits of randomness in 64bit mmaps, 40 address space bits
>> - */
>> if (mmap_is_ia32())
>> - rnd = (unsigned long)get_random_int() % (1<<8);
>> +#ifdef CONFIG_COMPAT
>> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
>> +#else
>> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>> +#endif
>> else
>> - rnd = (unsigned long)get_random_int() % (1<<28);
>> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>>
>> return rnd << PAGE_SHIFT;
>> }
>>
>
> Now, you and I know that both variants can be implemented with a simple
> AND, but I have a strong suspicion that once this is turned into a
> variable, this will in fact be changed from an AND to a divide.
>
> So I'd prefer to use the
> "get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead.
Good point. Will change in v7 across patch-set.
Thank You,
Dan
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