Re: [PATCH v4 09/10] powerpc/mm: Convert to default topdown mmap layout

Christophe Leroy
2021-12-09T10:22:11Z

Le 09/12/2021 à 11:15, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of December 9, 2021 3:18 am:
>> Select CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and
>> remove arch/powerpc/mm/mmap.c
>>
>> This change provides standard randomisation of mmaps.
>>
>> See commit 8b8addf891de ("x86/mm/32: Enable full randomization on i386
>> and X86_32") for all the benefits of mmap randomisation.
>
> The justification seems pretty reasonable.
>
>>
>> Comparison between powerpc implementation and the generic one:
>> - mmap_is_legacy() is identical.
>> - arch_mmap_rnd() does exactly the same allthough it's written
>> slightly differently.
>> - MIN_GAP and MAX_GAP are identical.
>> - mmap_base() does the same but uses STACK_RND_MASK which provides
>> the same values as stack_maxrandom_size().
>> - arch_pick_mmap_layout() is almost identical. The only difference
>> is that it also adds the random factor to mm->mmap_base in legacy mode.
>>
>> That last point is what provides the standard randomisation of mmaps.
>
> Thanks for describing it. Could you add random_factor to mmap_base for
> the legacy path for powerpc as a 2-line change that adds the legacy
> randomisation. And then this bigger patch would be closer to a no-op.
>

You mean you would like to see the following patch before doing the
convert ?