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

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Date:   Thu, 16 Dec 2021 14:27:53 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nicholas Piggin <npiggin@...il.com>,
        "alex@...ti.fr" <alex@...ti.fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v4 09/10] powerpc/mm: Convert to default topdown mmap
 layout



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.
> 

Ok, in v5 I added that change in patch 10 then switched this patch with 
that patch.

Christophe