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

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Message-Id: <1639044621.jeow25j0pr.astroid@bobo.none>
Date:   Thu, 09 Dec 2021 20:15:42 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     "alex@...ti.fr" <alex@...ti.fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        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

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.

Thanks,
Nick