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