Excerpts from Christophe Leroy's message of December 9, 2021 8:22 pm:

Le 09/12/2021 à 11:15, Nicholas Piggin a écrit :

quoted

Excerpts from Christophe Leroy's message of December 9, 2021 3:18 am:

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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.

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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 ?

https://patchwork.ozlabs.org/project/linuxppc-dev/patch/7dabf1cbde67a346a187881d4f0bd17347e0334a.1533732583.git.christophe.leroy@c-s.fr/

Yes.

Thanks,
Nick