linux-kernel - Re: [PATCH 6/8] mm: Allow arch specific arch_randomize_brk() with CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP

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Date:   Mon, 22 Nov 2021 12:47:13 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Alex Ghiti <alex@...ti.fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 6/8] mm: Allow arch specific arch_randomize_brk() with
 CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT

Le 22/11/2021 à 12:22, Alex Ghiti a écrit :
> Hi Christophe,
> 
> Le 22/11/2021 à 09:48, Christophe Leroy a écrit :
>> Commit e7142bf5d231 ("arm64, mm: make randomization selected by
>> generic topdown mmap layout") introduced a default version of
>> arch_randomize_brk() provided when
>> CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT is selected.
>>
>> powerpc could select CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
>> but needs to provide its own arch_randomize_brk().
>>
>> In order to allow that, don't make
>> CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT select
>> CONFIG_ARCH_HAS_ELF_RANDOMIZE. Instead, ensure that
>> selecting CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and
>> selecting CONFIG_ARCH_HAS_ELF_RANDOMIZE has the same effect.
> 
> This feels weird to me since if CONFIG_ARCH_HAS_ELF_RANDOMIZE is used 
> somewhere else at some point, it is not natural to add 
> CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT: can't we use a __weak 
> function or a new CONFIG_ARCH_HAS_RANDOMIZE_BRK?

Yes I also found things a bit weird.

CONFIG_ARCH_HAS_RANDOMIZE_BRK could be an idea but how different would 
it be from CONFIG_ARCH_HAS_ELF_RANDOMIZE ? In fact I find it weird that 
CONFIG_ARCH_HAS_ELF_RANDOMIZE is selected by 
CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and not by the arch itself.

On the other hand CONFIG_ARCH_HAS_ELF_RANDOMIZE also handles 
arch_mmap_rnd() and here we are talking about arch_randomize_brk() only.

In the begining I was thinking about adding a 
CONFIG_ARCH_WANT_DEFAULT_RANDOMIZE_BRK, but it was meaning adding it to 
the few other arches selecting CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT.

So I think I will go for the __weak function option.

Thanks
Christophe