linux-kernel - Re: [PATCH v7 5/6] powerpc/64: Make COMPAT user-selectable disabled on littleendian by default.

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Message-ID: <87h85us0xy.fsf@mpe.ellerman.id.au>
Date:   Tue, 03 Sep 2019 10:00:57 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Michal Suchánek <msuchanek@...e.de>
Cc:     linuxppc-dev@...ts.ozlabs.org,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Breno Leitao <leitao@...ian.org>,
        Michael Neuling <mikey@...ling.org>,
        Diana Craciun <diana.craciun@....com>,
        Firoz Khan <firoz.khan@...aro.org>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Joel Stanley <joel@....id.au>, Arnd Bergmann <arnd@...db.de>,
        Nicholas Piggin <npiggin@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v7 5/6] powerpc/64: Make COMPAT user-selectable disabled on littleendian by default.

Michal Suchánek <msuchanek@...e.de> writes:
> On Mon, 02 Sep 2019 12:03:12 +1000
> Michael Ellerman <mpe@...erman.id.au> wrote:
>
>> Michal Suchanek <msuchanek@...e.de> writes:
>> > On bigendian ppc64 it is common to have 32bit legacy binaries but much
>> > less so on littleendian.  
>> 
>> I think the toolchain people will tell you that there is no 32-bit
>> little endian ABI defined at all, if anything works it's by accident.
>
> I have seen a piece of software that workarounds code issues on 64bit
> by always compiling 32bit code. So it does work in some way.

What software is that?

> Also it has been pointed out that you can still switch to BE even with
> the 'fast-switch' removed.

Yes we have a proper syscall for endian switching, sys_switch_endian(),
which is definitely supported.

But that *only* switches the endian-ness of the process, it does nothing
to the syscall layer. So any process that switches to the other endian
must endian flip syscall arguments (that aren't in registers), or flip
back to the native endian before calling syscalls.

>> So I think we should not make this selectable, unless someone puts their
>> hand up to say they want it and are willing to test it and keep it
>> working.
>
> I don't really care either way.

Sure. We'll see if anyone else speaks up.

cheers