linux-kernel - Changing the ARM variant reported by 'uname -m'

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Message-ID: <e681bef0-353b-cdcb-18a1-361105eb1fe9@iki.fi>
Date:   Wed, 18 Oct 2017 18:07:22 +0300
From:   Tuomas Tynkkynen <tuomas.tynkkynen@....fi>
To:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Subject: Changing the ARM variant reported by 'uname -m'

Hi,

For our distro (NixOS), I've experimented with building packages for 
ARMv6 on ARMv7 hardware. This has generally worked fine, except that 
some build scripts (in coreutils, for example) notice that `uname -m` 
returns 'armv7l' and thus decide to e.g. add some ARMv7-specific gcc 
flags, which makes the binaries not run on actual ARMv6 devices.

So to avoid that, it would be necessary to have a way to make `uname -m` 
return a different ARM architecture version that the hardware has. After 
all, there is precedent for doing this: calling personality(PER_LINUX32) 
turns 'x86_64' to 'i686' and so on for other architectures.

I can see two ways of implementing this:

1) Keep using the personality() call by reserving some PER_* constants 
for architectures to use and decide what to map them to. E.g. the ARM 
implementation would require 8 values for choosing one of:
        armv4 armv4t armv5t armv5te armv5tej armv6 armv7 armv7m
plus automatically appending 'b' or 'l' depending on endianness.

2) Add a new setmachine() system call that allows setting the machine 
    name to an arbitrary string inside a non-initial UTS namespace.