linux-kernel - Re: [PATCH] um: make personality(PER_LINUX32) work on x86

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Message-ID: <CANP3RGdxrKHsERYG+yW5fpRUrahkBJbHCKD24v182ZNKuJgfwg@mail.gmail.com>
Date: Mon, 19 Aug 2024 11:46:31 -0700
From: Maciej Żenczykowski <maze@...gle.com>
To: Johannes Berg <johannes@...solutions.net>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Richard Weinberger <richard@....at>, 
	Anton Ivanov <anton.ivanov@...bridgegreys.com>, linux-um@...ts.infradead.org
Subject: Re: [PATCH] um: make personality(PER_LINUX32) work on x86_64

On Mon, Aug 19, 2024 at 5:23 AM Johannes Berg <johannes@...solutions.net> wrote:
>
> On Tue, 2024-08-13 at 16:47 -0700, Maciej Żenczykowski wrote:
> > Without this patch:
> >   #!/usr/bin/python3
> >   import ctypes
> >   import os
> >   personality = ctypes.CDLL(None).personality
> >   personality.restype = ctypes.c_int
> >   personality.argtypes = [ctypes.c_ulong]
> >   PER_LINUX32=8
> >   personality(PER_LINUX32)
> >   print(os.uname().machine)
> > returns:
> >   x86_64
> > instead of the desired:
> >   i686
> >
>
> But ... why should it work? UML has no 32-bit compat support anyway.

Well, that's certainly a fair point.
On 'native' x86_64 this works even for 64-bit processes though.
I wonder if that, in itself, is a feature or a bug...

In my case I was writing some debug code (to print the environment
some test code is running in, since I think it was failing due to
running 32-bit code in PER_LINUX32 on 64-bit arm) and testing (the
test code) on x86_64 UML.  I was surprised to discover the difference
in UML vs my host desktop.

> johannes

--
Maciej Żenczykowski, Kernel Networking Developer @ Google