linux-kernel - Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list

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Date:   Thu, 14 Oct 2021 12:44:27 +0000 (UTC)
From:   Michael Matz <matz@...e.de>
To:     Borislav Petkov <bp@...en8.de>
cc:     Willy Tarreau <w@....eu>,
        Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the
 clobber list

Hello,

On Wed, 13 Oct 2021, Borislav Petkov wrote:

> > > In short:  Ammars initial claim:
> > > 
> > > > Linux x86-64 syscall only clobbers rax, rcx and r11 (and "memory").
> > > > 
> > > >   - rax for the return value.
> > > >   - rcx to save the return address.
> > > >   - r11 to save the rflags.
> > > > 
> > > > Other registers are preserved.
> > > 
> > > is accurate and I will clarify the psABI to make that explicit.
> > 
> > Many thanks for this very detailed explanation! Ammar, I'll take your
> > patch.
> 
> Great, why are we dealing with some funky document when the law is in
> glibc sources?!

In theory, theory and practice are the same, in practice, they are not.

Usually it's good to resolve a conflict towards what the document says, or 
intended to say.  But glibc of course provides a huge amount of pressure 
to resolve toward it ;-)

(laws are also changed toward practice when the latter overtakes :) )


Ciao,
Michael.