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

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Message-ID: <8147f571-0da2-f20c-31dd-72ad56eb1ba0@zytor.com>
Date:   Sat, 23 Oct 2021 13:40:59 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Borislav Petkov <bp@...en8.de>,
        Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>
Cc:     Willy Tarreau <w@....eu>, 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
Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the
 clobber list



On 10/13/21 05:43, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 05:23:11AM +0700, Ammar Faizi wrote:
>> "Figure 3.4: Register Usage" is not the answer, if it were, nolibc.h
>> would be broken as it is missing "rdi", "rsi", "rdx" in the clobber list.
> 
> It is not about what happens in practice but what the contract is:
> syscall argument registers can potentially get clobbered and userspace
> should treat them as such. Because if the kernel decides to actually
> clobber them for whatever reason and some userspace thing thinks
> otherwise, then it is the userspace thing's problem as it doesn't adhere
> to the well known ABI.
> 

Currently the kernel doesn't, but some past kernels have zeroed some of 
these registers rather than preserving them.

	-hpa