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Date:   Sat, 23 Oct 2021 13:40:59 -0700
From:   "H. Peter Anvin" <>
To:     Borislav Petkov <>,
        Ammar Faizi <>
Cc:     Willy Tarreau <>, Paul Walmsley <>,
        Palmer Dabbelt <>,
        Albert Ou <>,
        Linux Kernel Mailing List <>,
        Andy Lutomirski <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,
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.


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