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Date:   Sat, 23 Oct 2021 13:40:59 -0700
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Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the
 clobber list
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To:     Borislav Petkov <bp@alien8.de>,
        Ammar Faizi <ammar.faizi@students.amikom.ac.id>
Cc:     Willy Tarreau <w@1wt.eu>, Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org
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 <20211012222311.578581-1-ammar.faizi@students.amikom.ac.id>
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From:   "H. Peter Anvin" <hpa@zytor.com>
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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