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Date:   Fri, 17 Feb 2017 09:03:26 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Dmitry Safonov <dsafonov@...tuozzo.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>, X86 ML <x86@...nel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCHv3] x86/selftests: add clobbers for int80 on x86_64

On Mon, Feb 13, 2017 at 2:18 PM,  <hpa@...or.com> wrote:
> On February 13, 2017 11:06:04 AM PST, Andy Lutomirski <luto@...capital.net> wrote:
>>On Mon, Feb 13, 2017 at 2:13 AM, Dmitry Safonov
>><dsafonov@...tuozzo.com> wrote:
>>> Kernel erases R8..R11 registers prior returning to userspace
>>> from int80: https://lkml.org/lkml/2009/10/1/164
>>>
>>> GCC can reuse this registers and doesn't expect them to change
>>> during syscall invocation. I met this kind of bug in CRIU once
>>> gcc 6.1 and clang stored local variables in those registers
>>> and the kernel zerofied them during syscall:
>>>
>>https://github.com/xemul/criu/commit/990d33f1a1cdd17bca6c2eb059ab3be2564f7fa2
>>>
>>> By that reason I suggest to add those registers to clobbers
>>> in selftests.  Also, as notted by Andy - removed unneeded clobber
>>> for flags in int 0x80 inline asm.
>>
>>Acked-by: Andy Lutomirski <luto@...nel.org>
>>
>>Although these days we could preserve these regs in the kernel at the
>>cost of (I think) 4 bytes of text and zero runtime overhead.
>>
>>--Andy
>
> If we can do so that cheaply I think we should.

Sure, but we should still apply Dmitry's patches first.

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