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Date:   Fri, 17 Feb 2017 09:03:26 -0800
From:   Andy Lutomirski <>
To:     "H. Peter Anvin" <>
Cc:     Dmitry Safonov <>,
        "" <>,
        Dmitry Safonov <>,
        Shuah Khan <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,
        Andy Lutomirski <>,
        Borislav Petkov <>, X86 ML <>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
Subject: Re: [PATCHv3] x86/selftests: add clobbers for int80 on x86_64

On Mon, Feb 13, 2017 at 2:18 PM,  <> wrote:
> On February 13, 2017 11:06:04 AM PST, Andy Lutomirski <> wrote:
>>On Mon, Feb 13, 2017 at 2:13 AM, Dmitry Safonov
>><> wrote:
>>> Kernel erases R8..R11 registers prior returning to userspace
>>> from int80:
>>> 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:
>>> 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 <>
>>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.
> 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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