linux-kernel - Re: [PATCHv3] x86/selftests: add clobbers for int80 on x86

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Date:   Mon, 13 Feb 2017 20:30:34 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>
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>,
        "H. Peter Anvin" <hpa@...or.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


* 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.

I think we should try that...

Thanks,

	Ingo