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

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Message-ID: <da7dd44a-fe2a-8e2f-e301-02e8d6e7fb11@virtuozzo.com>
Date:   Fri, 10 Feb 2017 19:28:59 +0300
From:   Dmitry Safonov <dsafonov@...tuozzo.com>
To:     Andy Lutomirski <luto@...capital.net>
CC:     "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: [PATCH] x86/selftests: add clobbers for int80 on x86_64

On 02/10/2017 07:13 PM, Andy Lutomirski wrote:
> On Fri, Feb 10, 2017 at 3:52 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.
>
> Seems reasonable, but presumably INT80_CLOBBERS should be defined the
> same way in all the tests.  IOW, if the "flags" clobber is actually
> needed, it should be "flags", INT80_CLOBBERS (possibly without the
> comma if it's problematic).
>

Well, that was my initial attempt: I've defined it as:
+# define INT80_CLOBBERS , "r8", "r9", "r10", "r11"

But that hanging comma looks awful, so I added "flags" there.
And if I do define it without coma and leave it in asm statement,
32-bit version would be unhappy.
So, I found that it's easier to define it with flags included.

-- 
              Dmitry