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

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Date:   Fri, 10 Feb 2017 08:45:17 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dmitry Safonov <dsafonov@...tuozzo.com>
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 Fri, Feb 10, 2017 at 8:28 AM, Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
> 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.
>

Woudl the right answer be to get rid of "flags" in the test where it
appears?  I'm not sure it's needed in the first place.