Subject: Re: [PATCHv2] x86/mm: set x32 syscall bit in SET_PERSONALITY()
To: Andy Lutomirski <luto@amacapital.net>
References: <20170321163712.20334-1-dsafonov@virtuozzo.com>
 <20170321171723.GB21564@uranus.lan>
 <CALCETrXoxRBTon8+jrYcbruYVUZASwgd-kzH-A96DGvT7gLXVA@mail.gmail.com>
 <6648805c-e0d8-5e27-9e19-602ab47937a7@virtuozzo.com>
 <CALCETrWvYERYaNscyQ3Q9rBUvVdzm1do86mMccnZzHsTMEn1HQ@mail.gmail.com>
CC: Cyrill Gorcunov <gorcunov@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Adam Borowski <kilobyte@angband.pl>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrei Vagin <avagin@gmail.com>, Borislav Petkov <bp@suse.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
From: Dmitry Safonov <dsafonov@virtuozzo.com>
Message-ID: <3ff42889-4ba3-15e5-0e77-b3bd1db7619f@virtuozzo.com>
Date: Tue, 21 Mar 2017 22:42:07 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.7.1
MIME-Version: 1.0
In-Reply-To: <CALCETrWvYERYaNscyQ3Q9rBUvVdzm1do86mMccnZzHsTMEn1HQ@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Transfer-Encoding: 7bit
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 21 Mar 2017 19:45:51.8155 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM5PR0801MB1731
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2910
Lines: 70

On 03/21/2017 10:31 PM, Andy Lutomirski wrote:
> On Tue, Mar 21, 2017 at 11:09 AM, Dmitry Safonov <dsafonov@virtuozzo.com> wrote:
>> On 03/21/2017 08:45 PM, Andy Lutomirski wrote:
>>>
>>> On Tue, Mar 21, 2017 at 10:17 AM, Cyrill Gorcunov <gorcunov@gmail.com>
>>> wrote:
>>>>
>>>> On Tue, Mar 21, 2017 at 07:37:12PM +0300, Dmitry Safonov wrote:
>>>> ...
>>>>>
>>>>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>>>>> index d6b784a5520d..d3d4d9abcaf8 100644
>>>>> --- a/arch/x86/kernel/process_64.c
>>>>> +++ b/arch/x86/kernel/process_64.c
>>>>> @@ -519,8 +519,14 @@ void set_personality_ia32(bool x32)
>>>>>               if (current->mm)
>>>>>                       current->mm->context.ia32_compat = TIF_X32;
>>>>>               current->personality &= ~READ_IMPLIES_EXEC;
>>>>> -             /* in_compat_syscall() uses the presence of the x32
>>>>> -                syscall bit flag to determine compat status */
>>>>> +             /*
>>>>> +              * in_compat_syscall() uses the presence of the x32
>>>>> +              * syscall bit flag to determine compat status.
>>>>> +              * On the bitness of syscall relies x86 mmap() code,
>>>>> +              * so set x32 syscall bit right here to make
>>>>> +              * in_compat_syscall() work during exec().
>>>>> +              */
>>>>> +             task_pt_regs(current)->orig_ax |= __X32_SYSCALL_BIT;
>>>>>               current->thread.status &= ~TS_COMPAT;
>>>>
>>>>
>>>> Hi! I must admit I didn't follow close the overall series (so can't
>>>> comment much here :) but I have a slightly unrelated question -- is
>>>> there a way to figure out if task is running in x32 mode say with
>>>> some ptrace or procfs sign?
>>>
>>>
>>> You should be able to figure out of a *syscall* is x32 by simply
>>> looking at bit 30 in the syscall number.  (This is unlike i386, which
>>> is currently not reflected in ptrace.)
>>
>>
>> The process could be stopped with PTRACE_SEIZE and I think, it'll not
>> have x32 syscall bit at that moment.
>>
>> I guess the question comes from that we're releasing CRIU 3.0 with
>> 32-bit C/R and some other cool stuff, but we don't support x32 yet.
>> As we don't want release a thing that we aren't properly testing.
>> So for a while we should error on dumping x32 applications.
>
> I'm curious: shouldn't x32 CRIU just work?  What goes wrong?

I also think, it should be quite easy to add, as we have arch_prctl() 
for vdso and etc.
But there are things, which will not work if we just dump application
as 64-bit.

For example, what comes to mind: sys_get_robust_list(), it has different 
pointers for 64-bit or for x32/ia32 applications: robust_list
and compat_robust_list. So during C/R we should sometimes call
compatible syscalls for x32 applications to dump/restore, as for futex
list e.g., native will return NULL or empty list.

>
> --Andy
>


-- 
              Dmitry