Re: [PATCHv2] x86/mm: set x32 syscall bit in SET_PERSONALITY()

[Andy Lutomirski]

On Tue, Mar 21, 2017 at 11:09 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote:
> On 03/21/2017 08:45 PM, Andy Lutomirski wrote:
>>
>> On Tue, Mar 21, 2017 at 10:17 AM, Cyrill Gorcunov <gorcunov@xxxxxxxxx>
>> 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?

--Andy