linux-kernel - Re: [PATCHv2] x86/mm: set x32 syscall bit in SET

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Date:   Tue, 21 Mar 2017 10:45:57 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     Dmitry Safonov <dsafonov@...tuozzo.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Adam Borowski <kilobyte@...band.pl>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrei Vagin <avagin@...il.com>, Borislav Petkov <bp@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv2] x86/mm: set x32 syscall bit in SET_PERSONALITY()

On Tue, Mar 21, 2017 at 10:17 AM, Cyrill Gorcunov <gorcunov@...il.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.)

Do we actually have an x32 per-task mode at all?  If so, maybe we can
just remove it on top of Dmitry's series.