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

From: hpa@zytor.com
To: Andy Lutomirski <luto@amacapital.net>,
	Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Dmitry Safonov <dsafonov@virtuozzo.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>, Andy Lutomirski <luto@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCHv2] x86/mm: set x32 syscall bit in SET_PERSONALITY()
Date: Tue, 21 Mar 2017 11:49:37 -0700	[thread overview]
Message-ID: <13EAF4BE-144F-47D6-8A38-3B6D97ACFF8A@zytor.com> (raw)
In-Reply-To: <CALCETrXoxRBTon8+jrYcbruYVUZASwgd-kzH-A96DGvT7gLXVA@mail.gmail.com>

On March 21, 2017 10:45:57 AM PDT, Andy Lutomirski <luto@amacapital.net> 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.)
>
>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.

We do, for things like signal delivery mostly.  We have tried relying on it as little as possible, intentionally.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.