Re: New vsyscall emulation breaks JITs

["H. Peter Anvin" <hpa@zytor.com>]

Greg Lueck <lueckintel@yahoo.com> wrote:

>Yes, this sounds like a cleaner solution.  What happens, though, if the
>system call is interrupted by a signal or by ptrace(ATTACH)?  Does RIP
>point at the target of the RET instruction?  Is it moved back to the
>entry of the vsyscall page?  Does it point immediately after the
>SYSCALL instruction?  GDB might also care about these details.
>
>> That's a fun corner case.  Is the problem that you might receive a
>> signal while single-stepping?
>
>
>Actually, the situation is more difficult.  The application may have
>received a signal while inside the gate, sometime before the SYSENTER
>trap.  The signal context frame on the application's stack now has RIP
>pointing someplace inside the gate.  At this point, Pin attaches to the
>native process, and it has no reasonable way to know about the saved
>context with this RIP value.  Later, the application (running under
>Pin) will return from its handler and resume execution in the middle of
>the gate code.  What can Pin do here?  It' s too late to execute
>natively at the start of the gate.  If Pin executes natively at the
>signal return point, Pin will lose control of the application and it
>will execute natively from that point forward.
>
>-- Greg
>
>
>
>
>________________________________
>From: Linus Torvalds <torvalds@linux-foundation.org>
>To: Andrew Lutomirski <luto@mit.edu>
>Cc: Greg Lueck <lueckintel@yahoo.com>; H. Peter Anvin <hpa@zytor.com>;
>Andi Kleen <andi@firstfloor.org>; "x86@kernel.org" <x86@kernel.org>;
>"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>;
>"kimwooyoung@gmail.com" <kimwooyoung@gmail.com>
>Sent: Tuesday, August 9, 2011 6:36 PM
>Subject: Re: New vsyscall emulation breaks JITs
>
>On Tue, Aug 9, 2011 at 2:04 PM, Andrew Lutomirski <luto@mit.edu> wrote:
>>
>> Here's a different proposal, then:
>>
>> What if the kernel had the sequence:
>>
>> mov $__NR_whatever,%eax
>> syscall
>> ret
>>
>> in the vsyscall page but marked the vsyscall page NX.
>
>This sounds like a sound idea. And then the difference between "fast
>and native" and "slow and trapping" ends up literally being just the
>NX bit.
>
>                        Linus

The logical answer is that rip will point to the entry to the vsyscall page.
-- 
Sent from my mobile phone. Please excuse my brevity and lack of formatting.