Re: [CRIU] [PATCH] Add VDSO time function support for x86 32-bit kernel

From: "H. Peter Anvin" <hpa@zytor.com>
To: John Stultz <john.stultz@linaro.org>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>,
	Andy Lutomirski <luto@amacapital.net>,
	aarcange@redhat.com, ak@linux.intel.com,
	Pavel Emelyanov <xemul@parallels.com>,
	Stefani Seibold <stefani@seibold.net>,
	x86@kernel.org, linux-kernel@vger.kernel.org, criu@openvz.org,
	mingo@redhat.com, tglx@linutronix.de
Subject: Re: [CRIU] [PATCH] Add VDSO time function support for x86 32-bit kernel
Date: Fri, 14 Dec 2012 15:55:04 -0800	[thread overview]
Message-ID: <50CBBC58.4080307@zytor.com> (raw)
In-Reply-To: <50CBBACA.2070108@linaro.org>

On 12/14/2012 03:48 PM, John Stultz wrote:
> On 12/14/2012 02:48 PM, H. Peter Anvin wrote:
>> On 12/14/2012 02:43 PM, Cyrill Gorcunov wrote:
>>> On Fri, Dec 14, 2012 at 02:27:08PM -0800, H. Peter Anvin wrote:
>>>
>>>
>>> This won't help in case of scenario you've been pointing in
>>> previous email (where c/r happens in a middle of vdso),
>>> would it? Because we still need somehow to be sure we're not
>>> checkpointing in a middle of signal handler which will return
>>> to some vdso place.
>> It is okay if and only if those vdso places never change... which I
>> think is doable if they only contain trival system call wrappers, i.e.
>> something like:
>>
>>     movl $__SYS_gettimeofday, %eax
>>     syscall
>>     ret
> 
> Though doesn't this make it easier for exploits (somewhat undoing ASLR)?
> I know Andi always wanted to avoid having syscall instructions at a
> fixed location for the old vsyscall code (though I know we had it
> none-the-less for awhile).   But maybe I'm confusing issues here?
> 

They aren't in fixed addresses across processes... the vdso location can
still be randomized.  It just has to be the same across the
checkpoint/restart operation, just like all the other instructions.

	-hpa