Re: [PATCH 3/4] x86,mm: Improve _install_special_mapping and fix x86 vdso naming

From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>, X86 ML <x86@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sasha Levin <sasha.levin@oracle.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Dave Jones <davej@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Pavel Emelyanov <xemul@parallels.com>
Subject: Re: [PATCH 3/4] x86,mm: Improve _install_special_mapping and fix x86 vdso naming
Date: Tue, 20 May 2014 11:27:50 -0700	[thread overview]
Message-ID: <537B9EA6.3030103@zytor.com> (raw)
In-Reply-To: <CALCETrWmKvox1poGK5fBw2OBip7zMpjb-bpYrzd4EGHPDvZEHg@mail.gmail.com>

On 05/20/2014 11:24 AM, Andy Lutomirski wrote:
> On Tue, May 20, 2014 at 11:18 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 05/20/2014 11:01 AM, Cyrill Gorcunov wrote:
>>>>
>>>> This patch should fix this issue, at least.  If there's still a way to
>>>> get a native vdso that doesn't say "[vdso]", please let me know/
>>>
>>> Yes, having a native procfs way to detect vdso is much preferred!
>>>
>>
>> Is there any path by which we can end up with [vdso] without a leading
>> slash in /proc/self/maps?  Otherwise, why is that not "native"?
> 
> Dunno.  But before this patch the reverse was possible: we can end up
> with a vdso that doesn't say [vdso].
> 

That's a bug, which is being fixed.  We can't go back in time and create
new interfaces on old kernels.

>>
>>>>>   The situation get worse when task was dumped on one kernel and
>>>>> then restored on another kernel where vdso content is different
>>>>> from one save in image -- is such case as I mentioned we need
>>>>> that named vdso proxy which redirect calls to vdso of the system
>>>>> where task is restoring. And when such "restored" task get checkpointed
>>>>> second time we don't dump new living vdso but save only old vdso
>>>>> proxy on disk (detecting it is a different story, in short we
>>>>> inject a unique mark into elf header).
>>>>
>>>> Yuck.  But I don't know whether the kernel can help much here.
>>>
>>> Some prctl which would tell kernel to put vdso at specifed address.
>>> We can live without it for now so not a big deal (yet ;)
>>
>> mremap() will do this for you.
> 
> Except that it's buggy: it doesn't change mm->context.vdso.  For
> 64-bit tasks, the only consumer outside exec was arch_vma_name, and
> this patch removes even that.  For 32-bit tasks, though, it's needed
> for signal delivery.
> 

Again, a bug, let's fix it rather than saying we need a new interface.

	-hpa