linux-kernel - Re: [PATCH 3/4] x86,mm: Improve _install_special

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Message-ID: <CALCETrWmKvox1poGK5fBw2OBip7zMpjb-bpYrzd4EGHPDvZEHg@mail.gmail.com>
Date:	Tue, 20 May 2014 11:24:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>, X86 ML <x86@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Jones <davej@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH 3/4] x86,mm: Improve _install_special_mapping and fix x86
 vdso naming

On Tue, May 20, 2014 at 11:18 AM, H. Peter Anvin <hpa@...or.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].

>
>>>>   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.

--Andy
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