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

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Date:	Tue, 20 May 2014 22:39:07 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.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 11:24:56AM -0700, Andy Lutomirski wrote:
> 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].

I fear I don't understand the phrase "leading slash in /proc/self/maps".
Peter could you rephrase please?

> >>>>   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.

yes, fwiw we can deal with it currently but i'm not sure yet about
compat case simply because didn't look presicely.
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