linux-kernel - Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap

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Message-ID: <20150326141730.GA23060@gmail.com>
Date:	Thu, 26 Mar 2015 15:17:31 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Guan Xuetao <gxt@...c.pku.edu.cn>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Arnd Bergmann <arnd@...db.de>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net,
	user-mode-linux-user@...ts.sourceforge.net,
	linux-arch@...r.kernel.org, linux-mm@...ck.org, cov@...eaurora.org,
	criu@...nvz.org
Subject: Re: [PATCH v3 2/2] powerpc/mm: Tracking vDSO remap


* Laurent Dufour <ldufour@...ux.vnet.ibm.com> wrote:

> > I argue we should use the right condition to clear vdso_base: if 
> > the vDSO gets at least partially unmapped. Otherwise there's 
> > little point in the whole patch: either correctly track whether 
> > the vDSO is OK, or don't ...
> 
> That's a good option, but it may be hard to achieve in the case the 
> vDSO area has been splitted in multiple pieces.
>
> Not sure there is a right way to handle that, here this is a best 
> effort, allowing a process to unmap its vDSO and having the 
> sigreturn call done through the stack area (it has to make it 
> executable).
> 
> Anyway I'll dig into that, assuming that the vdso_base pointer 
> should be clear if a part of the vDSO is moved or unmapped. The 
> patch will be larger since I'll have to get the vDSO size which is 
> private to the vdso.c file.

At least for munmap() I don't think that's a worry: once unmapped 
(even if just partially), vdso_base becomes zero and won't ever be set 
again.

So no need to track the zillion pieces, should there be any: Humpty 
Dumpty won't be whole again, right?

> > There's also the question of mprotect(): can users mprotect() the 
> > vDSO on PowerPC?
> 
> Yes, mprotect() the vDSO is allowed on PowerPC, as it is on x86, and 
> certainly all the other architectures. Furthermore, if it is done on 
> a partial part of the vDSO it is splitting the vma...

btw., CRIU's main purpose here is to reconstruct a vDSO that was 
originally randomized, but whose address must now be reproduced as-is, 
right?

In that sense detecting the 'good' mremap() as your patch does should 
do the trick and is certainly not objectionable IMHO - I was just 
wondering whether we could make a perfect job very simply.

Thanks,

	Ingo
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