linux-kernel - Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start

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Date:	Mon, 25 Aug 2014 19:18:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Julien Tinnes <jln@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack
	check

On 08/25, Hugh Dickins wrote:
>
> On Mon, 25 Aug 2014, Oleg Nesterov wrote:
> > On 08/24, Hugh Dickins wrote:
> > >
> > > I'd say it comes earlier, from Christoph Rohland's 2.4.17-pre7's
> > > "Add missing checks on shmat()", though I didn't find more than that.
> > >
> > > We can all understand wanting to leave a gap below the growsdown stack,
> > > but of course could argue about growsup and 1 or 4 or 5 or whatever:
> >
> > And it is not clear to me why the kernel should care at all,
>
> Care about what exactly?  Leaving a gap between shm and stack?
>
> The man page says that (unless SHM_REMAP) shmat() will fail with
> EINVAL if a mapping already exists there, and I think it's fair
> to regard the vm_start of a VM_GROWSDOWN somewhat elastically.
>
> It may be that Linus's check_stack_guard_page() work in 2.6.36
> changed the importance of this shmat() check, but I'd still feel
> safer to leave it as is (while turning a blind eye to the
> VM_GROWSUP omission).
>
> >
> > > okay that we're all more interested in just removing that start_stack.
> >
> > so perhaps v2 should simply remove it? Or do you think it would be safer
> > to not do this?
>
> It would be safer to leave it, but replace the start_stack use as you did.

OK, thanks.

> And I think I'll let Linus's guard page justify your 4 (to match comment)
> in place of the original's mysterious 5.

Ah, thanks again. Yes, if we want to guarantee 4 pages we should check 5.

Although obviously this doesn't explain the original's 5, this was
written before check_stack_guard_page().

OK.

Oleg.

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