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

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Date:	Mon, 25 Aug 2014 11:51:47 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Hugh Dickins <hughd@...gle.com>,
	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 Mon, 25 Aug 2014, Oleg Nesterov wrote:
> On 08/25, Hugh Dickins wrote:
> 
> > 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().

I meant, you changed the coded 5 to 4, to match the comment above it;
and if I were insistent on maximizing back-compatible-safety, I would
say that 4 should go back to 5; but since Linus added the guard page
gap at next->vm_start in between, I don't mind if you stick with 4
there to match the comment.

Disclaimer: I may be mistaken in each clause of that paragraph,
and you may prefer to ignore everything I wrote there as worthless!
But I didn't understand your "to guarantee 4 we should check 5".

Hugh
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