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Date: Mon, 25 Aug 2014 21:09:56 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Hugh Dickins <hughd@google.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Manfred Spraul <manfred@colorfullife.com>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Kees Cook <keescook@chromium.org>, Tejun Heo <tj@kernel.org>,
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        Pavel Emelyanov <xemul@parallels.com>,
        Vasiliy Kulikov <segoon@openwall.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Julien Tinnes <jln@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack
	check
Message-ID: <20140825190956.GA25759@redhat.com>
References: <20140823144246.GA6281@redhat.com> <20140823144327.GA6299@redhat.com> <20140823152240.GH25918@moon> <20140823155832.GA9406@redhat.com> <alpine.LSU.2.11.1408241818530.8474@eggly.anvils> <20140825150301.GB3021@redhat.com> <alpine.LSU.2.11.1408250856020.24245@eggly.anvils> <20140825171820.GB8338@redhat.com> <alpine.LSU.2.11.1408251135540.24957@eggly.anvils>
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On 08/25, Hugh Dickins wrote:
>
> 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;

I already did this ;) I'll send v2 patch in a minute.

> But I didn't understand your "to guarantee 4 we should check 5".

I just tried to say that I fully agree with your point.

I didn't dare to preserve your acks although the patch is almost the
same, I'll appreciate it if you can ack v2 too.

Oleg.