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Date: Sat, 23 Aug 2014 19:22:40 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>,
        Manfred Spraul <manfred@colorfullife.com>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
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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: <20140823152240.GH25918@moon>
References: <20140823144246.GA6281@redhat.com>
 <20140823144327.GA6299@redhat.com>
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On Sat, Aug 23, 2014 at 04:43:27PM +0200, Oleg Nesterov wrote:
> The ->start_stack check in do_shmat() looks ugly and simply wrong.
> 
> 1. ->start_stack is only valid right after exec(), the application
>    can switch to another stack and even unmap this area.
> 
> 2. The reason for this check is not clear at all. The application
>    should know what it does. And why 4 pages? And why in fact it
>    requires 5 pages?
> 
> 3. This wrongly assumes that the stack can only grown down.
> 
> Personally I think we should simply kill this check, but I did not
> dare to do this. So the patch only fixes the 1st problem (mostly to
> avoid the usage of mm->start_stack) and ignores VM_GROWSUP.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>

I don't understand this check either, the comment above it says nothing
but only commits what code is doing not explaining why.