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Date: Sun, 24 Aug 2014 18:38:44 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
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To: Oleg Nesterov <oleg@redhat.com>
cc: Cyrill Gorcunov <gorcunov@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Manfred Spraul <manfred@colorfullife.com>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Kees Cook <keescook@chromium.org>, Tejun Heo <tj@kernel.org>,
        Andrew Vagin <avagin@openvz.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        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
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References: <20140823144246.GA6281@redhat.com> <20140823144327.GA6299@redhat.com> <20140823152240.GH25918@moon> <20140823155832.GA9406@redhat.com>
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On Sat, 23 Aug 2014, Oleg Nesterov wrote:
> On 08/23, Cyrill Gorcunov wrote:
> >
> > 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>

Yes, much better to use find_vma than have this strange stray use
of unreliable start_stack.

Acked-by: Hugh Dickins <hughd@google.com>

though like Manfred I didn't quite see how overflow was impossible
on unfamiliar architectures.

> 
> Thanks!
> 
> > I don't understand this check either, the comment above it says nothing
> > but only commits what code is doing not explaining why.
> 
> Yes, and this check predates the git history. I even looked into
> git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git but this
> change was added by the huge "v2.5.0.7 -> v2.5.0.8" update in 2002,
> and obviously without any explanation (apart from "fix up proper shmat
> semantics", but this connects SHM_REMAP itself).

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:
okay that we're all more interested in just removing that start_stack.

Hugh