linux-kernel - Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at

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Date:   Wed, 21 Jun 2017 22:56:17 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        kernel test robot <xiaolong.ye@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>
Subject: Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c

On 06/21, Linus Torvalds wrote:
>
> On Wed, Jun 21, 2017 at 1:27 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > Now __do_page_fault() tries to expand the stack itself, and this check
> > fails.
>
> But we want that check to trigger and cause the access to fail.
> Accessing the stack below the stack pointer is wrong.

I understand. My point is that this check was invalidated by stack-guard-page
a long ago, and this means that we add the user-visible change now.

> Do you have a pointer to the report for this regression? I must have missed it.

See http://marc.info/?t=149794523000001&r=1&w=2

this thread is a bit confusing, the most relevant emails are

	http://marc.info/?l=linux-kernel&m=149806256621440&w=2 (with test case)
	http://marc.info/?l=linux-kernel&m=149806452322233&w=2

Actually, I got another (redhat internal) bug report after this discussion,
and at first glance this is the same thing.

Just in case, I agree that mmap(MAP_GROWSDOWN) is mostly useless, perhaps
we do not even care. But still this is regression.

Oleg.