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

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Message-ID: <CA+55aFyTRPOuzo2-Q0xM4+D6naGxFtaE_7dbxXOOdwQ4QGPfpw@mail.gmail.com>
Date:   Wed, 21 Jun 2017 12:39:33 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Oleg Nesterov <oleg@...hat.com>
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 Wed, Jun 21, 2017 at 12:33 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> -               if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
> +if (0)         if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {

This smells bad.

That test is not about grow-down or even the guard page. That test is
that it's always wrong to grow down the stack below %esp.

Except we allow some slop, because certain instructions take the page
fault before actually updating %rsp.

So that patch is not correct. We want a page fault (and *not* expand
the stack) if somebody accesses below the stack pointer.

If we had a regression, it's due to something else.

                  Linus