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Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c

From Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups linux.kernel
Subject Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c
Date 2017-06-21 21:40 +0200
Message-ID <tUTDQ-2a7-1@gated-at.bofh.it> (permalink)
References <tUDIK-9t-5@gated-at.bofh.it> <tUDSp-cS-3@gated-at.bofh.it> <tUSy6-1pG-33@gated-at.bofh.it> <tUTDQ-2a7-3@gated-at.bofh.it>
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On Wed, Jun 21, 2017 at 12:33 PM, Oleg Nesterov <oleg@redhat.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

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Re: [lkp-robot] [mm]  1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-21 04:50 +0200
  Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-21 20:30 +0200
    Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-21 21:40 +0200
      Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-21 22:30 +0200
        Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-21 22:40 +0200
          Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-21 23:00 +0200
            Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-22 00:20 +0200
              Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-22 03:10 +0200
                Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-22 17:20 +0200
                Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-22 20:10 +0200
                Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-22 23:00 +0200
    Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-21 21:40 +0200
      Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-22 06:30 +0200
    Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-21 21:50 +0200

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