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

[Oleg Nesterov]

On 06/21, Linus Torvalds wrote:
>
> On Wed, Jun 21, 2017 at 1:27 PM, Oleg Nesterov <oleg@xxxxxxxxxx> 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.