Re: [PATCH v2] ipc/shm: fix the historical/wrong mm->start_stack check

[Oleg Nesterov <oleg@redhat.com>]

On 08/25, Hugh Dickins wrote:
>
> On Mon, 25 Aug 2014, Oleg Nesterov wrote:
>
> >    As Hugh pointed out, we actually need to require the additional
> >    guard page, but this code was written before linux had it.
> >
> > 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 the VM_GROWSUP case.
> >
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
>
> Sorry, I cannot ack this,

Hugh, I appreciate you nack even more. Thanks!

> because your comment below "at least 4 pages
> plus a guard page enforced by check_stack_guard_page()" makes no sense
> to me as an explanation for the 5.  The guard page (gap) enforced by
> check_stack_guard_page() is already at vma->vm_start
                              ^^^^^^^^^^^^^^^^^^^^^^^^
Ahh. Yes I misunderstood this logic. And yes, you tried to explain it
twice but I was too stupid.

> I did come across 2.6.34's 128k stack_expand inherited from 2.6.11's
> 20 page EXTRA_STACK_VM_PAGES.  With Linus's guard page enforcing a
> page gap since 2.6.36.

OK, and setup_arg_pages() still does expand_stack() although stack_expand
depends on RLIMIT_STACK. But I think this doesn't matter. The room was
already reserved, in general mm->start_stack points into the middle of
the stack.

So that check in do_shmat() can only help if the stack was not expanded
due to the low RLIMIT_STACK, then this application raises RLIMIT_STACK,
then does do_shmat(). But in this case it should likely crash when exec
returns to usermode.

> and our difficulty
> in communicating a sensible way of updating and describing the test,

I like your polite interpretation of my ignorance and inability to listen ;)

> now drives me to agree with you.  Please just rip out the start_stack
> test and the comment defending it.

Great. Will do.

Oleg.