Re: [PATCH 1/1] 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:
> > On 08/24, Hugh Dickins wrote:
> > >
> > > I'd say it comes earlier, from Christoph Rohland's 2.4.17-pre7's
> > > "Add missing checks on shmat()", though I didn't find more than that.
> > >
> > > We can all understand wanting to leave a gap below the growsdown stack,
> > > but of course could argue about growsup and 1 or 4 or 5 or whatever:
> >
> > And it is not clear to me why the kernel should care at all,
>
> Care about what exactly?  Leaving a gap between shm and stack?
>
> The man page says that (unless SHM_REMAP) shmat() will fail with
> EINVAL if a mapping already exists there, and I think it's fair
> to regard the vm_start of a VM_GROWSDOWN somewhat elastically.
>
> It may be that Linus's check_stack_guard_page() work in 2.6.36
> changed the importance of this shmat() check, but I'd still feel
> safer to leave it as is (while turning a blind eye to the
> VM_GROWSUP omission).
>
> >
> > > okay that we're all more interested in just removing that start_stack.
> >
> > so perhaps v2 should simply remove it? Or do you think it would be safer
> > to not do this?
>
> It would be safer to leave it, but replace the start_stack use as you did.

OK, thanks.

> And I think I'll let Linus's guard page justify your 4 (to match comment)
> in place of the original's mysterious 5.

Ah, thanks again. Yes, if we want to guarantee 4 pages we should check 5.

Although obviously this doesn't explain the original's 5, this was
written before check_stack_guard_page().

OK.

Oleg.