Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack check

[Hugh Dickins <hughd@google.com>]

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

Hugh