Re: [PATCH] mm: larger stack guard gap, between vmas

[Michal Hocko]

On Tue 04-07-17 13:59:59, Michal Hocko wrote:
> On Tue 04-07-17 12:36:11, Ben Hutchings wrote:
> > On Tue, 2017-07-04 at 12:42 +0200, Michal Hocko wrote:
> > > On Tue 04-07-17 11:47:28, Willy Tarreau wrote:
> > > > On Tue, Jul 04, 2017 at 11:35:38AM +0200, Michal Hocko wrote:
> > [...]
> > > > But wouldn't this completely disable the check in case such a guard page
> > > > is installed, and possibly continue to allow the collision when the stack
> > > > allocation is large enough to skip this guard page ?
> > > 
> > > Yes and but a PROT_NONE would fault and as the changelog says, we _hope_
> > > that userspace does the right thing.
> > 
> > It may well not be large enough, because of the same wrong assumptions
> > that resulted in the kernel's guard page not being large enough.  We
> > should count it as part of the guard gap but not a substitute.
> 
> yes, you are right of course. But isn't this a bug on their side
> considering they are managing their _own_ stack gap? Our stack gap
> management is a best effort thing and two such approaches competing will
> always lead to weird cornercases. That was my assumption when saying
> that I am not sure this is really _worth_ it. We should definitely try
> to workaround clashes but that's about it. If others think that we
> should do everything to prevent even those issues I will not oppose
> of course. It just adds more cycles to something that is a weird case
> already.

Forgot to mention another point. Currently we do not check other
previous vmas if prev->vm_flags & VM_GROWSDOWN. Consider that the stack
gap is implemented by mprotect. This wouldn't change the VM_GROWSDOWN
flag and we are back to square 1 because the gap might be too small. Do
we want/need to handle those cases. Are they too different from
MAP_FIXED gaps? I am not so sure but I would be inclined to say no.

-- 
Michal Hocko
SUSE Labs