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

From: Michal Hocko
Date: Tue Jul 04 2017 - 08:00:16 EST

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


> This *doesn't* fix the LibreOffice regression on i386.

Are there any details about this regression?

Michal Hocko