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

Michal Hocko
2017-07-04T12:33:20Z

On Tue 04-07-17 13:21:02, Ben Hutchings wrote:
> On Tue, 2017-07-04 at 14:00 +0200, 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?
>
> Yes it's their bug, but you know the rule - don't break user-space.

Absolutely, that is why I belive we should consider the prev VMA but
doing anything more just risks for new regressions. Or why do you think
that not-checking them would cause a regression?

> > 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.
>
> I don't want odd behaviour to weaken the stack guard.
>
> > [...]
> >
> > > This *doesn't* fix the LibreOffice regression on i386.
> >
> > Are there any details about this regression?
>
> Here:
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=865303#170
>
> I haven't reproduced it in Writer, but if I use Base to create a new
> HSQLDB database it reliably crashes (HSQLDB is implemented in Java).

I haven't read through previous 169 comments but I do not see any stack
trace. Ideally with info proc mapping that would tell us the memory
layout.

--
Michal Hocko
SUSE Labs