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

Ben Hutchings
2017-07-04T12:21:40Z

[-- Attachment #1: Type: text/plain, Size: 2034 bytes --]

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.

> 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).

Ben.

--
Ben Hutchings
The world is coming to an end. Please log off.

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]