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

Michal Hocko
2017-07-06T07:35:06Z

On Wed 05-07-17 08:36:45, Michal Hocko wrote:
> On Tue 04-07-17 16:31:52, Linus Torvalds wrote:
> > On Tue, Jul 4, 2017 at 4:01 PM, Ben Hutchings <ben@decadent.org.uk> wrote:
> > >
> > > We have:
> > >
> > > bottom = 0xff803fff
> > > sp = 0xffffb178
> > >
> > > The relevant mappings are:
> > >
> > > ff7fc000-ff7fd000 rwxp 00000000 00:00 0
> > > fffdd000-ffffe000 rw-p 00000000 00:00 0 [stack]
> >
> > Ugh. So that stack is actually 8MB in size, but the alloca() is about
> > to use up almost all of it, and there's only about 28kB left between
> > "bottom" and that 'rwx' mapping.
> >
> > Still, that rwx mapping is interesting: it is a single page, and it
> > really is almost exactly 8MB below the stack.
> >
> > In fact, the top of stack (at 0xffffe000) is *exactly* 8MB+4kB from
> > the top of that odd one-page allocation (0xff7fd000).
>
> Very interesting! I would be really curious whether changing ulimit to
> something bigger changes the picture.

It's public holiday today here and I haven't read all new emails and I
will be mostly offline today. I will catch up tomorrow. But before we go
to more tricky workarounds. Could you double check that simply
increasing the RLIMIT_STACK workarounds the problem here? Because if it
does and other workarounds require some manual intervention then
changing ulimit sounds like the least tricky one to me.
--
Michal Hocko
SUSE Labs