Re: [PATCH] Restrict initial stack space expansion to rlimit

[Helge Deller]

On 02/10/2010 06:31 AM, Michael Neuling wrote:
> In message<20100210141016.4D18.A69D9226@xxxxxxxxxxxxxx>  you wrote:
> > > On 02/09/2010 10:51 PM, Michael Neuling wrote:
> > > > > > I'd still like someone with a CONFIG_STACK_GROWSUP arch to test/ACK it
> > > > > > as well.
> > > > >
> > > > > There's only one CONFIG_GROWSUP arch - parisc.
> > > > > Could someone please test it on parisc?
> > >
> > > I did.
> > >
> > > > How about doing:
> > > >     'ulimit -s 15; ls'
> > > > before and after the patch is applied.  Before it's applied, 'ls' should
> > > > be killed.  After the patch is applied, 'ls' should no longer be killed.
> > > >
> > > > I'm suggesting a stack limit of 15KB since it's small enough to trigger
> > > > 20*PAGE_SIZE.  Also 15KB not a multiple of PAGE_SIZE, which is a trickier
> > > > case to handle correctly with this code.
> > > >
> > > > 4K pages on parisc should be fine to test with.
> > >
> > > Mikey, thanks for the suggested test plan.
> > >
> > > I'm not sure if your patch does it correct for parisc/stack-grows-up-case.
> > >
> > > I tested your patch on  a 4k pages kernel:
> > > root@c3000:~# uname -a
> > > Linux c3000 2.6.33-rc7-32bit #221 Tue Feb 9 23:17:06 CET 2010 parisc GNU/Li
> nux
> > > Without your patch:
> > > root@c3000:~# ulimit -s 15; ls
> > > Killed
> > > ->  correct.
> > >
> > > With your patch:
> > > root@c3000:~# ulimit -s 15; ls
> > > Killed
> > > _or_:
> > > root@c3000:~# ulimit -s 15; ls
> > > Segmentation fault
> > > ->  ??
> > >
> > > Any idea?
> >
> > My x86_64 box also makes segmentation fault. I think "ulimit -s 15" is too sm
> all stack for ls.
> > "ulimit -s 27; ls "  wroks perfectly fine.
>
> Arrh.  I asked Helge offline earlier to check what use to work on parisc
> on 2.6.31.
>
> I guess PPC has a nice clean non-bloated ABI :-D

Hi Mikey,

I tested again, and it works for me with "ulimit -s 27" as well (on a 4k, 32bit kernel).
Still, I'm not 100%  sure if your patch is correct.
Anyway, it seems to work.

But what makes me wonder is, why EXTRA_STACK_VM_PAGES is defined in pages at all.
You wrote in your patch description:
> This bug means that when limiting the stack to less the 20*PAGE_SIZE (eg.
> 80K on 4K pages or 'ulimit -s 79') all processes will be killed before
> they start.  This is particularly bad with 64K pages, where a ulimit below
> 1280K will kill every process.

Wouldn't it make sense to define and use EXTRA_STACK_VM_SIZE instead (e.g. as 20*4096 = 80k)?
This extra stack reservation should IMHO be independend of the actual kernel page size.

Helge
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