RFR(xxs): 8187230: [aix] Leave OS guard page size at default for non-java threads instead of explicitly setting it

[Thomas Stüfe]

Hi Guys,

On Fri, Sep 8, 2017 at 9:51 AM, Lindenmaier, Goetz <
goetz.lindenmaier at sap.com> wrote:

> Hi Chris,
>
> on linux the pthread implementation is a bit strange, or buggy.
> It takes the OS guard pages out of the stack size specified.
> We need to set it so we can predict the additional space
> that must be allocated for the stack.
>
> See also the comment in os_linux.cpp, create_thread().
>

Goetz, I know we talked about this off list yesterday, but now I am not
sure this is actually needed. Yes, to correctly calculate the stack size,
we need to know the OS guard page size, but we do not need to set it, we
just need to know it. So, for non-java threads (java threads get the OS
guard set to zero), it would probably be sufficient to:

- pthread_attr_init() (sets default thread attribute values to the
attribute structure) and then
- pthread_attr_getguardsize() to read the guard size from that structure.

That way we leave the OS guard page at the size glibc deems best. I think
that is a better option. Consider a situation where the glibc changes the
size of the OS guard pages, for whatever reason - we probably should follow
suit.

See e.g. this security issue - admittedly only loosely related, since the
fix for this issue seemed to be a fix to stack banging, not changing the OS
guard size: https://access.redhat.com/security/vulnerabilities/stackguard

So, in short, I think we could change this for Linux too. If you guys
agree, I'll add this to the patch. Since I am on vacation and the depot is
closed, it may take some time.

Kind Regards, Thomas





>
> Best regards,
>   Goetz.
>
> > -----Original Message-----
> > From: ppc-aix-port-dev [mailto:ppc-aix-port-dev-bounces at openjdk.java.net
> ]
> > On Behalf Of Chris Plummer
> > Sent: Thursday, September 07, 2017 11:07 PM
> > To: Thomas Stüfe <thomas.stuefe at gmail.com>; ppc-aix-port-
> > dev at openjdk.java.net
> > Cc: HotSpot Open Source Developers <hotspot-dev at openjdk.java.net>
> > Subject: Re: RFR(xxs): 8187230: [aix] Leave OS guard page size at
> default for
> > non-java threads instead of explicitly setting it
> >
> > Hi Thomas,
> >
> > Is there a reason this shouldn't also be done for linux?
> >
> > thanks,
> >
> > Chris
> >
> > On 9/7/17 3:02 AM, Thomas Stüfe wrote:
> > > Hi all,
> > >
> > > may I please have a review for this small change:
> > >
> > > Bug:
> > > https://bugs.openjdk.java.net/browse/JDK-8187230
> > >
> > > Webrev:
> > > http://cr.openjdk.java.net/~stuefe/webrevs/8187230-aix-
> > > leave-os-guard-page-size-at-default-for-non-java-
> > threads/webrev.00/webrev/
> > >
> > > The change is very subtle.
> > >
> > > Before, we would set the OS guard page size for every thread - for java
> > > threads disable them, for non-java threads we'd set them to 4K.
> > >
> > > Now, we still disable them for java threads but leave them at the OS
> > > default size for non-java threads.
> > >
> > > The really important part is the disabling of OS guard pages for java
> > > threads, where we have a VM guard pages in place and do not want to
> > spend
> > > more memory on OS guards. We do not really care for the exact size of
> the
> > > OS guard pages for non-java threads, and therefore should not set it -
> we
> > > should leave the size in place the OS deems sufficient. That also
> spares us
> > > the complexity of handling the thread stack page size, which on AIX
> may be
> > > different from os::vm_page_size().
> > >
> > > Thank you and Kind Regards, Thomas
> >
> >
>
>