RFR: 8221639: [x32] expand_exec_shield_cs_limit workaround is undefined code after JDK-8199717

[Severin Gehwolf]

Hi Florian,

On Wed, 2019-04-24 at 15:33 +0200, Florian Weimer wrote:
> * Severin Gehwolf:
> 
> > Hi,
> > 
> > Could I please get reviews for this Linux x32 fix? JDK-8199717 added a
> > performance optimization to only capture the initial stack size when
> > launched via non-java launchers. However, on Linux x32,
> 
> Do you mean actual x32, or i386?  These two are different.

i386

> Can you actually verify changes in this area?  I think it's pretty hard
> these days to find a machine that actually uses the CS hack to avoid
> universal read-implies-exec.  I don't think this was ever part of any
> mainline kernel.

No, I cannot really verify the workaround itself. I can, however, see
that the current code is undefined on i386. See the bug for details[1].
This snippet:

  char* hint = (char*)(Linux::initial_thread_stack_bottom() -
                       (JavaThread::stack_guard_zone_size() + page_size));

Assumes Linux::initial_thread_stack_bottom() != NULL. In this case it's
NULL and pointer arithmetic on a NULL pointer is UB. That's what this
fix resolves.

Thanks,
Severin

[1] https://bugs.openjdk.java.net/browse/JDK-8221639