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

[Severin Gehwolf]

Hi David,

On Wed, 2019-04-24 at 19:13 +1000, David Holmes wrote:
> Hi Severin,
> 
> Sorry for the delay, this slipped off my rader after the break.

No worries.

> On 19/04/2019 12:02 am, David Holmes wrote:
> > Hi Severin,
> > 
> > I'll have to delve into this more deeply (but I'm on Easter break for 
> > the next 4 days). I don't recall ever being aware of 
> > "suppress_primordial_thread_resolution"!
> 
> Well I have a bad memory as I reviewed that particular optimisation.
> 
> So, the exec_shield workaround requires knowing the process's primordial 
> thread stack regardless of whether the JVM is loaded on it or not. So it 
> assumes the initial_thread_stack has been determined, while we now avoid 
> doing that unnecessarily.
> 
> Okay the suggested fix is okay to achieve that - though alternatively it 
> could be handled inside workaround_expand_exec_shield_cs_limit() via:
> 
>      // Need to ensure we've determined the process's initial stack to
>      // perform the workaround
>      if (suppress_primordial_thread_resolution) {
>        Linux::capture_initial_stack(JavaThread::stack_size_at_create());
>      }
> 
> I would like to see a comment like the above regardless of where you 
> make the change.

Thanks for the review! Updated webrev with the comment:
http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8221639/02/webrev/

Would this be a trivial change or do I need a second reviewer? FWIW,
jdk/submit testing came back clean with this.

Thanks,
Severin

> Thanks,
> David
> ------
> 
> > David
> > 
> > On 18/04/2019 11:41 pm, Severin Gehwolf wrote:
> > > 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, there is old
> > > code being executed to work around the exec shield CS limit. That code
> > > depends on the initial stack size being captured. Right now this is
> > > undefined code: Pointer artithmetic on NULL pointer.
> > > 
> > > src/hotspot/os_cpu/linux_x86/os_linux_x86.cpp:884:73: runtime error: 
> > > pointer index expression with base 0x00000000 overflowed to 0xffffb000
> > > 
> > > I propose to not perform the optimization of JDK-8199717 for Linux x32.
> > > 
> > > Bug: https://bugs.openjdk.java.net/browse/JDK-8221639
> > > webrev: 
> > > http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8221639/01/webrev/
> > > 
> > > Testing: release/fastdebug builds on x32 Linux, inspecting
> > > -Xlog:os=info messages, currently running through jdkd/submit. Tier 1
> > > tests on Linux x86_64
> > > 
> > > Thoughts?
> > > 
> > > Thanks,
> > > Severin
> > >