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

[David Holmes]

Hi Severin,

Sorry for the delay, this slipped off my rader after the break.

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,
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
>>