RFR: 8294160: misc crash dump improvements [v2]

[Dean Long]

On Tue, 27 Sep 2022 06:17:18 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Dean Long has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   revert unneeded changes
>
> src/hotspot/os/posix/signals_posix.cpp line 629:
> 
>> 627:   // Check for UD trap caused by NOP patching.
>> 628:   // If it is, patch return address to be deopt handler.
>> 629:   if (!signal_was_handled && pc != NULL && os::is_readable_pointer(pc)) {
> 
> If these new conditions are not met then what happens? What code will get the unreadable, or null pc to process?

It will fall through and treat the signal as unhandled, resulting in a crash dump.  The old code was wrong for these reasons:

- It would assert in the signal handler if PC was 0
- It would crash in the signal handler if PC was not in readable memory
- It ignored the previously adjusted value of PC, which means trap-based NOP patching probably wouldn't have worked on platforms that need the PC adjusted, like s390

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PR: https://git.openjdk.org/jdk/pull/10387