RFR: 8292695: SIGQUIT and jcmd attaching mechanism does not work with signal chaining library

[Man Cao]

On Wed, 7 Sep 2022 06:53:12 GMT, Man Cao <manc at openjdk.org> wrote:

>> Hi all,
>> 
>> Could anyone review this bug fix? See https://bugs.openjdk.org/browse/JDK-8292695 for details.
>> 
>> I changed the temporary handler for SIGQUIT to use a dummy function, and use `os::signal()` to set it up, just as `os::initialize_jdk_signal_support()` does.
>> It is possible that just moving the `set_signal_handler(BREAK_SIGNAL, false);` in `install_signal_handlers()` outside of the window bounded by `JVM_{begin|end}_signal_setting()` could also fix this bug. However, `set_signal_handler()` and `JVM_HANDLE_XXX_SIGNAL()` are currently used for signals that support chaining and periodically check, which do not apply to SIGQUIT. I think it is cleaner to use different functions for SIGQUIT.
>> 
>> I also added a test to check that sending SIGQUIT should produce a thread dump on stdout, with and without using libjsig.so.
>> 
>> -Man
>
> Thanks for filing the other bug. Responded in that bug.

> One thing, libjsig does nothing to prevent user code from changing thread/process sigmask. @caoman has that never been a problem at Google?

We don't actually use libjsig directly. Our launcher reimplemented signal chaining support using the same JVM_begin_signal_setting/JVM_end_signal_setting/JVM_get_signal_action interface.

Our launcher's implementation has calls to pthread_sigmask, but the goal seems to make the overridden sigaction() async-signal-safe. It doesn't prevent user code from changing thread/process sigmask either. However, I haven't heard of issues with user code changing sigmask.

-------------

PR: https://git.openjdk.org/jdk/pull/9955