RFR: 8253795: Implementation of JEP 391: macOS/AArch64 Port [v9]

[Gerard Ziemski]

On Wed, 3 Feb 2021 22:44:18 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>> Thanks for your questions Gerard.
>> 
>>> Part of the comment said This work-around is not necessary for 10.5+, as CrashReporter no longer intercedes on caught fatal signals.
>> 
>> That comment can probably be deleted since minversion is anyway 10.9 (and soon 10.12 https://github.com/openjdk/jdk/pull/2268 ).
>> 
>>> Do you know if this also apply to lldb or is it gdb only specific? How do you run gdb on macOS nowadays anyhow?
>> 
>> `lldb` is shipped with Xcode, `gdb` isn't. You would need to build and sign it yourself, I haven't tried that in a while. So, we should update that comment to talk about `lldb` 🙂 
>> 
>>> a) why we need `EXC_MASK_ARITHMETIC` ?
>> 
>> I _believe_ this dates back to i386. As far as I can tell this isn't needed for x86_64 or aarch64. I guess we can remove it, the worst case is that it makes the debugging experience of the runtime a little bit worse. OTOH it doesn't hurt either to have it here.
>> 
>>> b) we hit signal SIGSEGV in debugger even with the code in place, any way to avoid that?
>> 
>> The equivalent for `handle SIGSEGV nostop noprint` (gdb) in lldb is `process handle -n false -p true -s false SIGSEGV`.
>> 
>>> c) does `BSD aarch6` need only `EXC_MASK_BAD_INSTRUCTION` or does it need `EXC_MASK_BAD_ACCESS` as well?
>> 
>> aarch64 needs `EXC_MASK_BAD_ACCESS` at least for implicit null checking, there might be other cases.
>> 
>>> d) can we `#ifdef` the `EXC_MASK_BAD_INSTRUCTION` part of the mask only to apply to `aarch64`?
>> 
>> Maybe. I don't see any value in it though, except making the code more complicated to read 🙂
>
> I don't like the idea of using masks on architectures that do not require them. How about something like this?
> 
> `#if defined(__APPLE__)`
> `  // lldb (gdb) installs both standard BSD signal handlers, and mach exception`
> `  // handlers. By replacing the existing task exception handler, we disable lldb's mach`
> `  // exception handling, while leaving the standard BSD signal handlers functional.`
> `  //`
> `  // EXC_MASK_BAD_ACCESS needed by all architectures for NULL ptr checking`
> `  // EXC_MASK_ARITHMETIC needed by i386`
> `  // EXC_MASK_BAD_INSTRUCTION needed by aarch64 to initiate deoptimization`
> `  kern_return_t kr;`
> `  kr = task_set_exception_ports(mach_task_self(),`
> `                                EXC_MASK_BAD_ACCESS`
> `                                NOT_LP64(| EXC_MASK_ARITHMETIC)`
> `                                AARCH64_ONLY(| EXC_MASK_BAD_INSTRUCTION),`
> `                                MACH_PORT_NULL,`
> `                                EXCEPTION_STATE_IDENTITY,`
> `                                MACHINE_THREAD_STATE);`
> ` `
> `  assert(kr == KERN_SUCCESS, "could not set mach task signal handler");`
> `#endif`
> 
> If I just knew why i386 needs `EXC_MASK_ARITHMETIC` and add that to the comment I would be personally happy with that chunk of code.

No idea how to insert spaces and make text align :-(

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PR: https://git.openjdk.java.net/jdk/pull/2200