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

[Bernhard Urban-Forster]

On Wed, 3 Feb 2021 20:29:48 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>> Part of the comment said `This work-around is not necessary for 10.5+, as CrashReporter no longer intercedes on caught fatal signals.` so I thought it was no longer needed, but it sounds like the part about `gdb` still applies then.
>> 
>> We should update the comment to just say the `gdb` relevant part perhaps (and evaluate which of the EXC_MASK_BAD_ACCESS | EXC_MASK_BAD_INSTRUCTION | EXC_MASK_ARITHMETIC) we actually need for gdb:
>> 
>>  `// gdb installs both standard BSD signal handlers, and mach exception`
>>  `// handlers. By replacing the existing task exception handler, we disable gdb's mach`
>>  `// exception handling, while leaving the standard BSD signal handlers functional.`
>> 
>> Do you know if this also apply to `lldb` or is it `gdb` only specific? How do you run `gdb` on macOS nowadays anyhow?
>
> To answer my own question, it seems that code is still needed on `x86_64` for `lldb` with `EXC_MASK_BAD_ACCESS` or we keep tripping over `EXC_BAD_ACCESS`
> 
> Remaining questions:
> 
> a) why we need `EXC_MASK_ARITHMETIC` ?
> b) we hit `signal SIGSEGV` in debugger even with the code in place, any way to avoid that?
> c) does `BSD aarch64` need only `EXC_MASK_BAD_INSTRUCTION` or does it need `EXC_MASK_BAD_ACCESS` as well?
> d) can we `#ifdef` the `EXC_MASK_BAD_INSTRUCTION` part of the mask only to apply to `aarch64`?

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 🙂

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