[ruby-core:107005] [Ruby master Bug#18464] RUBY_INTERNAL_EVENT_NEWOBJ tracepoint causes an interpreter crash when combined with Ractors

Issue #18464 has been reported by kjtsanaktsidis (KJ Tsanaktsidis).

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Bug #18464: RUBY_INTERNAL_EVENT_NEWOBJ tracepoint causes an interpreter crash when combined with Ractors
https://bugs.ruby-lang.org/issues/18464

* Author: kjtsanaktsidis (KJ Tsanaktsidis)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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When a Ractor is created whilst a tracepoint for `RUBY_INTERNAL_EVENT_NEWOBJ` is active (registered with `rb_tracepoint_new`/`rb_tracepoint_enabled`), the interpreter crashes with a null pointer dereference with the following backtrace:

```
[BUG] Segmentation fault at 0x0000000000000000
ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin20]

...

-- C level backtrace information -------------------------------------------
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(rb_print_backtrace+0xf) [0x10a15fadd] vm_dump.c:759
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(rb_vm_bugreport) vm_dump.c:1045
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(rb_vm_bugreport) (null):0
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(bug_report_end+0x0) [0x109f96b81] error.c:820
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(rb_bug_for_fatal_signal) error.c:820
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(sigsegv+0x52) [0x10a0be3a2] signal.c:964
/usr/lib/system/libsystem_platform.dylib(_sigtramp+0x1d) [0x7fff20934d7d]
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(gc_event_hook_body+0x4) [0x109fb9d21] gc.c:2214
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(newobj_slowpath) gc.c:2486
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(newobj_slowpath_wb_unprotected) gc.c:2507
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(newobj_fill+0x0) [0x109fac92e] gc.c:2543
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(newobj_of0) gc.c:2553
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(newobj_of) gc.c:2552
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(rb_wb_unprotected_newobj_of) gc.c:2567
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(io_alloc+0x12) [0x109fd341c] io.c:1047
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(prep_io) io.c:8483
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(prep_stdio) io.c:8514
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(rb_io_prep_stdin) io.c:8532
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(thread_start_func_2+0xf7) [0x10a1058a7] thread.c:802
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(rb_native_cond_initialize+0x0) [0x10a1055fb] ./thread_pthread.c:1047
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(register_cached_thread_and_wait) ./thread_pthread.c:1099
/Users/ktsanaktsidis/Code/zendesk/ruby/ruby(thread_start_func_1) ./thread_pthread.c:1054
/usr/lib/system/libsystem_pthread.dylib(_pthread_start+0xe0) [0x7fff208ef8fc]

```

(full output is attached). 

This seems to be because the new Ractor sets up stdio objects (`rb_io_prep_stdin` et. al.), which in turn allocate Ruby objects, before `rb_ec_initialize_vm_stack` is called to set up the initial stack frame.

I've attached a patch which works around this by not firing GC event hooks if there is no control frame on the execution context. The patch also includes a test which reproduces the issue using the `objspace` extension; creating a Ractor within an `ObjectSpace.trace_object_allocations` block is enough to trigger the crash. The patch seems to fix things, but if you folk prefer I can also try swapping around the order of `prep_stdio` and `rb_ec_initialize_vm_stack`.


---Files--------------------------------
0001-Fix-interpreter-crash-caused-by-RUBY_INTERNAL_EVENT_.patch (1.91 KB)
crash.log (26.1 KB)
ruby_2022-01-08-151326_8927-ktsanaktsidis.crash (18.8 KB)


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