Issue #21710 has been updated by ivoanjo (Ivo Anjo).
And since rb_gc_mark_weak isn't public API, there's no decent way to have weakrefs in a C extensions (except if you assume/enforce no compaction, then you can use just the address).
Ahhhhh I totally had missed that one. Ever since I spotted https://github.com/ruby/ruby/pull/8113 I've had an item on my TODO list on "move away from id2obj for heap profiling in the Datadog profiler" but I completely missed that `rb_gc_mark_weak` is not public. I'll open a separate issue for that -- definitely with the change to the object id implementation in Ruby 4.0 having `rb_gc_mark_weak` be public would be a great migration path. ---------------------------------------- Bug #21710: Segfault when reading object_id after it is set inside RUBY_INTERNAL_EVENT_NEWOBJ https://bugs.ruby-lang.org/issues/21710#change-115361 * Author: ivoanjo (Ivo Anjo) * Status: Open * Target version: 4.0 * ruby -v: ruby 4.0.0dev (2025-11-24T08:44:28Z master aeb7689e69) +PRISM [x86_64-linux] * Backport: 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED ---------------------------------------- Hey 👋. I caught a following segfault when running the [Datadog Ruby Profiler](https://github.com/datadog/dd-trace-rb) test suite on 4.0.0-preview2. The Datadog Ruby Profiler still uses object_ids in `RUBY_INTERNAL_EVENT_NEWOBJ` to simulate weak references. (We know this is deprecated and we plan to move to [`rb_gc_mark_weak`](https://bugs.ruby-lang.org/issues/19783) in the future). I was able to reproduce this bug with both 4.0.0-preview2 as well as a ruby-head build from 2025-11-24. Here's a [very simple reproducer](https://github.com/ivoanjo/lowlevel-toolkit/pull/1): ```c static void on_newobj_event(VALUE tpval, void *data) { VALUE obj = rb_tracearg_object(rb_tracearg_from_tracepoint(tpval)); if (!rb_objspace_internal_object_p(obj)) rb_obj_id(obj); } static VALUE add_object_id(RB_UNUSED_VAR(VALUE _)) { VALUE tp = rb_tracepoint_new(0, RUBY_INTERNAL_EVENT_NEWOBJ, on_newobj_event, NULL); rb_tracepoint_enable(tp); rb_yield(Qnil); rb_tracepoint_disable(tp); return Qnil; } ``` ```ruby puts RUBY_DESCRIPTION require "lowlevel-toolkit" foo = Struct.new(:foo) bar = nil LowlevelToolkit.add_object_id { bar = foo.new(1) } bar.object_id ``` and here's the segfault: ``` examples/add_object_id.rb:8: [BUG] Segmentation fault at 0x0000000000000000 ruby 4.0.0dev (2025-11-24T08:44:28Z master aeb7689e69) +PRISM [x86_64-linux] -- Control frame information ----------------------------------------------- c:0003 p:---- s:0012 e:000011 l:y b:---- CFUNC :object_id c:0002 p:0044 s:0008 E:002170 l:n b:---- EVAL examples/add_object_id.rb:8 [FINISH] c:0001 p:0000 s:0003 E:001dc0 l:y b:---- DUMMY [FINISH] -- Ruby level backtrace information ---------------------------------------- examples/add_object_id.rb:8:in '<main>' examples/add_object_id.rb:8:in 'object_id' -- Threading information --------------------------------------------------- Total ractor count: 1 Ruby thread count for this ractor: 1 -- Machine register context ------------------------------------------------ RIP: 0x000079c33b33e2cd RBP: 0x00007ffdcee65d10 RSP: 0x00007ffdcee65cd0 RAX: 0x0000000000000000 RBX: 0x000079c33a4ff058 RCX: 0x000079c339000000 RDX: 0x0000000000000000 RDI: 0x0000000000000000 RSI: 0x00000000000fe000 R8: 0x000079c33b35c078 R9: 0x000079c31f2814d0 R10: 0x000058e4893a6ee0 R11: 0xb84a1fbe2b4aab11 R12: 0x0000000000000002 R13: 0x0000000000000000 R14: 0x000058e489384b60 R15: 0x000079c33a5fefa0 EFL: 0x0000000000010246 -- C level backtrace information ------------------------------------------- .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_print_backtrace+0x24) [0x79c33b392149] .rvm/src/ruby-head/vm_dump.c:1105 .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_vm_bugreport+0x33f) [0x79c33b39291b] .rvm/src/ruby-head/vm_dump.c:1450 .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_bug_for_fatal_signal+0x147) [0x79c33b13a3e2] .rvm/rubies/ruby-head/lib/libruby.so.4.0(sigsegv+0x84) [0x79c33b2bea4d] .rvm/src/ruby-head/signal.c:948 .rvm/rubies/ruby-head/lib/libruby.so.4.0(sigill) (null):0 /lib/x86_64-linux-gnu/libc.so.6(0x79c33ac45330) [0x79c33ac45330] .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_obj_field_get+0x105) [0x79c33b33e2cd] .rvm/src/ruby-head/variable.c:1412 .rvm/rubies/ruby-head/lib/libruby.so.4.0(object_id_get+0x4e) [0x79c33b16ed28] .rvm/src/ruby-head/gc.c:1875 .rvm/rubies/ruby-head/lib/libruby.so.4.0(object_id0+0x46) [0x79c33b16ed78] .rvm/src/ruby-head/gc.c:1895 .rvm/rubies/ruby-head/lib/libruby.so.4.0(object_id+0xb1) [0x79c33b16ee93] .rvm/src/ruby-head/gc.c:1936 .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_find_object_id+0x43) [0x79c33b16f6ea] .rvm/src/ruby-head/gc.c:2179 .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_obj_id+0x2e) [0x79c33b16f75a] .rvm/src/ruby-head/gc.c:2234 .rvm/rubies/ruby-head/lib/libruby.so.4.0(ractor_safe_call_cfunc_0+0x30) [0x79c33b35c0a8] .rvm/src/ruby-head/vm_insnhelper.c:3718 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_call_cfunc_with_frame_+0x221) [0x79c33b35ccb1] .rvm/src/ruby-head/vm_insnhelper.c:3902 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_call_cfunc_with_frame+0x76) [0x79c33b35cf26] .rvm/src/ruby-head/vm_insnhelper.c:3948 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_call_cfunc_other+0x12b) [0x79c33b35d053] .rvm/src/ruby-head/vm_insnhelper.c:3974 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_call_cfunc+0x147) [0x79c33b35d49a] .rvm/src/ruby-head/vm_insnhelper.c:4056 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_call_method_each_type+0x180) [0x79c33b360161] .rvm/src/ruby-head/vm_insnhelper.c:4888 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_call_method+0xa1) [0x79c33b360c1d] .rvm/src/ruby-head/vm_insnhelper.c:5014 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_call_general+0x2f) [0x79c33b360e1f] .rvm/src/ruby-head/vm_insnhelper.c:5058 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_sendish+0x1d3) [0x79c33b363689] .rvm/src/ruby-head/vm_insnhelper.c:6124 .rvm/rubies/ruby-head/lib/libruby.so.4.0(vm_exec_core+0x3b1d) [0x79c33b36b42c] .rvm/src/ruby-head/insns.def:903 .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_vm_exec+0x140) [0x79c33b384d94] .rvm/src/ruby-head/vm.c:2784 .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_iseq_eval_main+0x3d) [0x79c33b385be2] .rvm/src/ruby-head/vm.c:3050 .rvm/rubies/ruby-head/lib/libruby.so.4.0(rb_ec_exec_node+0x128) [0x79c33b145eb4] .rvm/src/ruby-head/eval.c:283 .rvm/rubies/ruby-head/lib/libruby.so.4.0(ruby_run_node+0x8a) [0x79c33b146025] .rvm/src/ruby-head/eval.c:321 .rvm/rubies/ruby-head/bin/ruby(rb_main+0x4c) [0x58e47500e51e] ./main.c:42 .rvm/rubies/ruby-head/bin/ruby(main+0x62) [0x58e47500e596] ./main.c:62 ``` Let me know if I can provide any more info! ---Files-------------------------------- bug-21710.patch (1.25 KB) -- https://bugs.ruby-lang.org/