[ruby-core:86834] [Ruby trunk Bug#14561] Consistent 2.5.0 seg fault in GC, related to accessing an enumerator in a thread

[samuel@oriontransfer.org]

Issue #14561 has been updated by ioquatix (Samuel Williams).


By the way, it only happened once. Re-running the same spec several more times didn't generate any further odd output.

----------------------------------------
Bug #14561: Consistent 2.5.0 seg fault in GC, related to accessing an enumerator in a thread
https://bugs.ruby-lang.org/issues/14561#change-71797

* Author: dazuma (Daniel Azuma)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin17]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
This seg fault happens consistently on OSX (specifically I'm reproing it on a late 2015 Macbook pro running 10.13.3, but it seems to happen on similar machines as well). It happens only on Ruby 2.5.0.

Small repro case:

```ruby
enum = Enumerator.new { |y| y << 1 }
thread = Thread.new { enum.peek }  # enum.next also causes the segfault, but not enum.size
thread.join
GC.start   # <- seg fault here
```

The C-level backtrace identifies this as within the mark phase of GC:

```
-- C level backtrace information -------------------------------------------
0   ruby                                0x000000010f77ced7 rb_vm_bugreport + 135
1   ruby                                0x000000010f602628 rb_bug_context + 472
2   ruby                                0x000000010f6f1491 sigsegv + 81
3   libsystem_platform.dylib            0x00007fff6a779f5a _sigtramp + 26
4   ruby                                0x000000010f61bb93 rb_gc_mark_machine_stack + 99
5   ruby                                0x000000010f76bf39 rb_execution_context_mark + 137
6   ruby                                0x000000010f5ea32b cont_mark + 27
7   ruby                                0x000000010f626a02 gc_marks_rest + 146
8   ruby                                0x000000010f6253c0 gc_start + 2816
9   ruby                                0x000000010f61d628 garbage_collect + 184
10  ruby                                0x000000010f622215 gc_start_internal + 485
11  ruby                                0x000000010f7703be vm_call_cfunc + 286
12  ruby                                0x000000010f759af4 vm_exec_core + 12260
13  ruby                                0x000000010f76ac8e vm_exec + 142
14  ruby                                0x000000010f60c101 ruby_exec_internal + 177
15  ruby                                0x000000010f60bff8 ruby_run_node + 56
16  ruby                                0x000000010f592d1f main + 79

I also ran this against Ruby recompiled with -O0, and got a more detailed backtrace:

-- C level backtrace information -------------------------------------------
0   libruby.2.5.dylib                   0x000000010c416e19 rb_print_backtrace + 25
1   libruby.2.5.dylib                   0x000000010c416f28 rb_vm_bugreport + 136
2   libruby.2.5.dylib                   0x000000010c2096f2 rb_bug_context + 450
3   libruby.2.5.dylib                   0x000000010c35b4ee sigsegv + 94
4   libsystem_platform.dylib            0x00007fff6a779f5a _sigtramp + 26
5   libruby.2.5.dylib                   0x000000010c2395a1 mark_locations_array + 49
6   libruby.2.5.dylib                   0x000000010c22a5bb gc_mark_locations + 75
7   libruby.2.5.dylib                   0x000000010c22a7d9 mark_stack_locations + 41
8   libruby.2.5.dylib                   0x000000010c22a79f rb_gc_mark_machine_stack + 79
9   libruby.2.5.dylib                   0x000000010c3f8868 rb_execution_context_mark + 264
10  libruby.2.5.dylib                   0x000000010c1e263e cont_mark + 46
11  libruby.2.5.dylib                   0x000000010c1e2572 fiber_mark + 146
12  libruby.2.5.dylib                   0x000000010c22f4c6 gc_mark_children + 1094
13  libruby.2.5.dylib                   0x000000010c23734c gc_mark_stacked_objects + 108
14  libruby.2.5.dylib                   0x000000010c237a5b gc_mark_stacked_objects_all + 27
15  libruby.2.5.dylib                   0x000000010c236cb1 gc_marks_rest + 129
16  libruby.2.5.dylib                   0x000000010c238787 gc_marks + 103
17  libruby.2.5.dylib                   0x000000010c2352e2 gc_start + 802
18  libruby.2.5.dylib                   0x000000010c22ca18 garbage_collect + 56
19  libruby.2.5.dylib                   0x000000010c231f7d gc_start_internal + 493
20  libruby.2.5.dylib                   0x000000010c401f2a call_cfunc_m1 + 42
21  libruby.2.5.dylib                   0x000000010c400d1d vm_call_cfunc_with_frame + 605
22  libruby.2.5.dylib                   0x000000010c3fc41d vm_call_cfunc + 173
23  libruby.2.5.dylib                   0x000000010c3fb8fe vm_call_method_each_type + 190
24  libruby.2.5.dylib                   0x000000010c3fb690 vm_call_method + 160
25  libruby.2.5.dylib                   0x000000010c3fb5e5 vm_call_general + 53
26  libruby.2.5.dylib                   0x000000010c3e784e vm_exec_core + 8974
27  libruby.2.5.dylib                   0x000000010c3f6fe6 vm_exec + 182
28  libruby.2.5.dylib                   0x000000010c3f7d5b rb_iseq_eval_main + 43
29  libruby.2.5.dylib                   0x000000010c214208 ruby_exec_internal + 232
30  libruby.2.5.dylib                   0x000000010c214111 ruby_exec_node + 33
31  libruby.2.5.dylib                   0x000000010c2140d0 ruby_run_node + 64
32  ruby                                0x000000010c16ff2f main + 95
```

As far as I can tell, the C instruction triggering the segfault is here in gc.c (around line 4064):

```C
static void
mark_locations_array(rb_objspace_t *objspace, register const VALUE *x, register long n)
{
    VALUE v;
    while (n--) {
        v = *x;            // <----- Seems to be crashing here?
        gc_mark_maybe(objspace, v);
        x++;
    }
}
```

Indicating a bad pointer in the machine stack.

I'm not sufficiently familiar with the VM internals to make much further progress, but I hope the repro case is helpful. It seems to require accessing an `Enumerator` element within a separate thread, and then waiting for the thread to end.


---Files--------------------------------
ruby_2018-03-14-222035_Fukurou.crash (38.6 KB)
ruby_2018-03-14-205753_Fukurou.crash (38.6 KB)
dump.txt (51.4 KB)


-- 
https://bugs.ruby-lang.org/