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Bug #14561

Updated by nobu (Nobuyoshi Nakada) over 1 year ago

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` Enumerator element within a separate thread, and then waiting for the thread to end.

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