Issue #10206 has been updated by Koichi Sasada.



At first, Symbol is VALUE and it should be marked.

So that the following code should not be allowed.

```
id = SYM2ID(garbage_sym);
```

In this case, afeter sweeping, garbage_sym becomes freed VALUE.

What happen on it?


----------------------------------------
Bug #10206: garbage symbols crash symbol GC
https://bugs.ruby-lang.org/issues/10206#change-48705

* Author: Eric Wong
* Status: Open
* Priority: Normal
* Assignee: Koichi Sasada
* Category: core
* Target version: current: 2.2.0
* ruby -v: trunk
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
This is reproducible with just a test loop running for serveral minutes/hours:

	while make test-all TESTS=-j8; do :; done

It looks like SYM2ID/rb_sym2id interacts badly with dsymbol_check
when it encounters garbage objects.

dsymbol_check replaces an invalid object and returns a new object
for the caller, but the original arg for SYM2ID remains usable
to the caller:

        id = SYM2ID(garbage_sym);
        do_something(garbage_sym); /* bad invalid object used */

Changing: rb_sym2id(VALUE) to rb_sym2id(VALUE *)
might solve the issue, but introduces many incompatibilities in existing
code:

        id = rb_sym2id(&garbage_sym);
        do_something(garbage_sym); /* id == garbage_sym, safe to use */

ref: ruby-core thread starting at [ruby-core:64671]
backtraces:
	http://80x24.org/r35240/rb-dump.txt
	http://80x24.org/r35240/gdb-bt.txt





-- 
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