fix tracepoint + backtrace SEGV

PC modification in gc_event_hook_body was careless.  There are (so
to say) abnormal iseqs stored in the cfp.  We have to check sanity
before we touch the PC.

This has not been fixed because there was no way to (ab)use the
setup from pure-Ruby.  However by using our official C APIs it is
possible to touch such frame(s), resulting in SEGV.

Fixes [Bug #14834].

1 files changed, 5 insertions, 2 deletions

diff --git a/gc.c b/gc.c
index 1b0f13efbb..fd30c75618 100644
--- a/gc.c
+++ b/gc.c
@@ -1925,8 +1925,11 @@ rb_objspace_set_event_hook(const rb_event_flag_t event)
 static void
 gc_event_hook_body(rb_execution_context_t *ec, rb_objspace_t *objspace, const rb_event_flag_t event, VALUE data)
 {
-    /* increment PC because source line is calculated with PC-1 */
-    const VALUE *pc = ec->cfp->pc++;
+    const VALUE *pc = ec->cfp->pc;
+    if (VM_FRAME_RUBYFRAME_P(ec->cfp)) {
+        /* increment PC because source line is calculated with PC-1 */
+        ec->cfp->pc++;
+    }
     EXEC_EVENT_HOOK(ec, event, ec->cfp->self, 0, 0, 0, data);
     ec->cfp->pc = pc;
 }