Fixed possible SEGV in `Kernel#block_given?`; ref #3593

src/kernel.c

@@ -136,44 +136,38 @@ mrb_f_block_given_p_m(mrb_state *mrb, mrb_value self)
 {
   mrb_callinfo *ci = mrb->c->ci;
   mrb_value *bp;
-  mrb_bool given_p;
 
   bp = ci->stackent + 1;
   ci--;
   if (ci <= mrb->c->cibase) {
-    given_p = FALSE;
+    return mrb_false_value();
   }
-  else {
-    /* block_given? called within block; check upper scope */
-    if (ci->proc->env) {
-      struct REnv *e = ci->proc->env;
-      mrb_value *sp;
+  /* block_given? called within block; check upper scope */
+  if (ci->proc->env) {
+    struct REnv *e = ci->proc->env;
 
-      while (e->c) {
-        e = (struct REnv*)e->c;
-      }
-      sp = e->stack;
-      if (sp) {
-        /* top-level does not have block slot (alway false) */
-        if (sp == mrb->c->stbase)
-          return mrb_false_value();
-        if (e->cioff < 0) {
-          /* use saved block arg position */
-          bp = &e->stack[-e->cioff];
-        }
-        else {
-          ci = mrb->c->cibase + e->cioff;
-          bp = ci[1].stackent + 1;
-        }
-      }
+    while (e->c) {
+      e = (struct REnv*)e->c;
     }
-    if (ci->argc > 0) {
-      bp += ci->argc;
+    /* top-level does not have block slot (always false) */
+    if (e->stack == mrb->c->stbase)
+      return mrb_false_value();
+    if (e->stack && e->cioff < 0) {
+      /* use saved block arg position */
+      bp = &e->stack[-e->cioff];
+      ci = 0;                 /* no callinfo available */
+    }
+    else {
+      ci = mrb->c->cibase + e->cioff;
+      bp = ci[1].stackent + 1;
     }
-    given_p = !mrb_nil_p(*bp);
   }
-
-  return mrb_bool_value(given_p);
+  if (ci && ci->argc > 0) {
+    bp += ci->argc;
+  }
+  if (mrb_nil_p(*bp))
+    return mrb_false_value();
+  return mrb_true_value();
 }
 
 /* 15.3.1.3.7  */