Ensure that vm_stack is cleared in `thread_cleanup_func_before_exec`.

If `vm_stack` is left dangling in a forked process, the gc attempts to scan
it, but it is invalid and will cause a segfault. Therefore, we clear it
before forking.

In order to simplify this, `rb_ec_clear_vm_stack` was introduced.

1 files changed, 12 insertions, 4 deletions

diff --git a/thread.c b/thread.c
index 7cb8062075..f7ed40f1df 100644
--- a/thread.c
+++ b/thread.c
@@ -596,7 +596,11 @@ thread_cleanup_func_before_exec(void *th_ptr)
 {
     rb_thread_t *th = th_ptr;
     th->status = THREAD_KILLED;
+    // The thread stack doesn't exist in the forked process:
     th->ec->machine.stack_start = th->ec->machine.stack_end = NULL;
+
+    // The vm_stack is `alloca`ed on the thread stack, so it's gone too:
+    rb_ec_clear_vm_stack(th->ec);
 }
 
 static void
@@ -713,19 +717,19 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start)
         rb_bug("thread_start_func_2 must not be used for main thread");
     }
 
+    thread_debug("thread start: %p\n", (void *)th);
+    VM_ASSERT((size * sizeof(VALUE)) <= th->ec->machine.stack_maxsize);
+
     vm_stack = alloca(size * sizeof(VALUE));
     VM_ASSERT(vm_stack);
 
     gvl_acquire(th->vm, th);
 
     rb_ec_initialize_vm_stack(th->ec, vm_stack, size);
-
-    ruby_thread_set_native(th);
-
     th->ec->machine.stack_start = STACK_DIR_UPPER(vm_stack + size, vm_stack);
     th->ec->machine.stack_maxsize -= size * sizeof(VALUE);
 
-    thread_debug("thread start: %p\n", (void *)th);
+    ruby_thread_set_native(th);
 
     {
 	thread_debug("thread start (get lock): %p\n", (void *)th);
@@ -806,7 +810,10 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start)
 
         rb_fiber_close(th->ec->fiber_ptr);
     }
+
     thread_cleanup_func(th, FALSE);
+    VM_ASSERT(th->ec->vm_stack == NULL);
+
     gvl_release(th->vm);
 
     return 0;
@@ -2253,6 +2260,7 @@ rb_threadptr_execute_interrupts(rb_thread_t *th, int blocking_timing)
 	    if (th->status == THREAD_RUNNABLE)
 		th->running_time_us += TIME_QUANTUM_USEC;
 
+            VM_ASSERT(th->ec->cfp);
 	    EXEC_EVENT_HOOK(th->ec, RUBY_INTERNAL_EVENT_SWITCH, th->ec->cfp->self,
 			    0, 0, 0, Qundef);