From dbc2b89bc042d73e60aeb7d56686aa28d82e2622 Mon Sep 17 00:00:00 2001
From: Samuel Williams <samuel.williams@oriontransfer.co.nz>
Date: Thu, 20 Jun 2019 13:41:18 +1200
Subject: Ensure `vm_stack` is cleared after fork.

---
 cont.c    | 12 ++----------
 thread.c  |  4 ++++
 vm.c      | 36 ++++++++++++++++++++++++++----------
 vm_core.h |  4 ++++
 4 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/cont.c b/cont.c
index c0f9c1c720..b83a0ad94c 100644
--- a/cont.c
+++ b/cont.c
@@ -268,13 +268,6 @@ fiber_status_set(rb_fiber_t *fib, enum fiber_status s)
     fib->status = s;
 }
 
-void
-rb_ec_set_vm_stack(rb_execution_context_t *ec, VALUE *stack, size_t size)
-{
-    ec->vm_stack = stack;
-    ec->vm_stack_size = size;
-}
-
 static inline void
 ec_switch(rb_thread_t *th, rb_fiber_t *fib)
 {
@@ -705,7 +698,7 @@ cont_capture(volatile int *volatile stat)
     cont->saved_vm_stack.ptr = ALLOC_N(VALUE, ec->vm_stack_size);
     MEMCPY(cont->saved_vm_stack.ptr, ec->vm_stack, VALUE, ec->vm_stack_size);
 #endif
-    rb_ec_set_vm_stack(&cont->saved_ec, NULL, 0);
+    rb_ec_clear_vm_stack(&cont->saved_ec);
     cont_save_machine_stack(th, cont);
 
     /* backup ensure_list to array for search in another context */
@@ -1793,8 +1786,7 @@ rb_fiber_close(rb_fiber_t *fib)
         }
     }
 
-    rb_ec_set_vm_stack(ec, NULL, 0);
-    ec->cfp = NULL;
+    rb_ec_clear_vm_stack(ec);
 
 #if !FIBER_USE_NATIVE
     /* should not mark machine stack any more */
diff --git a/thread.c b/thread.c
index 492722b085..6c4e6506e8 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
diff --git a/vm.c b/vm.c
index 1f09411d39..495d5ddcb6 100644
--- a/vm.c
+++ b/vm.c
@@ -2514,7 +2514,7 @@ rb_execution_context_mark(const rb_execution_context_t *ec)
 #endif
 
     /* mark VM stack */
-    if (ec->vm_stack && ec->cfp) {
+    if (ec->vm_stack) {
 	VALUE *p = ec->vm_stack;
 	VALUE *sp = ec->cfp->sp;
 	rb_control_frame_t *cfp = ec->cfp;
@@ -2685,20 +2685,36 @@ thread_alloc(VALUE klass)
     return obj;
 }
 
+inline void
+rb_ec_set_vm_stack(rb_execution_context_t *ec, VALUE *stack, size_t size)
+{
+    ec->vm_stack = stack;
+    ec->vm_stack_size = size;
+}
+
 void
 rb_ec_initialize_vm_stack(rb_execution_context_t *ec, VALUE *stack, size_t size)
 {
-  rb_ec_set_vm_stack(ec, stack, size);
+    rb_ec_set_vm_stack(ec, stack, size);
+
+    ec->cfp = (void *)(ec->vm_stack + ec->vm_stack_size);
 
-  ec->cfp = (void *)(ec->vm_stack + ec->vm_stack_size);
+    rb_vm_push_frame(ec,
+        NULL /* dummy iseq */,
+        VM_FRAME_MAGIC_DUMMY | VM_ENV_FLAG_LOCAL | VM_FRAME_FLAG_FINISH | VM_FRAME_FLAG_CFRAME /* dummy frame */,
+        Qnil /* dummy self */, VM_BLOCK_HANDLER_NONE /* dummy block ptr */,
+        0 /* dummy cref/me */,
+        0 /* dummy pc */, ec->vm_stack, 0, 0
+    );
+}
+
+void
+rb_ec_clear_vm_stack(rb_execution_context_t *ec)
+{
+    rb_ec_set_vm_stack(ec, NULL, 0);
 
-  rb_vm_push_frame(ec,
-      NULL /* dummy iseq */,
-      VM_FRAME_MAGIC_DUMMY | VM_ENV_FLAG_LOCAL | VM_FRAME_FLAG_FINISH | VM_FRAME_FLAG_CFRAME /* dummy frame */,
-      Qnil /* dummy self */, VM_BLOCK_HANDLER_NONE /* dummy block ptr */,
-      0 /* dummy cref/me */,
-      0 /* dummy pc */, ec->vm_stack, 0, 0
-  );
+    // Avoid dangling pointers:
+    ec->cfp = NULL;
 }
 
 static void
diff --git a/vm_core.h b/vm_core.h
index 99dee425ab..d199fcdff0 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -907,6 +907,10 @@ void rb_ec_set_vm_stack(rb_execution_context_t *ec, VALUE *stack, size_t size);
 // @param size the size of the stack, as in `VALUE stack[size]`.
 void rb_ec_initialize_vm_stack(rb_execution_context_t *ec, VALUE *stack, size_t size);
 
+// Clear (set to `NULL`) the vm_stack pointer and frame pointer in the execution context.
+// @param ec the execution context to update.
+void rb_ec_clear_vm_stack(rb_execution_context_t *ec);
+
 typedef struct rb_thread_struct {
     struct list_node vmlt_node;
     VALUE self;
-- 
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