Bysyncify: Assertion improvements (#2193)

Add assertions on stack overflow in all 4 Bysyncify API calls (previously only 2 did it).

Also add a check that those assertions are hit.

9 files changed, 375 insertions, 132 deletions

diff --git a/src/passes/Bysyncify.cpp b/src/passes/Bysyncify.cpp
index cb85f0b0a..0897375fe 100644
--- a/src/passes/Bysyncify.cpp
+++ b/src/passes/Bysyncify.cpp
@@ -174,6 +174,17 @@
 // we are recreating the call stack. At that point you should call
 // bysyncify_stop_rewind and then execution can resume normally.
 //
+// Note that the bysyncify_* API calls will verify that the data structure
+// is valid, checking if the current location is past the end. If so, they
+// will throw a wasm unreachable. No check is done during unwinding or
+// rewinding, to keep things fast - in principle, from when unwinding begins
+// and until it ends there should be no memory accesses aside from the
+// bysyncify code itself (and likewise when rewinding), so there should be
+// no chance of memory corruption causing odd errors. However, the low
+// overhead of this approach does cause an error only to be shown when
+// unwinding/rewinding finishes, and not at the specific spot where the
+// stack end was exceeded.
+//
 // By default this pass is very carefuly: it assumes that any import and
 // any indirect call may start an unwind/rewind operation. If you know more
 // specific information you can inform bysyncify about that, which can reduce
@@ -1056,38 +1067,36 @@ private:
 
   void addFunctions(Module* module) {
     Builder builder(*module);
-    auto makeFunction = [&](Name name,
-                            bool setData,
-                            State state,
-                            Expression* extra) {
+    auto makeFunction = [&](Name name, bool setData, State state) {
       std::vector<Type> params;
       if (setData) {
         params.push_back(i32);
       }
       auto* body = builder.makeBlock();
-      if (setData) {
-        // Verify the data is valid.
-        auto* stackPos = builder.makeLoad(4,
-                                          false,
-                                          int32_t(DataOffset::BStackPos),
-                                          4,
-                                          builder.makeLocalGet(0, i32),
-                                          i32);
-        auto* stackEnd = builder.makeLoad(4,
-                                          false,
-                                          int32_t(DataOffset::BStackEnd),
-                                          4,
-                                          builder.makeLocalGet(0, i32),
-                                          i32);
-        body->list.push_back(
-          builder.makeIf(builder.makeBinary(GtUInt32, stackPos, stackEnd),
-                         builder.makeUnreachable()));
-      }
       body->list.push_back(builder.makeGlobalSet(
         BYSYNCIFY_STATE, builder.makeConst(Literal(int32_t(state)))));
-      if (extra) {
-        body->list.push_back(extra);
+      if (setData) {
+        body->list.push_back(
+          builder.makeGlobalSet(BYSYNCIFY_DATA, builder.makeLocalGet(0, i32)));
       }
+      // Verify the data is valid.
+      auto* stackPos =
+        builder.makeLoad(4,
+                         false,
+                         int32_t(DataOffset::BStackPos),
+                         4,
+                         builder.makeGlobalGet(BYSYNCIFY_DATA, i32),
+                         i32);
+      auto* stackEnd =
+        builder.makeLoad(4,
+                         false,
+                         int32_t(DataOffset::BStackEnd),
+                         4,
+                         builder.makeGlobalGet(BYSYNCIFY_DATA, i32),
+                         i32);
+      body->list.push_back(
+        builder.makeIf(builder.makeBinary(GtUInt32, stackPos, stackEnd),
+                       builder.makeUnreachable()));
       body->finalize();
       auto* func = builder.makeFunction(
         name, std::move(params), none, std::vector<Type>{}, body);
@@ -1095,18 +1104,10 @@ private:
       module->addExport(builder.makeExport(name, name, ExternalKind::Function));
     };
 
-    makeFunction(
-      BYSYNCIFY_START_UNWIND,
-      true,
-      State::Unwinding,
-      builder.makeGlobalSet(BYSYNCIFY_DATA, builder.makeLocalGet(0, i32)));
-    makeFunction(BYSYNCIFY_STOP_UNWIND, false, State::Normal, nullptr);
-    makeFunction(
-      BYSYNCIFY_START_REWIND,
-      true,
-      State::Rewinding,
-      builder.makeGlobalSet(BYSYNCIFY_DATA, builder.makeLocalGet(0, i32)));
-    makeFunction(BYSYNCIFY_STOP_REWIND, false, State::Normal, nullptr);
+    makeFunction(BYSYNCIFY_START_UNWIND, true, State::Unwinding);
+    makeFunction(BYSYNCIFY_STOP_UNWIND, false, State::Normal);
+    makeFunction(BYSYNCIFY_START_REWIND, true, State::Rewinding);
+    makeFunction(BYSYNCIFY_STOP_REWIND, false, State::Normal);
   }
 };
 
diff --git a/test/passes/bysyncify.txt b/test/passes/bysyncify.txt
index fbbc09d45..a9a780972 100644
--- a/test/passes/bysyncify.txt
+++ b/test/passes/bysyncify.txt
@@ -297,52 +297,74 @@
   (nop)
  )
  (func $bysyncify_start_unwind (; 6 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 7 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 8 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 8 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 9 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
 (module
@@ -2808,52 +2830,74 @@
   (nop)
  )
  (func $bysyncify_start_unwind (; 19 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 20 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 21 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 21 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 22 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
 (module
@@ -2865,51 +2909,73 @@
  (export "bysyncify_start_rewind" (func $bysyncify_start_rewind))
  (export "bysyncify_stop_rewind" (func $bysyncify_stop_rewind))
  (func $bysyncify_start_unwind (; 0 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 1 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 2 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 2 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 3 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
diff --git a/test/passes/bysyncify_optimize-level=1.txt b/test/passes/bysyncify_optimize-level=1.txt
index b09368ec4..7b1093964 100644
--- a/test/passes/bysyncify_optimize-level=1.txt
+++ b/test/passes/bysyncify_optimize-level=1.txt
@@ -1564,51 +1564,73 @@
   )
  )
  (func $bysyncify_start_unwind (; 19 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 20 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 21 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 21 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 22 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
diff --git a/test/passes/bysyncify_pass-arg=bysyncify-ignore-imports.txt b/test/passes/bysyncify_pass-arg=bysyncify-ignore-imports.txt
index 515fa365a..96133d648 100644
--- a/test/passes/bysyncify_pass-arg=bysyncify-ignore-imports.txt
+++ b/test/passes/bysyncify_pass-arg=bysyncify-ignore-imports.txt
@@ -211,51 +211,73 @@
   )
  )
  (func $bysyncify_start_unwind (; 7 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 8 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 9 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 9 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 10 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
diff --git a/test/passes/bysyncify_pass-arg=bysyncify-ignore-indirect.txt b/test/passes/bysyncify_pass-arg=bysyncify-ignore-indirect.txt
index 64e516cf8..846e05035 100644
--- a/test/passes/bysyncify_pass-arg=bysyncify-ignore-indirect.txt
+++ b/test/passes/bysyncify_pass-arg=bysyncify-ignore-indirect.txt
@@ -517,51 +517,73 @@
   (nop)
  )
  (func $bysyncify_start_unwind (; 7 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 8 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 9 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 9 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 10 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
diff --git a/test/passes/bysyncify_pass-arg=bysyncify-imports@env.import,env.import2.txt b/test/passes/bysyncify_pass-arg=bysyncify-imports@env.import,env.import2.txt
index b22e7d82a..02d353a7a 100644
--- a/test/passes/bysyncify_pass-arg=bysyncify-imports@env.import,env.import2.txt
+++ b/test/passes/bysyncify_pass-arg=bysyncify-imports@env.import,env.import2.txt
@@ -295,52 +295,74 @@
   (nop)
  )
  (func $bysyncify_start_unwind (; 6 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 7 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 8 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 8 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 9 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
 (module
@@ -1961,51 +1983,73 @@
   (nop)
  )
  (func $bysyncify_start_unwind (; 19 ;) (param $0 i32)
+  (global.set $__bysyncify_state
+   (i32.const 1)
+  )
+  (global.set $__bysyncify_data
+   (local.get $0)
+  )
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
-  (global.set $__bysyncify_state
-   (i32.const 1)
-  )
-  (global.set $__bysyncify_data
-   (local.get $0)
-  )
  )
  (func $bysyncify_stop_unwind (; 20 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
- )
- (func $bysyncify_start_rewind (; 21 ;) (param $0 i32)
   (if
    (i32.gt_u
     (i32.load
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
     (i32.load offset=4
-     (local.get $0)
+     (global.get $__bysyncify_data)
     )
    )
    (unreachable)
   )
+ )
+ (func $bysyncify_start_rewind (; 21 ;) (param $0 i32)
   (global.set $__bysyncify_state
    (i32.const 2)
   )
   (global.set $__bysyncify_data
    (local.get $0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
  (func $bysyncify_stop_rewind (; 22 ;)
   (global.set $__bysyncify_state
    (i32.const 0)
   )
+  (if
+   (i32.gt_u
+    (i32.load
+     (global.get $__bysyncify_data)
+    )
+    (i32.load offset=4
+     (global.get $__bysyncify_data)
+    )
+   )
+   (unreachable)
+  )
  )
 )
diff --git a/test/unit/input/bysyncify-stackOverflow.wast b/test/unit/input/bysyncify-stackOverflow.wast
new file mode 100644
index 000000000..a36a06b40
--- /dev/null
+++ b/test/unit/input/bysyncify-stackOverflow.wast
@@ -0,0 +1,22 @@
+(module
+  (memory 1 2)
+  (import "env" "sleep" (func $sleep))
+  (export "memory" (memory 0))
+  (func "many_locals" (param $x i32) (result i32)
+    (local $y i32)
+    (local $z i32)
+    (local.set $y
+      (i32.add (local.get $x) (i32.const 10))
+    )
+    (local.set $z
+      (i32.add (local.get $y) (i32.const 20))
+    )
+    (call $sleep)
+    (select
+      (local.get $y)
+      (local.get $z)
+      (local.get $x)
+    )
+  )
+)
+
diff --git a/test/unit/input/bysyncify.js b/test/unit/input/bysyncify.js
index 2e1bb4895..be6d32903 100644
--- a/test/unit/input/bysyncify.js
+++ b/test/unit/input/bysyncify.js
@@ -252,10 +252,53 @@ function coroutineTests() {
   ]), 'check yielded values')
 }
 
+function stackOverflowAssertTests() {
+  console.log('\nstack overflow assertion tests\n\n');
+
+  // Get and compile the wasm.
+
+  var binary = fs.readFileSync('c.wasm');
+
+  var module = new WebAssembly.Module(binary);
+
+  var DATA_ADDR = 4;
+
+  var instance = new WebAssembly.Instance(module, {
+    env: {
+      sleep: function() {
+        console.log('sleep...');
+        exports.bysyncify_start_unwind(DATA_ADDR);
+        view[DATA_ADDR >> 2] = DATA_ADDR + 8;
+        // The end of the stack will be reached as the stack is tiny.
+        view[DATA_ADDR + 4 >> 2] = view[DATA_ADDR >> 2] + 1;
+      }
+    }
+  });
+
+  var exports = instance.exports;
+  var view = new Int32Array(exports.memory.buffer);
+  exports.many_locals();
+  assert(view[DATA_ADDR >> 2] > view[DATA_ADDR + 4 >> 2], 'should have wrote past the end of the stack');
+  // All API calls should now fail, since we wrote past the end of the
+  // stack
+  var fails = 0;
+  ['bysyncify_stop_unwind', 'bysyncify_start_rewind', 'bysyncify_stop_rewind', 'bysyncify_start_unwind'].forEach(function(name) {
+    try {
+      exports[name](DATA_ADDR);
+      console.log('no fail on', name);
+    } catch (e) {
+      console.log('expected fail on', name);
+      fails++;
+    }
+  });
+  assert(fails == 4, 'all 4 should have failed');
+}
+
 // Main
 
 sleepTests();
 coroutineTests();
+stackOverflowAssertTests();
 
 console.log('\ntests completed successfully');
 
diff --git a/test/unit/test_bysyncify.py b/test/unit/test_bysyncify.py
index 8d5a780e8..407f3f7f9 100644
--- a/test/unit/test_bysyncify.py
+++ b/test/unit/test_bysyncify.py
@@ -10,6 +10,7 @@ class BysyncifyTest(BinaryenTestCase):
       print(args)
       run_process(WASM_OPT + args + [self.input_path('bysyncify-sleep.wast'), '--bysyncify', '-o', 'a.wasm'])
       run_process(WASM_OPT + args + [self.input_path('bysyncify-coroutine.wast'), '--bysyncify', '-o', 'b.wasm'])
+      run_process(WASM_OPT + args + [self.input_path('bysyncify-stackOverflow.wast'), '--bysyncify', '-o', 'c.wasm'])
       print('  file size: %d' % os.path.getsize('a.wasm'))
       run_process([NODEJS, self.input_path('bysyncify.js')])