runtime (wasm): scan the system stack

This saves the system stack pointer into a global and uses it to scan.
It should fix some use-after-free issues?

Author: niaow

src/internal/task/task_asyncify.go

@@ -108,6 +108,10 @@ func (*stackState) unwind()
 func (t *Task) Resume() {
 	// The current task must be saved and restored because this can nest on WASM with JS.
 	prevTask := currentTask
+	if prevTask == nil {
+		// Save the system stack pointer.
+		saveStackPointer()
+	}
 	t.gcData.swap()
 	currentTask = t
 	if !t.state.launched {
@@ -123,6 +127,9 @@ func (t *Task) Resume() {
 	}
 }
 
+//go:linkname saveStackPointer runtime.saveStackPointer
+func saveStackPointer()
+
 //export tinygo_rewind
 func (*state) rewind()
 

src/runtime/arch_tinygowasm.go

@@ -73,6 +73,14 @@ func align(ptr uintptr) uintptr {
 //export tinygo_getCurrentStackPointer
 func getCurrentStackPointer() uintptr
 
+// savedStackPointer is used to save the system stack pointer while in a goroutine.
+var savedStackPointer uintptr
+
+// saveStackPointer is called by internal/task.state.Resume() to save the stack pointer before entering a goroutine from the system stack.
+func saveStackPointer() {
+	savedStackPointer = getCurrentStackPointer()
+}
+
 // growHeap tries to grow the heap size. It returns true if it succeeds, false
 // otherwise.
 func growHeap() bool {

src/runtime/gc_stack_portable.go

@@ -29,16 +29,6 @@ type stackChainObject struct {
 //   - The system stack (aka startup stack) is not heap allocated, so even
 //     though it may be referenced it will not be scanned by default.
 //
-// Therefore, we only need to scan the system stack.
-// It is relatively easy to scan the system stack while we're on it: we can
-// simply read __stack_pointer and __global_base and scan the area in between.
-// Unfortunately, it's hard to get the system stack pointer while we're on a
-// goroutine stack. But when we're on a goroutine stack, the system stack is in
-// the scheduler which means there shouldn't be anything on the system stack
-// anyway.
-// ...I hope this assumption holds, otherwise we will need to store the system
-// stack in a global or something.
-//
 // The compiler also inserts code to store all globals in a chain via
 // stackChainStart. Luckily we don't need to scan these, as these globals are
 // stored on the goroutine stack and are therefore already getting scanned.
@@ -50,9 +40,18 @@ func markStack() {
 	// live.
 	volatile.LoadUint32((*uint32)(unsafe.Pointer(&stackChainStart)))
 
+	// Scan the system stack.
+	var sysSP uintptr
 	if task.OnSystemStack() {
-		markRoots(getCurrentStackPointer(), stackTop)
+		// We are on the system stack.
+		// Use the current stack pointer.
+		sysSP = getCurrentStackPointer()
+	} else {
+		// We are in a goroutine.
+		// Use the saved stack pointer.
+		sysSP = savedStackPointer
 	}
+	markRoots(sysSP, stackTop)
 }
 
 // trackPointer is a stub function call inserted by the compiler during IR