From 0524cede37508068b3031912989493e10eb0544a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A9ter=20Szil=C3=A1gyi?= <peterke@gmail.com>
Date: Tue, 11 May 2021 16:23:54 +0300
Subject: [PATCH] eth/tracers: do the JSON serialization via .js to capture C
 faults

---
 eth/tracers/tracer.go      | 24 +++++++++++++++++++-----
 eth/tracers/tracer_test.go | 14 +++++++++-----
 2 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/eth/tracers/tracer.go b/eth/tracers/tracer.go
index ba6592537..218903dd9 100644
--- a/eth/tracers/tracer.go
+++ b/eth/tracers/tracer.go
@@ -505,7 +505,7 @@ func (jst *Tracer) Stop(err error) {
 
 // call executes a method on a JS object, catching any errors, formatting and
 // returning them as error objects.
-func (jst *Tracer) call(method string, args ...string) (json.RawMessage, error) {
+func (jst *Tracer) call(noret bool, method string, args ...string) (json.RawMessage, error) {
 	// Execute the JavaScript call and return any error
 	jst.vm.PushString(method)
 	for _, arg := range args {
@@ -519,7 +519,21 @@ func (jst *Tracer) call(method string, args ...string) (json.RawMessage, error)
 		return nil, errors.New(err)
 	}
 	// No error occurred, extract return value and return
-	return json.RawMessage(jst.vm.JsonEncode(-1)), nil
+	if noret {
+		return nil, nil
+	}
+	// Push a JSON marshaller onto the stack. We can't marshal from the out-
+	// side because duktape can crash on large nestings and we can't catch
+	// C++ exceptions ourselves from Go. TODO(karalabe): Yuck, why wrap?!
+	jst.vm.PushString("(JSON.stringify)")
+	jst.vm.Eval()
+
+	jst.vm.Swap(-1, -2)
+	if code = jst.vm.Pcall(1); code != 0 {
+		err := jst.vm.SafeToString(-1)
+		return nil, errors.New(err)
+	}
+	return json.RawMessage(jst.vm.SafeToString(-1)), nil
 }
 
 func wrapError(context string, err error) error {
@@ -578,7 +592,7 @@ func (jst *Tracer) CaptureState(env *vm.EVM, pc uint64, op vm.OpCode, gas, cost
 		*jst.errorValue = err.Error()
 	}
 
-	if _, err := jst.call("step", "log", "db"); err != nil {
+	if _, err := jst.call(true, "step", "log", "db"); err != nil {
 		jst.err = wrapError("step", err)
 	}
 }
@@ -592,7 +606,7 @@ func (jst *Tracer) CaptureFault(env *vm.EVM, pc uint64, op vm.OpCode, gas, cost
 	jst.errorValue = new(string)
 	*jst.errorValue = err.Error()
 
-	if _, err := jst.call("fault", "log", "db"); err != nil {
+	if _, err := jst.call(true, "fault", "log", "db"); err != nil {
 		jst.err = wrapError("fault", err)
 	}
 }
@@ -640,7 +654,7 @@ func (jst *Tracer) GetResult() (json.RawMessage, error) {
 	jst.vm.PutPropString(jst.stateObject, "ctx")
 
 	// Finalize the trace and return the results
-	result, err := jst.call("result", "ctx", "db")
+	result, err := jst.call(false, "result", "ctx", "db")
 	if err != nil {
 		jst.err = wrapError("result", err)
 	}
diff --git a/eth/tracers/tracer_test.go b/eth/tracers/tracer_test.go
index 7cda2e533..033824474 100644
--- a/eth/tracers/tracer_test.go
+++ b/eth/tracers/tracer_test.go
@@ -78,7 +78,7 @@ func runTrace(tracer *Tracer, vmctx *vmContext) (json.RawMessage, error) {
 }
 
 func TestTracer(t *testing.T) {
-	execTracer := func(code string) []byte {
+	execTracer := func(code string) ([]byte, string) {
 		t.Helper()
 		ctx := &vmContext{blockCtx: vm.BlockContext{BlockNumber: big.NewInt(1)}, txCtx: vm.TxContext{GasPrice: big.NewInt(100000)}}
 		tracer, err := New(code, ctx.txCtx)
@@ -87,13 +87,14 @@ func TestTracer(t *testing.T) {
 		}
 		ret, err := runTrace(tracer, ctx)
 		if err != nil {
-			t.Fatal(err)
+			return nil, err.Error() // Stringify to allow comparison without nil checks
 		}
-		return ret
+		return ret, ""
 	}
 	for i, tt := range []struct {
 		code string
 		want string
+		fail string
 	}{
 		{ // tests that we don't panic on bad arguments to memory access
 			code: "{depths: [], step: function(log) { this.depths.push(log.memory.slice(-1,-2)); }, fault: function() {}, result: function() { return this.depths; }}",
@@ -116,10 +117,13 @@ func TestTracer(t *testing.T) {
 		}, { // tests intrinsic gas
 			code: "{depths: [], step: function() {}, fault: function() {}, result: function(ctx) { return ctx.gasPrice+'.'+ctx.gasUsed+'.'+ctx.intrinsicGas; }}",
 			want: `"100000.6.21000"`,
+		}, { // tests too deep object / serialization crash
+			code: "{step: function() {}, fault: function() {}, result: function() { var o={}; var x=o; for (var i=0; i<1000; i++){	o.foo={}; o=o.foo; } return x; }}",
+			fail: "RangeError: json encode recursion limit    in server-side tracer function 'result'",
 		},
 	} {
-		if have := execTracer(tt.code); tt.want != string(have) {
-			t.Errorf("testcase %d: expected return value to be %s got %s\n\tcode: %v", i, tt.want, string(have), tt.code)
+		if have, err := execTracer(tt.code); tt.want != string(have) || tt.fail != err {
+			t.Errorf("testcase %d: expected return value to be '%s' got '%s', error to be '%s' got '%s'\n\tcode: %v", i, tt.want, string(have), tt.fail, err, tt.code)
 		}
 	}
 }