encoding/json: make error capture logic in recover more type safe

Rather than only ignoring runtime.Error panics, which are a very
narrow set of possible panic values, switch it such that the json
package only captures panic values that have been properly wrapped
in a jsonError struct. This ensures that only intentional panics
originating from the json package are captured.

Fixes #23012

Change-Id: I5e85200259edd2abb1b0512ce6cc288849151a6d
Reviewed-on: https://go-review.googlesource.com/94019
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 5 insertions, 7 deletions

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 1e45e445d9..68512d0225 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -17,7 +17,6 @@ import (
 	"fmt"
 	"math"
 	"reflect"
-	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -286,21 +285,20 @@ func newEncodeState() *encodeState {
 func (e *encodeState) marshal(v interface{}, opts encOpts) (err error) {
 	defer func() {
 		if r := recover(); r != nil {
-			if _, ok := r.(runtime.Error); ok {
+			if je, ok := r.(jsonError); ok {
+				err = je.error
+			} else {
 				panic(r)
 			}
-			if s, ok := r.(string); ok {
-				panic(s)
-			}
-			err = r.(error)
 		}
 	}()
 	e.reflectValue(reflect.ValueOf(v), opts)
 	return nil
 }
 
+// error aborts the encoding by panicking with err wrapped in jsonError.
 func (e *encodeState) error(err error) {
-	panic(err)
+	panic(jsonError{err})
 }
 
 func isEmptyValue(v reflect.Value) bool {