cmd/vet: detect non-pointer arguments for unmarshal and decode

Checks usage of Unmarshal and Decode functions in json, gob and
xml packages to detect attempts to decode into non-pointer types.

Fixes #27564

Change-Id: I07bbd5be82d61834ffde9af9937329d7fb1f05d0
Reviewed-on: https://go-review.googlesource.com/c/139997
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/vet/testdata/unmarshal.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains tests for the unmarshal checker.
+
+package testdata
+
+import (
+	"bytes"
+	"encoding/gob"
+	"encoding/json"
+	"encoding/xml"
+	"errors"
+	"fmt"
+)
+
+func _() {
+	type t struct {
+		a int
+	}
+	var v t
+	var r io.Reader
+
+	json.Unmarshal([]byte{}, v) // ERROR "call of Unmarshal passes non-pointer as second argument"
+	json.Unmarshal([]byte{}, &v)
+	json.NewDecoder(r).Decode(v) // ERROR "call of Decode passes non-pointer"
+	json.NewDecoder(r).Decode(&v)
+	gob.NewDecoder(r).Decode(v) // ERROR "call of Decode passes non-pointer"
+	gob.NewDecoder(r).Decode(&v)
+	xml.Unmarshal([]byte{}, v) // ERROR "call of Unmarshal passes non-pointer as second argument"
+	xml.Unmarshal([]byte{}, &v)
+	xml.NewDecoder(r).Decode(v) // ERROR "call of Decode passes non-pointer"
+	xml.NewDecoder(r).Decode(&v)
+
+	var p *t
+	json.Unmarshal([]byte{}, p)
+	json.Unmarshal([]byte{}, *p) // ERROR "call of Unmarshal passes non-pointer as second argument"
+	json.NewDecoder(r).Decode(p)
+	json.NewDecoder(r).Decode(*p) // ERROR "call of Decode passes non-pointer"
+	gob.NewDecoder(r).Decode(p)
+	gob.NewDecoder(r).Decode(*p) // ERROR "call of Decode passes non-pointer"
+	xml.Unmarshal([]byte{}, p)
+	xml.Unmarshal([]byte{}, *p) // ERROR "call of Unmarshal passes non-pointer as second argument"
+	xml.NewDecoder(r).Decode(p)
+	xml.NewDecoder(r).Decode(*p) // ERROR "call of Decode passes non-pointer"
+
+	var i interface{}
+	json.Unmarshal([]byte{}, i)
+	json.NewDecoder(r).Decode(i)
+
+	json.Unmarshal([]byte{}, nil)               // ERROR "call of Unmarshal passes non-pointer as second argument"
+	json.Unmarshal([]byte{}, []t{})             // ERROR "call of Unmarshal passes non-pointer as second argument"
+	json.Unmarshal([]byte{}, map[string]int{})  // ERROR "call of Unmarshal passes non-pointer as second argument"
+	json.NewDecoder(r).Decode(nil)              // ERROR "call of Decode passes non-pointer"
+	json.NewDecoder(r).Decode([]t{})            // ERROR "call of Decode passes non-pointer"
+	json.NewDecoder(r).Decode(map[string]int{}) // ERROR "call of Decode passes non-pointer"
+
+	json.Unmarshal(func() ([]byte, interface{}) { return []byte{}, v }())
+}

src/cmd/vet/unmarshal.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file defines the check for passing non-pointer or non-interface
+// types to unmarshal and decode functions.
+
+package main
+
+import (
+	"go/ast"
+	"go/types"
+	"strings"
+)
+
+func init() {
+	register("unmarshal",
+		"check for passing non-pointer or non-interface types to unmarshal and decode functions",
+		checkUnmarshalArg,
+		callExpr)
+}
+
+var pointerArgFuncs = map[string]int{
+	"encoding/json.Unmarshal":         1,
+	"(*encoding/json.Decoder).Decode": 0,
+	"(*encoding/gob.Decoder).Decode":  0,
+	"encoding/xml.Unmarshal":          1,
+	"(*encoding/xml.Decoder).Decode":  0,
+}
+
+func checkUnmarshalArg(f *File, n ast.Node) {
+	call, ok := n.(*ast.CallExpr)
+	if !ok {
+		return // not a call statement
+	}
+	fun := unparen(call.Fun)
+
+	if f.pkg.types[fun].IsType() {
+		return // a conversion, not a call
+	}
+
+	info := &types.Info{Uses: f.pkg.uses, Selections: f.pkg.selectors}
+	name := callName(info, call)
+
+	arg, ok := pointerArgFuncs[name]
+	if !ok {
+		return // not a function we are interested in
+	}
+
+	if len(call.Args) < arg+1 {
+		return // not enough arguments, e.g. called with return values of another function
+	}
+
+	typ := f.pkg.types[call.Args[arg]]
+
+	if typ.Type == nil {
+		return // type error prevents further analysis
+	}
+
+	switch typ.Type.Underlying().(type) {
+	case *types.Pointer, *types.Interface:
+		return
+	}
+
+	shortname := name[strings.LastIndexByte(name, '.')+1:]
+	switch arg {
+	case 0:
+		f.Badf(call.Lparen, "call of %s passes non-pointer", shortname)
+	case 1:
+		f.Badf(call.Lparen, "call of %s passes non-pointer as second argument", shortname)
+	}
+}