syscall: check for invalid characters in Setenv on Unix

On POSIX, '=' in key is explicitly invalid, and '\x00' in key/value is implicitly invalid.

R=golang-dev, iant, bradfitz
CC=golang-dev
https://golang.org/cl/7311061

src/pkg/os/env_unix_test.go

+// Copyright 2013 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.
+
+// +build darwin freebsd linux netbsd openbsd
+
+package os_test
+
+import (
+	. "os"
+	"testing"
+)
+
+var setenvEinvalTests = []struct {
+	k, v string
+}{
+	{"", ""},      // empty key
+	{"k=v", ""},   // '=' in key
+	{"\x00", ""},  // '\x00' in key
+	{"k", "\x00"}, // '\x00' in value
+}
+
+func TestSetenvUnixEinval(t *testing.T) {
+	for _, tt := range setenvEinvalTests {
+		err := Setenv(tt.k, tt.v)
+		if err == nil {
+			t.Errorf(`Setenv(%q, %q) == nil, want error`, tt.k, tt.v)
+		}
+	}
+}

src/pkg/syscall/env_unix.go

@@ -71,6 +71,16 @@ func Setenv(key, value string) error {
 	if len(key) == 0 {
 		return EINVAL
 	}
+	for i := 0; i < len(key); i++ {
+		if key[i] == '=' || key[i] == 0 {
+			return EINVAL
+		}
+	}
+	for i := 0; i < len(value); i++ {
+		if value[i] == 0 {
+			return EINVAL
+		}
+	}
 
 	envLock.Lock()
 	defer envLock.Unlock()