os/user: handle large 32-bit uid/gid values when stringifying User.Uid/Gid

Fixes #22739

Change-Id: I374c29d237c498c9e5ac848b01f6d49d7c41b31f
Reviewed-on: https://go-review.googlesource.com/77930
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/user/cgo_lookup_unix.go

@@ -114,8 +114,8 @@ func lookupUnixUid(uid int) (*User, error) {
 
 func buildUser(pwd *C.struct_passwd) *User {
 	u := &User{
-		Uid:      strconv.Itoa(int(pwd.pw_uid)),
-		Gid:      strconv.Itoa(int(pwd.pw_gid)),
+		Uid:      strconv.FormatUint(uint64(pwd.pw_uid), 10),
+		Gid:      strconv.FormatUint(uint64(pwd.pw_gid), 10),
 		Username: C.GoString(pwd.pw_name),
 		Name:     C.GoString(pwd.pw_gecos),
 		HomeDir:  C.GoString(pwd.pw_dir),
@@ -269,3 +269,11 @@ const maxBufferSize = 1 << 20
 func isSizeReasonable(sz int64) bool {
 	return sz > 0 && sz <= maxBufferSize
 }
+
+// Because we can't use cgo in tests:
+func structPasswdForNegativeTest() C.struct_passwd {
+	sp := C.struct_passwd{}
+	sp.pw_uid = 1<<32 - 2
+	sp.pw_gid = 1<<32 - 3
+	return sp
+}

src/os/user/cgo_unix_test.go

+// Copyright 2017 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 dragonfly freebsd !android,linux netbsd openbsd solaris
+// +build cgo
+
+package user
+
+import (
+	"testing"
+)
+
+// Issue 22739
+func TestNegativeUid(t *testing.T) {
+	sp := structPasswdForNegativeTest()
+	u := buildUser(&sp)
+	if g, w := u.Uid, "4294967294"; g != w {
+		t.Errorf("Uid = %q; want %q", g, w)
+	}
+	if g, w := u.Gid, "4294967293"; g != w {
+		t.Errorf("Gid = %q; want %q", g, w)
+	}
+}