util: Add better user lookup functions

When looking up users in fscrypt, we often want to proceed even if the
requested uid doesn't appear to be a valid user on the system. This
mainly occurs when a user is deleted, but they still have a login
protector on disk.

Thus, GetUser() with a bad uid creates a fake user with a pretty
placeholder name. The corresponding call sites of util.EffectiveUser are
changed (often simplifying logic). Various documentation is updated and
typos are fixed.

2 files changed, 54 insertions, 9 deletions

diff --git a/util/users.go b/util/users.go
new file mode 100644
index 0000000..92affa8
--- /dev/null
+++ b/util/users.go
@@ -0,0 +1,50 @@
+/*
+ * util.go - Functions for dealing with users
+ *
+ * Copyright 2017 Google Inc.
+ * Author: Joe Richey (joerichey@google.com)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package util
+
+import (
+	"fmt"
+	"os"
+	"os/user"
+	"strconv"
+)
+
+// CurrentUserID returns the uid of the effective user.
+func CurrentUserID() int {
+	return os.Geteuid()
+}
+
+// GetUser returns the user entry corresponding to the provided uid.
+func GetUser(uid int) *user.User {
+	uidString := strconv.Itoa(uid)
+	foundUser, err := user.LookupId(uidString)
+	if err != nil {
+		return &user.User{
+			Uid:      uidString,
+			Username: fmt.Sprintf("[uid=%d]", uid),
+		}
+	}
+	return foundUser
+}
+
+// CurrentUser returns the user entry for the effective user.
+func CurrentUser() *user.User {
+	return GetUser(CurrentUserID())
+}
diff --git a/util/util.go b/util/util.go
index 3de4a1a..ed78519 100644
--- a/util/util.go
+++ b/util/util.go
@@ -19,15 +19,15 @@
 
 // Package util contains useful components for simplifying Go code.
 //
-// The package contains common error types (errors.go) and functions for
-// converting arrays to pointers.
+// The package contains functions missing from the standard library, error
+// utility functions, functions for converting arrays to pointers, and functions
+// for dealing with users and uids.
 package util
 
 import (
 	"bufio"
 	"math"
 	"os"
-	"os/user"
 	"strconv"
 	"unsafe"
 )
@@ -118,12 +118,7 @@ func AtoiOrPanic(input string) int {
 	return i
 }
 
-// EffectiveUser returns the user entry corresponding to the effective user.
-func EffectiveUser() (*user.User, error) {
-	return user.LookupId(strconv.Itoa(os.Geteuid()))
-}
-
 // IsUserRoot checks if the effective user is root.
 func IsUserRoot() bool {
-	return os.Geteuid() == 0
+	return CurrentUserID() == 0
 }