user: lookup: added os/user-like lookup API

This patch adds an os/user-like user lookup API, implemented in pure Go.
It also has some features not present in the standard library
implementation (such as group lookups).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)

namespaces/init.go

@@ -174,7 +174,17 @@ func SetupUser(u string) error {
 		Home: "/",
 	}
 
-	execUser, err := user.GetExecUserFile(u, &defaultExecUser, "/etc/passwd", "/etc/group")
+	passwdFile, err := user.GetPasswdFile()
+	if err != nil {
+		return err
+	}
+
+	groupFile, err := user.GetGroupFile()
+	if err != nil {
+		return err
+	}
+
+	execUser, err := user.GetExecUserFile(u, &defaultExecUser, passwdFile, groupFile)
 	if err != nil {
 		return fmt.Errorf("get supplementary groups %s", err)
 	}

user/lookup.go

@@ -0,0 +1,108 @@
+package user
+
+import (
+	"errors"
+	"fmt"
+	"syscall"
+)
+
+var (
+	// The current operating system does not provide the required data for user lookups.
+	ErrUnsupported = errors.New("user lookup: operating system does not provide passwd-formatted data")
+)
+
+func lookupUser(filter func(u User) bool) (User, error) {
+	// Get operating system-specific passwd reader-closer.
+	passwd, err := GetPasswd()
+	if err != nil {
+		return User{}, err
+	}
+	defer passwd.Close()
+
+	// Get the users.
+	users, err := ParsePasswdFilter(passwd, filter)
+	if err != nil {
+		return User{}, err
+	}
+
+	// No user entries found.
+	if len(users) == 0 {
+		return User{}, fmt.Errorf("no matching entries in passwd file")
+	}
+
+	// Assume the first entry is the "correct" one.
+	return users[0], nil
+}
+
+// CurrentUser looks up the current user by their user id in /etc/passwd. If the
+// user cannot be found (or there is no /etc/passwd file on the filesystem),
+// then CurrentUser returns an error.
+func CurrentUser() (User, error) {
+	return LookupUid(syscall.Getuid())
+}
+
+// LookupUser looks up a user by their username in /etc/passwd. If the user
+// cannot be found (or there is no /etc/passwd file on the filesystem), then
+// LookupUser returns an error.
+func LookupUser(username string) (User, error) {
+	return lookupUser(func(u User) bool {
+		return u.Name == username
+	})
+}
+
+// LookupUid looks up a user by their user id in /etc/passwd. If the user cannot
+// be found (or there is no /etc/passwd file on the filesystem), then LookupId
+// returns an error.
+func LookupUid(uid int) (User, error) {
+	return lookupUser(func(u User) bool {
+		return u.Uid == uid
+	})
+}
+
+func lookupGroup(filter func(g Group) bool) (Group, error) {
+	// Get operating system-specific group reader-closer.
+	group, err := GetGroup()
+	if err != nil {
+		return Group{}, err
+	}
+	defer group.Close()
+
+	// Get the users.
+	groups, err := ParseGroupFilter(group, filter)
+	if err != nil {
+		return Group{}, err
+	}
+
+	// No user entries found.
+	if len(groups) == 0 {
+		return Group{}, fmt.Errorf("no matching entries in group file")
+	}
+
+	// Assume the first entry is the "correct" one.
+	return groups[0], nil
+}
+
+// CurrentGroup looks up the current user's group by their primary group id's
+// entry in /etc/passwd. If the group cannot be found (or there is no
+// /etc/group file on the filesystem), then CurrentGroup returns an error.
+func CurrentGroup() (Group, error) {
+	return LookupGid(syscall.Getgid())
+}
+
+// LookupGroup looks up a group by its name in /etc/group. If the group cannot
+// be found (or there is no /etc/group file on the filesystem), then LookupGroup
+// returns an error.
+func LookupGroup(groupname string) (Group, error) {
+	return lookupGroup(func(g Group) bool {
+		return g.Name == groupname
+	})
+}
+
+// LookupGid looks up a group by its group id in /etc/group. If the group cannot
+// be found (or there is no /etc/group file on the filesystem), then LookupGid
+// returns an error.
+func LookupGid(gid int) (Group, error) {
+	return lookupGroup(func(g Group) bool {
+		return g.Gid == gid
+	})
+}

user/lookup_unix.go

@@ -0,0 +1,30 @@
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package user
+
+import (
+	"io"
+	"os"
+)
+
+// Unix-specific path to the passwd and group formatted files.
+const (
+	unixPasswdFile = "/etc/passwd"
+	unixGroupFile  = "/etc/group"
+)
+
+func GetPasswdFile() (string, error) {
+	return unixPasswdFile, nil
+}
+
+func GetPasswd() (io.ReadCloser, error) {
+	return os.Open(unixPasswdFile)
+}
+
+func GetGroupFile() (string, error) {
+	return unixGroupFile, nil
+}
+
+func GetGroup() (io.ReadCloser, error) {
+	return os.Open(unixGroupFile)
+}

user/lookup_unsupported.go

@@ -0,0 +1,21 @@
+// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris
+
+package user
+
+import "io"
+
+func GetPasswdFile() (string, error) {
+	return "", ErrUnsupported
+}
+
+func GetPasswd() (io.ReadCloser, error) {
+	return nil, ErrUnsupported
+}
+
+func GetGroupFile() (string, error) {
+	return "", ErrUnsupported
+}
+
+func GetGroup() (io.ReadCloser, error) {
+	return nil, ErrUnsupported
+}