Rewrite chuser() for simplicity and correctness (#1203)

- Use unambiguous variable names (w/o package name conflict).
- Fail on invalid input such as the empty string or `:`.
- Do not change group without user, i.e. fail on `:group`.
- Parse input using mnemonic APIs.
- Do not juggle between integer types.
- Unset supplementary groups.
- Use set[ug]id(2) to follow the idiom of OpenBSD base programs.
  (cannot use setres[ug]id(2) as macOS does not have them.)

Includes/Supersedes #1202.
Fixes #927.

I only tested on OpenBSD (so far), but other systems should just work.

cmd/yggdrasil/chuser_unix.go

@@ -4,89 +4,53 @@
 package main
 
 import (
-	"errors"
 	"fmt"
-	"math"
-	osuser "os/user"
+	"os/user"
 	"strconv"
 	"strings"
-	"syscall"
+
+	"golang.org/x/sys/unix"
 )
 
-func chuser(user string) error {
-	group := ""
-	if i := strings.IndexByte(user, ':'); i >= 0 {
-		user, group = user[:i], user[i+1:]
-	}
+func chuser(input string) error {
+	givenUser, givenGroup, _ := strings.Cut(input, ":")
 
-	u := (*osuser.User)(nil)
-	g := (*osuser.Group)(nil)
+	var (
+		err      error
+		usr      *user.User
+		grp      *user.Group
+		uid, gid int
+	)
 
-	if user != "" {
-		if _, err := strconv.ParseUint(user, 10, 32); err == nil {
-			u, err = osuser.LookupId(user)
-			if err != nil {
-				return fmt.Errorf("failed to lookup user by id %q: %v", user, err)
-			}
-		} else {
-			u, err = osuser.Lookup(user)
-			if err != nil {
-				return fmt.Errorf("failed to lookup user by name %q: %v", user, err)
-			}
+	if usr, err = user.LookupId(givenUser); err != nil {
+		if usr, err = user.Lookup(givenUser); err != nil {
+			return err
 		}
 	}
-	if group != "" {
-		if _, err := strconv.ParseUint(group, 10, 32); err == nil {
-			g, err = osuser.LookupGroupId(group)
-			if err != nil {
-				return fmt.Errorf("failed to lookup group by id %q: %v", user, err)
-			}
-		} else {
-			g, err = osuser.LookupGroup(group)
-			if err != nil {
-				return fmt.Errorf("failed to lookup group by name %q: %v", user, err)
-			}
-		}
+	if uid, err = strconv.Atoi(usr.Uid); err != nil {
+		return err
 	}
 
-	if g != nil {
-		gid, _ := strconv.ParseUint(g.Gid, 10, 32)
-		var err error
-		if gid < math.MaxInt {
-			if err := syscall.Setgroups([]int{int(gid)}); err != nil {
-				return fmt.Errorf("failed to setgroups %d: %v", gid, err)
+	if givenGroup != "" {
+		if grp, err = user.LookupGroupId(givenGroup); err != nil {
+			if grp, err = user.LookupGroup(givenGroup); err != nil {
+				return err
 			}
-			err = syscall.Setgid(int(gid))
-		} else {
-			err = errors.New("gid too big")
 		}
 
-		if err != nil {
-			return fmt.Errorf("failed to setgid %d: %v", gid, err)
-		}
-	} else if u != nil {
-		gid, _ := strconv.ParseUint(u.Gid, 10, 32)
-		if err := syscall.Setgroups([]int{int(uint32(gid))}); err != nil {
-			return fmt.Errorf("failed to setgroups %d: %v", gid, err)
-		}
-		err := syscall.Setgid(int(uint32(gid)))
-		if err != nil {
-			return fmt.Errorf("failed to setgid %d: %v", gid, err)
-		}
+		gid, _ = strconv.Atoi(grp.Gid)
+	} else {
+		gid, _ = strconv.Atoi(usr.Gid)
 	}
 
-	if u != nil {
-		uid, _ := strconv.ParseUint(u.Uid, 10, 32)
-		var err error
-		if uid < math.MaxInt {
-			err = syscall.Setuid(int(uid))
-		} else {
-			err = errors.New("uid too big")
-		}
-
-		if err != nil {
-			return fmt.Errorf("failed to setuid %d: %v", uid, err)
-		}
+	if err := unix.Setgroups([]int{gid}); err != nil {
+		return fmt.Errorf("setgroups: %d: %v", gid, err)
+	}
+	if err := unix.Setgid(gid); err != nil {
+		return fmt.Errorf("setgid: %d: %v", gid, err)
+	}
+	if err := unix.Setuid(uid); err != nil {
+		return fmt.Errorf("setuid: %d: %v", uid, err)
 	}
 
 	return nil