1// Copyright 2021 The Go Authors. All rights reserved.2// Use of this source code is governed by a BSD-style3// license that can be found in the LICENSE file.45//go:build (((unix && !android) || (js && wasm) || wasip1) && ((!cgo && !darwin) || osusergo)) || aix67package user89import (10	"bufio"11	"bytes"12	"errors"13	"fmt"14	"io"15	"os"16	"strconv"17)1819func listGroupsFromReader(u *User, r io.Reader) ([]string, error) {20	if u.Username == "" {21		return nil, errors.New("user: list groups: empty username")22	}23	primaryGid, err := strconv.Atoi(u.Gid)24	if err != nil {25		return nil, fmt.Errorf("user: list groups for %s: invalid gid %q", u.Username, u.Gid)26	}2728	userCommas := []byte("," + u.Username + ",")  // ,john,29	userFirst := userCommas[1:]                   // john,30	userLast := userCommas[:len(userCommas)-1]    // ,john31	userOnly := userCommas[1 : len(userCommas)-1] // john3233	// Add primary Gid first.34	groups := []string{u.Gid}3536	rd := bufio.NewReader(r)37	done := false38	for !done {39		line, err := rd.ReadBytes('\n')40		if err != nil {41			if err == io.EOF {42				done = true43			} else {44				return groups, err45			}46		}4748		// Look for username in the list of users. If user is found,49		// append the GID to the groups slice.5051		// There's no spec for /etc/passwd or /etc/group, but we try to follow52		// the same rules as the glibc parser, which allows comments and blank53		// space at the beginning of a line.54		line = bytes.TrimSpace(line)55		if len(line) == 0 || line[0] == '#' ||56			// If you search for a gid in a row where the group57			// name (the first field) starts with "+" or "-",58			// glibc fails to find the record, and so should we.59			line[0] == '+' || line[0] == '-' {60			continue61		}6263		// Format of /etc/group is64		// 	groupname:password:GID:user_list65		// for example66		// 	wheel:x:10:john,paul,jack67		//	tcpdump:x:72:68		listIdx := bytes.LastIndexByte(line, ':')69		if listIdx == -1 || listIdx == len(line)-1 {70			// No commas, or empty group list.71			continue72		}73		if bytes.Count(line[:listIdx], colon) != 2 {74			// Incorrect number of colons.75			continue76		}77		list := line[listIdx+1:]78		// Check the list for user without splitting or copying.79		if !(bytes.Equal(list, userOnly) || bytes.HasPrefix(list, userFirst) || bytes.HasSuffix(list, userLast) || bytes.Contains(list, userCommas)) {80			continue81		}8283		// groupname:password:GID84		parts := bytes.Split(line[:listIdx], colon)85		if len(parts) != 3 || len(parts[0]) == 0 {86			continue87		}88		gid := string(parts[2])89		// Make sure it's numeric and not the same as primary GID.90		numGid, err := strconv.Atoi(gid)91		if err != nil || numGid == primaryGid {92			continue93		}9495		groups = append(groups, gid)96	}9798	return groups, nil99}100101func listGroups(u *User) ([]string, error) {102	f, err := os.Open(groupFile)103	if err != nil {104		return nil, err105	}106	defer f.Close()107108	return listGroupsFromReader(u, f)109}