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group.go
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group.go
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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2017-2019 Canonical Ltd
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 3 as
* published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
package osutil
import (
"bytes"
"fmt"
"os/exec"
"os/user"
"strconv"
)
// FindUid returns the identifier of the given UNIX user name. It will
// automatically fallback to use "getent" if needed.
var FindUid = findUid
// FindGid returns the identifier of the given UNIX group name. It will
// automatically fallback to use "getent" if needed.
var FindGid = findGid
// getent returns the identifier of the given UNIX user or group name as
// determined by the specified database
func getent(database, name string) (uint64, error) {
if database != "passwd" && database != "group" {
return 0, fmt.Errorf(`unsupported getent database "%q"`, database)
}
cmdStr := []string{
"getent",
database,
name,
}
cmd := exec.Command(cmdStr[0], cmdStr[1:]...)
output, err := cmd.CombinedOutput()
if err != nil {
// according to getent(1) the exit value of "2" means:
// "One or more supplied key could not be found in the
// database."
exitCode, _ := ExitCode(err)
if exitCode == 2 {
if database == "passwd" {
return 0, user.UnknownUserError(name)
}
return 0, user.UnknownGroupError(name)
}
return 0, fmt.Errorf("getent failed with: %v", OutputErr(output, err))
}
// passwd has 7 entries and group 4. In both cases, parts[2] is the id
parts := bytes.Split(output, []byte(":"))
if len(parts) < 4 {
return 0, fmt.Errorf("malformed entry: %q", output)
}
return strconv.ParseUint(string(parts[2]), 10, 64)
}
// TODO: both findUidNoGetentFallback and findGidNoGetentFallback should return
// a more qualified default value than uint64, because currently the
// default return value for findUid is "0" as per Go conventions, which is
// unfortunately also the uid of root, so if a caller ignored the error
// from this function and used that to perform access authorization, then
// the caller would accidentally provide the same access level as root in
// the error case. This is excaberated by the fact that the error case is
// very difficult to positively identify correctly as "not found", see the
// comments inside the functions for more details.
// Note: there is a similar implementation in overlord/snapshotstate which
// should be similarly adjusted when resolving the above TODO.
var findUidNoGetentFallback = func(username string) (uint64, error) {
myuser, err := user.Lookup(username)
if err != nil {
// Treat all non-nil errors as user.Unknown{User,Group}Error's, as
// currently Go's handling of returned errno from get{pw,gr}nam_r
// in the cgo implementation of user.Lookup is lacking, and thus
// user.Unknown{User,Group}Error is returned only when errno is 0
// and the list of users/groups is empty, but as per the man page
// for get{pw,gr}nam_r, there are many other errno's that typical
// systems could return to indicate that the user/group wasn't
// found, however unfortunately the POSIX standard does not actually
// dictate what errno should be used to indicate "user/group not
// found", and so even if Go is more robust, it may not ever be
// fully robust. See from the man page:
//
// > It [POSIX.1-2001] does not call "not found" an error, hence
// > does not specify what value errno might have in this situation.
// > But that makes it impossible to recognize errors.
//
// See upstream Go issue: https://github.com/golang/go/issues/40334
// if there is a real problem finding the user/group then presumably
// other things will fail upon trying to create the user, etc. which
// will give more useful and specific errors
return 0, user.UnknownUserError(username)
}
return strconv.ParseUint(myuser.Uid, 10, 64)
}
var findGidNoGetentFallback = func(groupname string) (uint64, error) {
group, err := user.LookupGroup(groupname)
if err != nil {
// Treat all non-nil errors as user.Unknown{User,Group}Error's, as
// currently Go's handling of returned errno from get{pw,gr}nam_r
// in the cgo implementation of user.Lookup is lacking, and thus
// user.Unknown{User,Group}Error is returned only when errno is 0
// and the list of users/groups is empty, but as per the man page
// for get{pw,gr}nam_r, there are many other errno's that typical
// systems could return to indicate that the user/group wasn't
// found, however unfortunately the POSIX standard does not actually
// dictate what errno should be used to indicate "user/group not
// found", and so even if Go is more robust, it may not ever be
// fully robust. See from the man page:
//
// > It [POSIX.1-2001] does not call "not found" an error, hence
// > does not specify what value errno might have in this situation.
// > But that makes it impossible to recognize errors.
//
// See upstream Go issue: https://github.com/golang/go/issues/40334
// if there is a real problem finding the user/group then presumably
// other things will fail upon trying to create the user, etc. which
// will give more useful and specific errors
return 0, user.UnknownGroupError(groupname)
}
return strconv.ParseUint(group.Gid, 10, 64)
}
// findUidWithGetentFallback returns the identifier of the given UNIX user name with
// getent fallback
func findUidWithGetentFallback(username string) (uint64, error) {
// first do the cheap os/user lookup
myuser, err := findUidNoGetentFallback(username)
switch err.(type) {
case nil:
// found it!
return myuser, nil
case user.UnknownUserError:
// user unknown, let's try getent
return getent("passwd", username)
default:
// something weird happened with the lookup, just report it
return 0, err
}
}
// findGidWithGetentFallback returns the identifier of the given UNIX group name with
// getent fallback
func findGidWithGetentFallback(groupname string) (uint64, error) {
// first do the cheap os/user lookup
group, err := findGidNoGetentFallback(groupname)
switch err.(type) {
case nil:
// found it!
return group, nil
case user.UnknownGroupError:
// group unknown, let's try getent
return getent("group", groupname)
default:
// something weird happened with the lookup, just report it
return 0, err
}
}
func IsUnknownUser(err error) bool {
_, ok := err.(user.UnknownUserError)
return ok
}
func IsUnknownGroup(err error) bool {
_, ok := err.(user.UnknownGroupError)
return ok
}