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@@ -0,0 +1,139 @@
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+// Copyright (c) Tailscale Inc & AUTHORS
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+// SPDX-License-Identifier: BSD-3-Clause
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+
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+// Package osuser implements OS user lookup. It's a wrapper around os/user that
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+// works on non-cgo builds.
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+package osuser
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+
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+import (
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+ "context"
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+ "errors"
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+ "log"
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+ "os/exec"
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+ "os/user"
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+ "runtime"
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+ "strings"
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+ "time"
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+ "unicode/utf8"
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+
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+ "tailscale.com/version/distro"
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+)
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+
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+// LookupByUIDWithShell is like os/user.LookupId but handles a few edge cases
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+// like gokrazy and non-cgo lookups, and returns the user shell. The user shell
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+// lookup is best-effort and may be empty.
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+func LookupByUIDWithShell(uid string) (u *user.User, shell string, err error) {
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+ return lookup(uid, user.LookupId, true)
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+}
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+
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+// LookupByUsernameWithShell is like os/user.Lookup but handles a few edge
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+// cases like gokrazy and non-cgo lookups, and returns the user shell. The user
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+// shell lookup is best-effort and may be empty.
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+func LookupByUsernameWithShell(username string) (u *user.User, shell string, err error) {
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+ return lookup(username, user.Lookup, true)
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+}
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+
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+// LookupByUID is like os/user.LookupId but handles a few edge cases like
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+// gokrazy and non-cgo lookups.
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+func LookupByUID(uid string) (*user.User, error) {
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+ u, _, err := lookup(uid, user.LookupId, false)
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+ return u, err
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+}
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+
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+// LookupByUsername is like os/user.Lookup but handles a few edge cases like
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+// gokrazy and non-cgo lookups.
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+func LookupByUsername(username string) (*user.User, error) {
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+ u, _, err := lookup(username, user.Lookup, false)
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+ return u, err
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+}
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+
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+// lookupStd is either user.Lookup or user.LookupId.
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+type lookupStd func(string) (*user.User, error)
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+
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+func lookup(usernameOrUID string, std lookupStd, wantShell bool) (*user.User, string, error) {
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+ // TODO(awly): we should use genet on more platforms, like FreeBSD.
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+ if runtime.GOOS != "linux" {
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+ u, err := std(usernameOrUID)
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+ return u, "", err
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+ }
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+
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+ // No getent on Gokrazy. So hard-code the login shell.
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+ if distro.Get() == distro.Gokrazy {
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+ var shell string
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+ if wantShell {
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+ shell = "/tmp/serial-busybox/ash"
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+ }
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+ u, err := std(usernameOrUID)
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+ if err != nil {
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+ return &user.User{
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+ Uid: "0",
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+ Gid: "0",
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+ Username: "root",
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+ Name: "Gokrazy",
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+ HomeDir: "/",
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+ }, shell, nil
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+ }
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+ return u, shell, nil
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+ }
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+
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+ // Start with getent if caller wants to get the user shell.
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+ if wantShell {
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+ return userLookupGetent(usernameOrUID, std)
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+ }
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+ // If shell is not required, try os/user.Lookup* first and only use getent
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+ // if that fails. This avoids spawning a child process when os/user lookup
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+ // succeeds.
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+ if u, err := std(usernameOrUID); err == nil {
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+ return u, "", nil
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+ }
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+ return userLookupGetent(usernameOrUID, std)
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+}
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+
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+func checkGetentInput(usernameOrUID string) bool {
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+ maxUid := 32
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+ if runtime.GOOS == "linux" {
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+ maxUid = 256
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+ }
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+ if len(usernameOrUID) > maxUid || len(usernameOrUID) == 0 {
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+ return false
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+ }
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+ for _, r := range usernameOrUID {
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+ if r < ' ' || r == 0x7f || r == utf8.RuneError { // TODO(bradfitz): more?
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+ return false
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+ }
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+ }
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+ return true
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+}
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+
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+// userLookupGetent uses "getent" to look up users so that even with static
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+// tailscaled binaries without cgo (as we distribute), we can still look up
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+// PAM/NSS users which the standard library's os/user without cgo won't get
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+// (because of no libc hooks). If "getent" fails, userLookupGetent falls back
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+// to the standard library.
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+func userLookupGetent(usernameOrUID string, std lookupStd) (*user.User, string, error) {
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+ // Do some basic validation before passing this string to "getent", even though
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+ // getent should do its own validation.
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+ if !checkGetentInput(usernameOrUID) {
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+ return nil, "", errors.New("invalid username or UID")
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+ }
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+ ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
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+ defer cancel()
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+ out, err := exec.CommandContext(ctx, "getent", "passwd", usernameOrUID).Output()
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+ if err != nil {
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+ log.Printf("error calling getent for user %q: %v", usernameOrUID, err)
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+ u, err := std(usernameOrUID)
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+ return u, "", err
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+ }
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+ // output is "alice:x:1001:1001:Alice Smith,,,:/home/alice:/bin/bash"
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+ f := strings.SplitN(strings.TrimSpace(string(out)), ":", 10)
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+ for len(f) < 7 {
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+ f = append(f, "")
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+ }
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+ return &user.User{
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+ Username: f[0],
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+ Uid: f[2],
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+ Gid: f[3],
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+ Name: f[4],
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+ HomeDir: f[5],
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+ }, f[6], nil
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+}
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Andrew Lytvynov