mesh-catalog/modules/pacman/cmd/pacman-tools/root.go

package main

// The root helper (novox/hq ADR 0510): the module's acts that need root go through a second process of the same
// build, `pacman-root`, running as root with no bus account, never through a sudo rule. It serves a closed list
// of verbs on a unix socket only the keeper's account opens, the peer's account checked on every connection:
//
//   - transaction: start a run's full upgrade as a unit of the service manager, named by the run, leaving out
//     the packages the run holds back;
//   - rollback: start a rollback's transaction as such a unit, carrying out the plan sent with the verb;
//   - maps: the processes still mapping a file an upgrade replaced, by unit or container and executable;
//   - journal: the last lines a transaction unit of the module logged.
//
// Every other verb is refused, and every argument that is not a unit name of the module's own transactions, a
// package name, a version, a run's id, or a plan item's location in the package cache or the Arch Linux
// Archive. The helper's program is root's (ADR 0510 §3): the node-engine places a process's program in a
// directory its own account owns, and that account is root.
//
// A rollback's unit runs this same program (`pacman-tools root-rollback <plan>`) on a plan file the helper
// wrote into a directory only root reads: it finds each package of the dated repository by its file's name,
// installs the plan's packages in one pacman transaction, then removes the ones the rollback was asked to
// remove. Nothing is changed when a package cannot be found.

import (
	"bufio"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"net/http"
	"os"
	"os/exec"
	"os/signal"
	"os/user"
	"path"
	"path/filepath"
	"regexp"
	"runtime"
	"strconv"
	"strings"
	"syscall"
	"time"

	"golang.org/x/sys/unix"
)

// The helper's verbs.
const (
	VerbTransaction = "transaction"
	VerbRollback    = "rollback"
	VerbMaps        = "maps"
	VerbJournal     = "journal"
	VerbMirrorlist  = "mirrorlist"
)

// The helper's environment, and the keeper's word for where the helper listens.
const (
	EnvRootSocket = "PACMAN_ROOT_SOCKET" // the helper's socket, for the helper and the keeper
	EnvRootPeer   = "PACMAN_ROOT_PEER"   // the only account the helper answers: the keeper's
	// RootSocketPath is the socket as the manifest names it: in a directory of the module's state the helper
	// makes, root's.
	RootSocketPath = "${dir:state}/root/helper.sock"
)

// rootTimeout bounds one exchange with the helper: every verb answers at once (a transaction is started, not
// waited for).
const rootTimeout = 30 * time.Second

// mostJournalLines is the most lines the journal verb reads.
const mostJournalLines = 400

// RootRequest is one verb sent to the helper, one per connection.
type RootRequest struct {
	Verb   string         `json:"verb"`
	Kind   string         `json:"kind,omitempty"`
	Run    string         `json:"run,omitempty"`
	Ignore []string       `json:"ignore,omitempty"`
	Plan   []RollbackItem `json:"plan,omitempty"`
	Unit   string         `json:"unit,omitempty"`
	Lines  int            `json:"lines,omitempty"`
	// Servers are the mirror list's servers, in order, for the verb mirrorlist.
	Servers []string `json:"servers,omitempty"`
}

// RootAnswer is the helper's answer: an error, or what the verb read.
type RootAnswer struct {
	Error  string   `json:"error,omitempty"`
	Lines  []string `json:"lines,omitempty"`
	Mapped []Mapped `json:"mapped,omitempty"`
}

// RootHelper is the helper: what it runs, the process table it reads, where it keeps plans, its own program,
// and the one account it answers.
type RootHelper struct {
	Run      Runner
	Proc     ProcFS
	ProcRoot string
	PlanDir  string
	Self     string
	// MirrorlistPath is the mirror list file the verb mirrorlist writes: Mirrorlist, a test's own elsewhere.
	MirrorlistPath string
	Peer           int
	// PeerOf is the account at the other end of a connection; nil reads it from the socket (SO_PEERCRED).
	PeerOf func(net.Conn) (int, error)
	Logf   func(format string, a ...any)
}

var (
	// runIDShape is a run's id (RunID): its start in UTC to the second.
	runIDShape = regexp.MustCompile(`^[0-9]{8}T[0-9]{6}Z$`)
	// ownUnit is a transaction unit of the module's own runs (TransactionUnit).
	ownUnit = regexp.MustCompile(`^` + unitPrefix + `(` + RunUpgrade + `|` + RunRollback + `)-[0-9]{8}t[0-9]{6}z$`)
	// archArg is a package's architecture in a file's name.
	archArg = regexp.MustCompile(`^[a-z0-9_]+$`)
	// datedRepository is the archive's repository as it stood on one day, its architecture left to pacman.
	datedRepository = regexp.MustCompile(`^` + regexp.QuoteMeta(ArchiveURL) + `/repos/[0-9]{4}/[0-9]{2}/[0-9]{2}/[A-Za-z0-9_-]+/os/\$arch$`)
)

func refused(format string, a ...any) RootAnswer {
	return RootAnswer{Error: "refused: " + fmt.Sprintf(format, a...)}
}

func checkRun(run string) error {
	if !runIDShape.MatchString(run) {
		return fmt.Errorf("%q is not a run's id", run)
	}
	return nil
}

// checkItem refuses a plan item that is not a package brought back from the cache or the archive, or removed.
func checkItem(i RollbackItem) error {
	if !packageName.MatchString(i.Name) {
		return fmt.Errorf("%q is not a package's name", i.Name)
	}
	if i.Source == SourceRemoved {
		if i.To != "" || i.Location != "" {
			return fmt.Errorf("%s is removed, and names a version or a place to take it from", i.Name)
		}
		return nil
	}
	if !packageVersion.MatchString(i.To) {
		return fmt.Errorf("%q is not a version of %s", i.To, i.Name)
	}
	switch i.Source {
	case SourceCache:
		f := cacheFile.FindStringSubmatch(path.Base(i.Location))
		if path.Clean(i.Location) != i.Location || path.Dir(i.Location) != CacheDir || f == nil || f[1] != i.Name || f[2] != i.To {
			return fmt.Errorf("%q is not %s %s's file in the package cache", i.Location, i.Name, i.To)
		}
	case SourceArchive:
		prefix := ArchiveURL + "/packages/" + i.Name[:1] + "/" + i.Name + "/" + i.Name + "-" + i.To + "-"
		arch, ok := strings.CutPrefix(i.Location, prefix)
		if !ok || !strings.HasSuffix(arch, ".pkg.tar.zst") || !archArg.MatchString(strings.TrimSuffix(arch, ".pkg.tar.zst")) {
			return fmt.Errorf("%q is not %s %s's file in the archive", i.Location, i.Name, i.To)
		}
	case SourceRepository:
		if !datedRepository.MatchString(i.Location) {
			return fmt.Errorf("%q is not a dated repository of the archive", i.Location)
		}
	default:
		return fmt.Errorf("%q is not a source a package is brought back from", i.Source)
	}
	return nil
}

// Handle carries out one verb, or refuses it naming why; nothing runs for a refused verb.
func (h *RootHelper) Handle(req RootRequest) RootAnswer {
	switch req.Verb {
	case VerbTransaction:
		if req.Kind != RunUpgrade {
			return refused("a transaction is a full upgrade, not %q", req.Kind)
		}
		if err := checkRun(req.Run); err != nil {
			return refused("%v", err)
		}
		for _, n := range req.Ignore {
			if !packageName.MatchString(n) {
				return refused("%q is not a package's name to leave out", n)
			}
		}
		args := append([]string{"pacman"}, UpgradeArgs...)
		if len(req.Ignore) > 0 {
			args = append(args, "--ignore", strings.Join(req.Ignore, ","))
		}
		return h.startUnit(TransactionUnit(RunUpgrade, req.Run), "pacman full upgrade of run "+req.Run, args)
	case VerbRollback:
		if err := checkRun(req.Run); err != nil {
			return refused("%v", err)
		}
		if len(req.Plan) == 0 {
			return refused("a rollback with no plan")
		}
		for _, i := range req.Plan {
			if err := checkItem(i); err != nil {
				return refused("%v", err)
			}
		}
		raw, err := json.Marshal(req.Plan)
		if err != nil {
			return RootAnswer{Error: err.Error()}
		}
		if err := os.MkdirAll(h.PlanDir, 0o700); err != nil {
			return RootAnswer{Error: "the plan's directory: " + err.Error()}
		}
		file := filepath.Join(h.PlanDir, req.Run+".json")
		if err := os.WriteFile(file, raw, 0o600); err != nil {
			return RootAnswer{Error: "writing the plan: " + err.Error()}
		}
		return h.startUnit(TransactionUnit(RunRollback, req.Run), "pacman rollback of run "+req.Run, []string{h.Self, "root-rollback", file})
	case VerbMaps:
		mapped, err := MappedDeleted(h.Proc, h.ProcRoot)
		if err != nil {
			return RootAnswer{Error: "reading the process table: " + err.Error()}
		}
		return RootAnswer{Mapped: mapped}
	case VerbJournal:
		if !ownUnit.MatchString(req.Unit) {
			return refused("%q is not a transaction unit of the module", req.Unit)
		}
		if req.Lines < 1 || req.Lines > mostJournalLines {
			return refused("a journal is read 1 to %d lines at a time, not %d", mostJournalLines, req.Lines)
		}
		r := h.Run(context.Background(), "journalctl", "--no-pager", "-o", "cat", "-n", strconv.Itoa(req.Lines), "-u", req.Unit)
		if r.Status != 0 || r.Err != "" {
			return RootAnswer{Error: failure("journalctl", "journalctl", r).Error()}
		}
		return RootAnswer{Lines: lines(r.Stdout)}
	case VerbMirrorlist:
		if err := checkServers(req.Servers); err != nil {
			return refused("%v", err)
		}
		if err := writeWhole(h.MirrorlistPath, RenderMirrorlist(req.Servers), 0o644); err != nil {
			return RootAnswer{Error: "writing the mirror list: " + err.Error()}
		}
		return RootAnswer{}
	}
	return refused("%q is not a verb of the root helper (%s, %s, %s, %s, %s)", req.Verb, VerbTransaction, VerbRollback, VerbMaps,
		VerbJournal, VerbMirrorlist)
}

// startUnit starts command as a transient unit of the service manager and answers once it started.
func (h *RootHelper) startUnit(unit, description string, command []string) RootAnswer {
	args := append([]string{"--unit=" + unit, "--description=" + description + ", started by the mesh's pacman keeper", "--quiet"}, command...)
	r := h.Run(context.Background(), "systemd-run", args...)
	if r.Status != 0 || r.Err != "" {
		return RootAnswer{Error: failure("systemd-run", "systemd-run", r).Error()}
	}
	return RootAnswer{}
}

// peerCred is the account at the other end of a unix socket.
func peerCred(c net.Conn) (int, error) {
	uc, ok := c.(*net.UnixConn)
	if !ok {
		return -1, errors.New("not a unix socket")
	}
	raw, err := uc.SyscallConn()
	if err != nil {
		return -1, err
	}
	uid := -1
	var gerr error
	if err := raw.Control(func(fd uintptr) {
		cred, err := unix.GetsockoptUcred(int(fd), unix.SOL_SOCKET, unix.SO_PEERCRED)
		if err != nil {
			gerr = err
			return
		}
		uid = int(cred.Uid)
	}); err != nil {
		return -1, err
	}
	return uid, gerr
}

// Serve answers each connection on l, one request each, until ctx ends: a peer that is not the keeper's
// account is told so and nothing it sent is read.
func (h *RootHelper) Serve(ctx context.Context, l net.Listener) error {
	go func() { <-ctx.Done(); l.Close() }()
	peerOf := h.PeerOf
	if peerOf == nil {
		peerOf = peerCred
	}
	for {
		c, err := l.Accept()
		if err != nil {
			if ctx.Err() != nil {
				return nil
			}
			return err
		}
		go h.serveOne(c, peerOf)
	}
}

func (h *RootHelper) serveOne(c net.Conn, peerOf func(net.Conn) (int, error)) {
	defer c.Close()
	_ = c.SetDeadline(time.Now().Add(rootTimeout))
	answer := func(a RootAnswer) { _ = json.NewEncoder(c).Encode(a) }
	uid, err := peerOf(c)
	if err != nil || uid != h.Peer {
		if h.Logf != nil {
			h.Logf("[pacman] root helper: a connection from account %d refused: only the keeper's (%d) is answered", uid, h.Peer)
		}
		answer(refused("only the keeper's account is answered"))
		return
	}
	var req RootRequest
	if err := json.NewDecoder(io.LimitReader(c, 4<<20)).Decode(&req); err != nil {
		answer(refused("no request: %v", err))
		return
	}
	a := h.Handle(req)
	if h.Logf != nil {
		said := "carried out"
		if a.Error != "" {
			said = a.Error
		}
		h.Logf("[pacman] root helper: %s %s %s: %s", req.Verb, req.Kind, req.Run+req.Unit, said)
	}
	answer(a)
}

// rootClient is the keeper's side: the RootActs over the helper's socket.
type rootClient struct{ Socket string }

func (c rootClient) ask(req RootRequest) (RootAnswer, error) {
	conn, err := net.DialTimeout("unix", c.Socket, rootTimeout)
	if err != nil {
		return RootAnswer{}, fmt.Errorf("the root helper cannot be reached at %s: %w", c.Socket, err)
	}
	defer conn.Close()
	_ = conn.SetDeadline(time.Now().Add(rootTimeout))
	if err := json.NewEncoder(conn).Encode(req); err != nil {
		return RootAnswer{}, fmt.Errorf("asking the root helper: %w", err)
	}
	var a RootAnswer
	if err := json.NewDecoder(bufio.NewReader(conn)).Decode(&a); err != nil {
		return RootAnswer{}, fmt.Errorf("the root helper's answer: %w", err)
	}
	if a.Error != "" {
		return a, fmt.Errorf("the root helper: %s", a.Error)
	}
	return a, nil
}

func (c rootClient) Transaction(kind, run string, ignore []string) error {
	_, err := c.ask(RootRequest{Verb: VerbTransaction, Kind: kind, Run: run, Ignore: ignore})
	return err
}

func (c rootClient) Rollback(run string, plan []RollbackItem) error {
	_, err := c.ask(RootRequest{Verb: VerbRollback, Run: run, Plan: plan})
	return err
}

func (c rootClient) Mirrorlist(servers []string) error {
	_, err := c.ask(RootRequest{Verb: VerbMirrorlist, Servers: servers})
	return err
}

// writeWhole writes a file whole: into a file beside it, then renamed over it, so a reader never meets half of it.
func writeWhole(path string, content []byte, mode os.FileMode) error {
	tmp := filepath.Join(filepath.Dir(path), "."+filepath.Base(path)+".mesh")
	if err := os.WriteFile(tmp, content, mode); err != nil {
		return err
	}
	if err := os.Chmod(tmp, mode); err != nil {
		_ = os.Remove(tmp)
		return err
	}
	if err := os.Rename(tmp, path); err != nil {
		_ = os.Remove(tmp)
		return err
	}
	return nil
}

func (c rootClient) Journal(unit string, n int) ([]string, error) {
	a, err := c.ask(RootRequest{Verb: VerbJournal, Unit: unit, Lines: n})
	return a.Lines, err
}

func (c rootClient) Maps() ([]Mapped, error) {
	a, err := c.ask(RootRequest{Verb: VerbMaps})
	if a.Mapped == nil && err == nil {
		a.Mapped = []Mapped{}
	}
	return a.Mapped, err
}

// rootHelper is `pacman-tools root`: it listens on its socket, made the keeper's alone, until stopped.
func rootHelper() int {
	logf := func(f string, a ...any) { fmt.Fprintf(os.Stderr, f+"\n", a...) }
	if os.Getuid() != 0 {
		logf("[pacman] root helper: it runs as root, and runs as account %d", os.Getuid())
		return 2
	}
	sock, peerName := os.Getenv(EnvRootSocket), os.Getenv(EnvRootPeer)
	if sock == "" || peerName == "" {
		logf("[pacman] root helper was started without %s or %s", EnvRootSocket, EnvRootPeer)
		return 2
	}
	peer, err := user.Lookup(peerName)
	if err != nil {
		logf("[pacman] root helper: the keeper's account %s: %v", peerName, err)
		return 1
	}
	uid, err := strconv.Atoi(peer.Uid)
	if err != nil {
		logf("[pacman] root helper: account %s has no number: %v", peerName, err)
		return 1
	}
	self, err := os.Executable()
	if err != nil {
		logf("[pacman] root helper: its own program: %v", err)
		return 1
	}
	dir := filepath.Dir(sock)
	if err := os.MkdirAll(dir, 0o711); err != nil || os.Chown(dir, 0, 0) != nil || os.Chmod(dir, 0o711) != nil {
		logf("[pacman] root helper: its directory %s cannot be made root's: %v", dir, err)
		return 1
	}
	_ = os.Remove(sock)
	l, err := net.Listen("unix", sock)
	if err != nil {
		logf("[pacman] root helper: listening on %s: %v", sock, err)
		return 1
	}
	if err := os.Chown(sock, uid, 0); err != nil || os.Chmod(sock, 0o600) != nil {
		logf("[pacman] root helper: the socket cannot be made the keeper's alone: %v", err)
		return 1
	}
	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
	defer stop()
	h := &RootHelper{Run: ExecRunner, Proc: osProc{}, ProcRoot: "/proc", PlanDir: filepath.Join(dir, "plans"), Self: self,
		MirrorlistPath: Mirrorlist, Peer: uid, Logf: logf}
	logf("[pacman] root helper listening on %s for account %s (%d)", sock, peerName, uid)
	if err := h.Serve(ctx, l); err != nil {
		logf("[pacman] root helper stopped: %v", err)
		return 1
	}
	return 0
}

// machineArch is this machine's architecture as Arch Linux names it.
func machineArch() string {
	switch runtime.GOARCH {
	case "amd64":
		return "x86_64"
	case "arm64":
		return "aarch64"
	}
	return runtime.GOARCH
}

// archiveHas says whether the archive holds a file, by asking for its head.
func archiveHas(url string) bool {
	c := &http.Client{Timeout: 30 * time.Second}
	resp, err := c.Head(url)
	if err != nil {
		return false
	}
	resp.Body.Close()
	return resp.StatusCode == http.StatusOK
}

// rollbackUnit carries out a rollback plan inside its unit: every package of a dated repository found by its
// file's name (this machine's architecture, then any), the plan's packages installed in one transaction, then
// the packages to remove removed. Nothing is changed when a package is not found.
func rollbackUnit(file, arch string, run func(string, ...string) int, probe func(string) bool, log io.Writer) int {
	raw, err := os.ReadFile(file)
	if err != nil {
		fmt.Fprintf(log, "error: reading the plan: %v\n", err)
		return 1
	}
	var plan []RollbackItem
	if err := json.Unmarshal(raw, &plan); err != nil {
		fmt.Fprintf(log, "error: the plan is not one: %v\n", err)
		return 1
	}
	var targets, remove []string
	for _, i := range plan {
		if err := checkItem(i); err != nil {
			fmt.Fprintf(log, "error: %v\n", err)
			return 1
		}
		switch i.Source {
		case SourceRemoved:
			remove = append(remove, i.Name)
		case SourceRepository:
			base := strings.Replace(i.Location, "$arch", arch, 1)
			found := ""
			for _, a := range []string{arch, "any"} {
				if u := base + "/" + i.Name + "-" + i.To + "-" + a + ".pkg.tar.zst"; probe(u) {
					found = u
					break
				}
			}
			if found == "" {
				fmt.Fprintf(log, "error: %s %s is not in %s: nothing was changed\n", i.Name, i.To, base)
				return 1
			}
			targets = append(targets, found)
		default:
			targets = append(targets, i.Location)
		}
	}
	if len(targets) > 0 {
		if code := run("pacman", append([]string{"-U", "--noconfirm", "--"}, targets...)...); code != 0 {
			return code
		}
	}
	if len(remove) > 0 {
		return run("pacman", append([]string{"-R", "--noconfirm", "--"}, remove...)...)
	}
	return 0
}

// rootRollback is `pacman-tools root-rollback <plan>`, the rollback unit's command: pacman's output goes to the
// unit's journal.
func rootRollback(args []string) int {
	if len(args) != 1 || os.Getuid() != 0 {
		fmt.Fprintln(os.Stderr, "error: root-rollback takes one plan file, and runs as root in its unit")
		return 2
	}
	run := func(name string, a ...string) int {
		cmd := exec.Command(name, a...)
		cmd.Stdout, cmd.Stderr = os.Stdout, os.Stderr
		cmd.Env = append(os.Environ(), "LC_ALL=C")
		if err := cmd.Run(); err != nil {
			var exit *exec.ExitError
			if errors.As(err, &exit) {
				return exit.ExitCode()
			}
			fmt.Fprintf(os.Stderr, "error: %s: %v\n", name, err)
			return 1
		}
		return 0
	}
	return rollbackUnit(args[0], machineArch(), run, archiveHas, os.Stderr)
}