mesh-host/internal/apply/home_links.go

package apply

// A link under a person's home is never followed as root (novox/hq ADR 0266, the review of 2026-10-08).
//
// The node-engine runs as root and places files and directories under homes: the operator's shell
// configuration, the agent's `~/.claude`. Everything below a home is the account's to change. An account that
// replaces `~/.claude` with a symbolic link to /etc, between two applies, would have root change the owner and
// mode of /etc, or write a file into it, on the next one — and on a machine whose agents run as an account of
// their own, that account is exactly the one that must not become root.
//
// So for a path strictly below a home:
//
//   - **a symbolic link in any component below the home, the path's own included, refuses the resource**, said
//     in words, and nothing is done to it — before anything is read, written, chowned or chmodded;
//   - **the owner and mode are set through a descriptor opened without following a link**, each component
//     opened from the one above it (homes_linux.go), so a link put in place after the check is not followed
//     either; a link that is itself the thing to own is owned as a link, never its target;
//   - **a file is written through its directory's descriptor**, made with O_EXCL|O_NOFOLLOW and renamed within
//     that directory, so neither the file nor a parent can be swapped for a link between check and write.
//
// The home itself is the machine's, made by root, and is followed as it is. A path outside every home is
// handled exactly as before: a module's system paths may be links the machine set up (a resolver's file is).
//
// Which homes: every account the user database lists with a uid of 1000 or more, other than the overflow
// account, and every home under /home. A service account's home under /var/lib is a module's own directory,
// and is left as it was.

import (
	"bufio"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
)

// passwdFile is the user database the homes are read from; a test names its own.
var passwdFile = "/etc/passwd"

// homes is every home of a person's or an agent's account on this machine. A variable so a test can name the
// homes it made.
var homes = func() []string {
	f, err := os.Open(passwdFile)
	if err != nil {
		return nil
	}
	defer f.Close()
	var out []string
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		fields := strings.Split(sc.Text(), ":")
		if len(fields) < 6 {
			continue
		}
		uid, err := strconv.Atoi(fields[2])
		if err != nil {
			continue
		}
		home := filepath.Clean(fields[5])
		if home == "/" || home == "." || home == "" {
			continue
		}
		if (uid >= 1000 && uid != 65534) || strings.HasPrefix(home, "/home/") {
			out = append(out, home)
		}
	}
	return out
}

// homeAbove is the home a path lies strictly below, the deepest when homes nest; "" when none.
func homeAbove(path string) string {
	path = filepath.Clean(path)
	hs := homes()
	sort.Slice(hs, func(i, j int) bool { return len(hs[i]) > len(hs[j]) })
	for _, h := range hs {
		if strings.HasPrefix(path, h+string(os.PathSeparator)) {
			return h
		}
	}
	return ""
}

// LinkUnderHomeError is a resource refused because a component below a home is a symbolic link.
type LinkUnderHomeError struct {
	Path, Link, Home string
}

func (e *LinkUnderHomeError) Error() string {
	return fmt.Sprintf("%s is a symbolic link inside the home %s, and the node-engine runs as root: it never "+
		"follows a link an account can put there, so nothing was done to %s (novox/hq ADR 0266). Make %s a "+
		"directory or file again, or ask why something replaced it", e.Link, e.Home, e.Path, e.Link)
}

// refuseLinksUnderHome refuses a path below a home when any component below the home, its own included, is a
// symbolic link. Components that do not exist yet end the walk: what is missing is made without following.
func refuseLinksUnderHome(path string) error {
	home := homeAbove(path)
	if home == "" {
		return nil
	}
	rel, err := filepath.Rel(home, filepath.Clean(path))
	if err != nil {
		return err
	}
	at := home
	for _, part := range strings.Split(rel, string(os.PathSeparator)) {
		at = filepath.Join(at, part)
		info, err := os.Lstat(at)
		if errors.Is(err, fs.ErrNotExist) {
			return nil
		}
		if err != nil {
			return err
		}
		if info.Mode()&os.ModeSymlink != 0 {
			return &LinkUnderHomeError{Path: path, Link: at, Home: home}
		}
	}
	return nil
}

// statPath is os.Stat, except below a home, where it never follows a link.
func statPath(path string) (os.FileInfo, error) {
	if homeAbove(path) != "" {
		return os.Lstat(path)
	}
	return os.Stat(path)
}

// chmodPath sets a mode; below a home through a descriptor that followed no link.
func chmodPath(path string, mode os.FileMode) error {
	home := homeAbove(path)
	if home == "" {
		return os.Chmod(path, mode)
	}
	return chmodUnder(home, path, mode)
}

// chownPath sets an owner; below a home through a descriptor that followed no link, and a link itself is
// owned as a link.
func chownPath(path string, uid, gid int) error {
	home := homeAbove(path)
	if home == "" {
		return os.Chown(path, uid, gid)
	}
	if info, err := os.Lstat(path); err == nil && info.Mode()&os.ModeSymlink != 0 {
		return os.Lchown(path, uid, gid)
	}
	return chownUnder(home, path, uid, gid)
}

// readPath reads a file; below a home without following a link.
func readPath(path string) ([]byte, error) {
	home := homeAbove(path)
	if home == "" {
		return os.ReadFile(path)
	}
	return readUnder(home, path)
}