banger/internal/paths/paths.go

package paths

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"syscall"
)

type Layout struct {
	ConfigHome     string
	StateHome      string
	CacheHome      string
	RuntimeHome    string
	ConfigDir      string
	StateDir       string
	CacheDir       string
	RuntimeDir     string
	SocketPath     string
	DBPath         string
	DaemonLog      string
	VMsDir         string
	ImagesDir      string
	KernelsDir     string
	OCICacheDir    string
	SSHDir         string
	KnownHostsPath string

	// runtimeHomeFallback is true when we fabricated the RuntimeHome path
	// under /tmp because XDG_RUNTIME_DIR was unset. Ensure() uses the flag
	// to apply strict ownership + mode checks on the fallback parent (a
	// world-writable /tmp needs us to own and lock the subtree ourselves;
	// a systemd-provisioned /run/user/<uid> is already 0700 and trusted).
	runtimeHomeFallback bool
}

func Resolve() (Layout, error) {
	home, err := os.UserHomeDir()
	if err != nil {
		return Layout{}, err
	}
	configHome := getenvDefault("XDG_CONFIG_HOME", filepath.Join(home, ".config"))
	stateHome := getenvDefault("XDG_STATE_HOME", filepath.Join(home, ".local", "state"))
	cacheHome := getenvDefault("XDG_CACHE_HOME", filepath.Join(home, ".cache"))
	runtimeHome := os.Getenv("XDG_RUNTIME_DIR")
	runtimeFallback := false
	if runtimeHome == "" {
		runtimeHome = filepath.Join(os.TempDir(), fmt.Sprintf("banger-runtime-%d", os.Getuid()))
		runtimeFallback = true
	}

	layout := Layout{
		ConfigHome:          configHome,
		StateHome:           stateHome,
		CacheHome:           cacheHome,
		RuntimeHome:         runtimeHome,
		runtimeHomeFallback: runtimeFallback,
		ConfigDir:           filepath.Join(configHome, "banger"),
		StateDir:            filepath.Join(stateHome, "banger"),
		CacheDir:            filepath.Join(cacheHome, "banger"),
		RuntimeDir:          filepath.Join(runtimeHome, "banger"),
	}
	layout.SocketPath = filepath.Join(layout.RuntimeDir, "bangerd.sock")
	layout.DBPath = filepath.Join(layout.StateDir, "state.db")
	layout.DaemonLog = filepath.Join(layout.StateDir, "bangerd.log")
	layout.VMsDir = filepath.Join(layout.StateDir, "vms")
	layout.ImagesDir = filepath.Join(layout.StateDir, "images")
	layout.KernelsDir = filepath.Join(layout.StateDir, "kernels")
	layout.OCICacheDir = filepath.Join(layout.CacheDir, "oci")
	layout.SSHDir = filepath.Join(layout.StateDir, "ssh")
	layout.KnownHostsPath = filepath.Join(layout.SSHDir, "known_hosts")
	return layout, nil
}

func Ensure(layout Layout) error {
	// When we're using the /tmp fallback, we must create and own the
	// runtime-home parent ourselves and reject any pre-existing directory
	// that isn't 0700 + owned by the current uid. Otherwise a local
	// attacker could pre-create that path and have banger's control
	// sockets land inside a directory they control.
	if layout.runtimeHomeFallback && strings.TrimSpace(layout.RuntimeHome) != "" {
		if err := ensureSafeRuntimeHome(layout.RuntimeHome); err != nil {
			return err
		}
	}
	// RuntimeDir holds bangerd.sock + per-VM firecracker API + vsock
	// sockets. Lock it to 0700 unconditionally so even if the parent
	// runtime-home is traversable by others, none of our sockets are
	// reachable.
	if strings.TrimSpace(layout.RuntimeDir) != "" {
		if err := os.MkdirAll(layout.RuntimeDir, 0o700); err != nil {
			return err
		}
		if err := os.Chmod(layout.RuntimeDir, 0o700); err != nil {
			return err
		}
	}
	for _, dir := range []string{layout.ConfigDir, layout.StateDir, layout.CacheDir, layout.VMsDir, layout.ImagesDir, layout.KernelsDir, layout.OCICacheDir} {
		if strings.TrimSpace(dir) == "" {
			continue
		}
		if err := os.MkdirAll(dir, 0o755); err != nil {
			return err
		}
	}
	// SSH material (private key, known_hosts) — 0700 like ~/.ssh so
	// strict SSH clients don't complain and no other host user can
	// read it. Empty SSHDir means the caller built a Layout by hand
	// (tests) and doesn't need the subdir; skip silently.
	if strings.TrimSpace(layout.SSHDir) != "" {
		if err := os.MkdirAll(layout.SSHDir, 0o700); err != nil {
			return err
		}
	}
	return nil
}

// ensureSafeRuntimeHome creates path at 0700 if missing, or validates
// existing ownership + mode. Returns an error describing how to remediate
// when the existing directory doesn't meet the bar.
func ensureSafeRuntimeHome(path string) error {
	if err := os.MkdirAll(path, 0o700); err != nil {
		return err
	}
	info, err := os.Lstat(path)
	if err != nil {
		return err
	}
	// Must be a real directory, not a symlink an attacker could swap.
	if info.Mode()&os.ModeSymlink != 0 {
		return fmt.Errorf("runtime dir %s is a symlink; refusing to place sockets there — remove it or set XDG_RUNTIME_DIR", path)
	}
	if !info.IsDir() {
		return fmt.Errorf("runtime dir %s exists but is not a directory", path)
	}
	sys, ok := info.Sys().(*syscall.Stat_t)
	if ok && int(sys.Uid) != os.Getuid() {
		return fmt.Errorf("runtime dir %s is owned by uid %d, not %d; remove it or set XDG_RUNTIME_DIR", path, sys.Uid, os.Getuid())
	}
	if info.Mode().Perm() != 0o700 {
		if err := os.Chmod(path, 0o700); err != nil {
			return fmt.Errorf("runtime dir %s has insecure mode %#o and chmod failed: %w", path, info.Mode().Perm(), err)
		}
	}
	return nil
}

var executablePath = os.Executable

func BangerdPath() (string, error) {
	if env := os.Getenv("BANGER_DAEMON_BIN"); env != "" {
		return env, nil
	}
	exe, err := executablePath()
	if err != nil {
		return "", err
	}
	dir := filepath.Dir(exe)
	for _, candidate := range []string{
		filepath.Join(dir, "bangerd"),
		filepath.Join(dir, "bangerd.exe"),
	} {
		if _, err := os.Stat(candidate); err == nil {
			return candidate, nil
		}
	}
	return "", errors.New("bangerd binary not found next to banger; run `make build`")
}

func CompanionBinaryPath(name string) (string, error) {
	envNames := []string{
		"BANGER_" + strings.ToUpper(strings.NewReplacer("-", "_", ".", "_").Replace(name)) + "_BIN",
	}
	if trimmed, ok := strings.CutPrefix(name, "banger-"); ok {
		envNames = append(envNames, "BANGER_"+strings.ToUpper(strings.NewReplacer("-", "_", ".", "_").Replace(trimmed))+"_BIN")
	}
	for _, envName := range envNames {
		if env := strings.TrimSpace(os.Getenv(envName)); env != "" {
			return env, nil
		}
	}
	exe, err := executablePath()
	if err != nil {
		return "", err
	}
	exeDir := filepath.Dir(exe)
	for _, candidate := range []string{
		filepath.Join(exeDir, name),
		filepath.Join(exeDir, "..", "lib", "banger", name),
		filepath.Join(exeDir, "..", "libexec", "banger", name),
	} {
		if _, err := os.Stat(candidate); err == nil {
			return candidate, nil
		}
	}
	return "", fmt.Errorf("%s companion binary not found; run `make build` or reinstall banger", name)
}

func getenvDefault(key, fallback string) string {
	if value := strings.TrimSpace(os.Getenv(key)); value != "" {
		return value
	}
	return fallback
}