felhom-agent/internal/storage/durable_device.go

package storage

import (
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
)

// Durable BLOCK-DEVICE identity for the slice-10B operator-signed wipe (anti-retarget). The
// signed op binds a DURABLE id (a WWN / hardware serial, falling back to a filesystem UUID), and
// execution re-resolves it to the CURRENT /dev path — so "wipe device X" wipes that exact physical
// device, never whatever happens to be at /dev/sdb now. These reads are world-readable udev
// symlinks under /dev/disk/by-id and /dev/disk/by-uuid — NO privilege and NO subprocess (the
// root-CLI fence is untouched).
//
// devDiskRoot is overridable for tests (a fake /dev/disk layout).
var devDiskRoot = "/dev/disk"

// durable-id scheme prefixes. byid: a /dev/disk/by-id/<name> entry (preferred — wwn/serial,
// survives reformat + re-cabling); byuuid: a filesystem UUID (fallback — survives re-cabling but
// not a reformat, acceptable for a one-shot wipe resolved immediately before wiping).
const (
	durableByID   = "byid:"
	durableByUUID = "byuuid:"
)

// byIDPriority ranks /dev/disk/by-id link prefixes most-stable-first: wwn (hardware world-wide
// name) > nvme-eui (NVMe EUI) > nvme-/ata-/scsi-/usb- (model+serial). dm-/lvm-/md- names are
// excluded (they are mapper constructs, not the physical disk we want to bind a wipe to).
var byIDPriority = []string{"wwn-", "nvme-eui.", "nvme-", "ata-", "scsi-", "usb-"}

// DeviceDurableID derives a stable durable identifier for a block device by scanning the
// world-readable udev symlinks. Prefers a by-id (wwn/serial) link, then a filesystem UUID. Errors
// when neither is resolvable (a device with no durable identity cannot be safely wipe-bound).
func DeviceDurableID(device string) (string, error) {
	target, err := filepath.EvalSymlinks(device)
	if err != nil {
		return "", fmt.Errorf("storage: resolve %s: %w", device, err)
	}
	// 1. best by-id link pointing at this device.
	if name := bestByIDLink(target); name != "" {
		return durableByID + name, nil
	}
	// 2. fallback: a filesystem UUID symlink.
	if uuid := byUUIDFor(target); uuid != "" {
		return durableByUUID + uuid, nil
	}
	return "", fmt.Errorf("storage: no durable id (wwn/serial/uuid) for device %s", device)
}

// ResolveDurableDevice resolves a durable id back to the CURRENT canonical /dev path, or errors if
// it no longer resolves (the device was physically removed/replaced — the wipe must then refuse).
func ResolveDurableDevice(durableID string) (string, error) {
	switch {
	case strings.HasPrefix(durableID, durableByID):
		name := strings.TrimPrefix(durableID, durableByID)
		if !safeLinkName(name) {
			return "", fmt.Errorf("storage: unsafe durable id %q", durableID)
		}
		return filepath.EvalSymlinks(filepath.Join(devDiskRoot, "by-id", name))
	case strings.HasPrefix(durableID, durableByUUID):
		uuid := strings.TrimPrefix(durableID, durableByUUID)
		if !safeLinkName(uuid) {
			return "", fmt.Errorf("storage: unsafe durable id %q", durableID)
		}
		return filepath.EvalSymlinks(filepath.Join(devDiskRoot, "by-uuid", uuid))
	default:
		// A bare path or unknown scheme is REFUSED — a wipe must bind to a durable id, never a
		// mutable /dev path (the anti-retarget invariant).
		return "", fmt.Errorf("storage: durable id %q is not a durable scheme (byid:/byuuid:) — refusing path-only binding", durableID)
	}
}

// bestByIDLink returns the highest-priority /dev/disk/by-id link name whose target is `device`
// (already symlink-resolved), or "" if none.
func bestByIDLink(device string) string {
	dir := filepath.Join(devDiskRoot, "by-id")
	entries, err := os.ReadDir(dir)
	if err != nil {
		return ""
	}
	// collect matching names, then pick by priority (then lexically for determinism).
	var matches []string
	for _, e := range entries {
		name := e.Name()
		if tgt, err := filepath.EvalSymlinks(filepath.Join(dir, name)); err == nil && tgt == device {
			matches = append(matches, name)
		}
	}
	if len(matches) == 0 {
		return ""
	}
	sort.Strings(matches)
	for _, pfx := range byIDPriority {
		for _, name := range matches {
			if strings.HasPrefix(name, pfx) {
				return name
			}
		}
	}
	return matches[0] // some by-id link exists but not a recognized prefix — still durable enough
}

// byUUIDFor returns the filesystem UUID whose by-uuid link targets `device`, or "".
func byUUIDFor(device string) string {
	dir := filepath.Join(devDiskRoot, "by-uuid")
	entries, err := os.ReadDir(dir)
	if err != nil {
		return ""
	}
	for _, e := range entries {
		if tgt, err := filepath.EvalSymlinks(filepath.Join(dir, e.Name())); err == nil && tgt == device {
			return e.Name()
		}
	}
	return ""
}

// safeLinkName rejects a durable-id component that could escape the by-id/by-uuid dir (path
// traversal / separators) — defense even though the value is operator-signed.
func safeLinkName(s string) bool {
	if s == "" || strings.ContainsAny(s, "/\x00") || s == "." || s == ".." {
		return false
	}
	return true
}