netwatch/backend/internal/reportbuf/datadir.go

package reportbuf

import (
	"errors"
	"io/fs"
	"os"
	"os/user"
	"path/filepath"
	"strconv"
	"syscall"
)

// ErrDataDirOutsideVolume is returned by PrepareDataDir for a DATA_DIR
// that is not the volume or a path below it, written in full.
var ErrDataDirOutsideVolume = errors.New(
	"must be /data or a path below it, with no '.', '..' or extra '/'")

// PrepareDataDir gets dir, the server's DATA_DIR, ready for owner, the
// user the server runs as, so that a host directory mounted at volume,
// /data in the image, needs no preparing: it creates dir, gives volume
// and everything in it to owner, and gives volume and dir the mode the
// server gives a directory it creates. dir must be volume or a path
// below it, with no '.', '..', empty part or '/' at the end.
//
// bin/entrypoint.sh runs this as root, which would follow a symbolic
// link anywhere, so every step goes through an os.Root opened on
// volume: it follows a link only when it is written as a relative
// path that stays inside volume, and refuses any other. A process on
// the host can swap a link onto a path in volume at any moment while
// this runs. Even then, the os.Root checks each link as it reaches it,
// MkdirAll never creates a directory through a link, Lchown never
// changes what a link points to, and the modes are set on directories
// already opened (see chmodDir), so the most that process can do is
// make a step fail or wait, or act on something else inside volume.
func PrepareDataDir(volume, dir string, owner *user.User) error {
	// rel is dir as a path from volume; IsLocal is false for one that
	// leads out of it.
	rel, err := filepath.Rel(volume, dir)
	if err != nil || dir != filepath.Clean(dir) || !filepath.IsLocal(rel) {
		return ErrDataDirOutsideVolume
	}

	uid, err := strconv.Atoi(owner.Uid)
	if err != nil {
		return err
	}

	gid, err := strconv.Atoi(owner.Gid)
	if err != nil {
		return err
	}

	root, err := os.OpenRoot(volume)
	if err != nil {
		return err
	}

	defer func() { _ = root.Close() }()

	err = root.MkdirAll(rel, dirPerms)
	if err != nil {
		return err
	}

	// WalkDir does not follow a symbolic link, and Lchown gives the
	// link itself to owner, not what it points to.
	err = fs.WalkDir(root.FS(), ".",
		func(name string, _ fs.DirEntry, err error) error {
			if err != nil {
				return err
			}

			return root.Lchown(name, uid, gid)
		})
	if err != nil {
		return err
	}

	err = chmodDir(root, ".")
	if err != nil {
		return err
	}

	return chmodDir(root, rel)
}

// chmodDir gives name, a directory inside root, the mode the server
// gives a directory it creates. Root.Chmod would not hold: on Linux it
// checks that name is not a symbolic link, then sets the mode by name,
// following a link swapped in between. So chmodDir opens name through
// root and sets the mode on the open directory. It refuses anything
// but a directory: a directory has no second name (hard link), so the
// one opened is inside root, where any other file could be a hard link
// to one outside.
func chmodDir(root *os.Root, name string) error {
	dir, err := root.Open(name)
	if err != nil {
		return err
	}

	defer func() { _ = dir.Close() }()

	info, err := dir.Stat()
	if err != nil {
		return err
	}

	if !info.IsDir() {
		return &fs.PathError{Op: "chmod", Path: name, Err: syscall.ENOTDIR}
	}

	return dir.Chmod(dirPerms)
}