mount_linux.go

package fuse

import (
	"errors"
	"fmt"
	"os"
	"os/exec"
	"strconv"
	"strings"
	"syscall"

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

func findFusermount() (string, error) {
	path, err := exec.LookPath("fusermount3")
	if err != nil {
		path, err = exec.LookPath("fusermount")
	}
	if err != nil {
		return "", err
	}
	return path, nil
}

func enableFunc(flag uintptr) func(uintptr) uintptr {
	return func(v uintptr) uintptr {
		return v | flag
	}
}

func disableFunc(flag uintptr) func(uintptr) uintptr {
	return func(v uintptr) uintptr {
		return v &^ flag
	}
}

// As per libfuse/fusermount.c:602: https://bit.ly/2SgtWYM#L602
var mountflagopts = map[string]func(uintptr) uintptr{
	"rw":      disableFunc(unix.MS_RDONLY),
	"ro":      enableFunc(unix.MS_RDONLY),
	"suid":    disableFunc(unix.MS_NOSUID),
	"nosuid":  enableFunc(unix.MS_NOSUID),
	"dev":     disableFunc(unix.MS_NODEV),
	"nodev":   enableFunc(unix.MS_NODEV),
	"exec":    disableFunc(unix.MS_NOEXEC),
	"noexec":  enableFunc(unix.MS_NOEXEC),
	"async":   disableFunc(unix.MS_SYNCHRONOUS),
	"sync":    enableFunc(unix.MS_SYNCHRONOUS),
	"atime":   disableFunc(unix.MS_NOATIME),
	"noatime": enableFunc(unix.MS_NOATIME),
	"dirsync": enableFunc(unix.MS_DIRSYNC),
}

var errFallback = errors.New("sentinel: fallback to fusermount(1)")

func directmount(dir string, cfg *MountConfig) (*os.File, error) {
	if cfg.DebugLogger != nil {
		cfg.DebugLogger.Println("Preparing for direct mounting")
	}
	// We use syscall.Open + os.NewFile instead of os.OpenFile so that the file
	// is opened in blocking mode. When opened in non-blocking mode, the Go
	// runtime tries to use poll(2), which does not work with /dev/fuse.
	fd, err := syscall.Open("/dev/fuse", syscall.O_RDWR, 0644)
	if err != nil {
		return nil, errFallback
	}
	dev := os.NewFile(uintptr(fd), "/dev/fuse")

	if cfg.DebugLogger != nil {
		cfg.DebugLogger.Println("Successfully opened the /dev/fuse in blocking mode")
	}
	// As per libfuse/fusermount.c:847: https://bit.ly/2SgtWYM#L847
	data := fmt.Sprintf("fd=%d,rootmode=40000,user_id=%d,group_id=%d",
		dev.Fd(), os.Getuid(), os.Getgid())
	// As per libfuse/fusermount.c:749: https://bit.ly/2SgtWYM#L749
	mountflag := uintptr(unix.MS_NODEV | unix.MS_NOSUID)
	opts := cfg.toMap()
	for k := range opts {
		fn, ok := mountflagopts[k]
		if !ok {
			continue
		}
		mountflag = fn(mountflag)
		delete(opts, k)
	}
	fsname := opts["fsname"]
	delete(opts, "fsname") // handled via fstype mount(2) parameter
	fstype := "fuse"
	if subtype, ok := opts["subtype"]; ok {
		fstype += "." + subtype
	}
	delete(opts, "subtype")
	data += "," + mapToOptionsString(opts)

	if cfg.DebugLogger != nil {
		cfg.DebugLogger.Println("Starting the unix mounting")
	}
	if err := unix.Mount(
		fsname,    // source
		dir,       // target
		fstype,    // fstype
		mountflag, // mountflag
		data,      // data
	); err != nil {
		if err == syscall.EPERM {
			return nil, errFallback

		}
		return nil, err
	}
	if cfg.DebugLogger != nil {
		cfg.DebugLogger.Println("Unix mounting completed successfully")
	}
	return dev, nil
}

// Begin the process of mounting at the given directory, returning a connection
// to the kernel. Mounting continues in the background, and is complete when an
// error is written to the supplied channel. The file system may need to
// service the connection in order for mounting to complete.
func mount(dir string, cfg *MountConfig, ready chan<- error) (*os.File, error) {
	// On linux, mounting is never delayed.
	ready <- nil

	if cfg.DebugLogger != nil {
		cfg.DebugLogger.Println("Parsing fuse file descriptor")
	}
	// If the mountpoint is /dev/fd/N, assume that the file descriptor N is an
	// already open FUSE channel. Parse it, cast it to an fd, and don't do any
	// other part of the mount dance.
	if fd, err := parseFuseFd(dir); err == nil {
		dev := os.NewFile(uintptr(fd), "/dev/fuse")
		return dev, nil
	}

	// Try mounting without fusermount(1) first: we might be running as root or
	// have the CAP_SYS_ADMIN capability.
	dev, err := directmount(dir, cfg)
	if err == errFallback {
		if cfg.DebugLogger != nil {
			cfg.DebugLogger.Println("Directmount failed. Trying fallback.")
		}
		fusermountPath, err := findFusermount()
		if err != nil {
			return nil, err
		}
		argv := []string{
			"-o", cfg.toOptionsString(),
			"--",
			dir,
		}
		return fusermount(fusermountPath, argv, []string{}, true, cfg.DebugLogger)
	}
	return dev, err
}

func parseFuseFd(dir string) (int, error) {
	if !strings.HasPrefix(dir, "/dev/fd/") {
		return -1, fmt.Errorf("not a /dev/fd path")
	}

	fd, err := strconv.ParseUint(strings.TrimPrefix(dir, "/dev/fd/"), 10, 32)
	if err != nil {
		return -1, fmt.Errorf("invalid /dev/fd/N path: N must be a positive integer")
	}

	return int(fd), nil
}