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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,
}
dev, err := fusermount(fusermountPath, argv, []string{}, true, cfg.DebugLogger)
if err == nil {
return dev, nil
}
// fusermount requires the mount user to have write access to the directory.
// However, it doesn't give a useful error on mount-failure if the access is missing.
// So, add this check and return a useful error if the user doesn't have write-access.
if err2 := unix.Access(dir, unix.W_OK); err2 != nil {
return nil, errors.Join(err, fmt.Errorf("the user doesn't have write-access on the mount point: %w", err2))
}
return dev, err
}
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
}
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