try mounting without fusermount(1) first

This requires root privileges or the CAP_SYS_ADMIN capability, but allows
supporting -o suid.

mount_config.go

@@ -223,10 +223,9 @@ func escapeOptionsKey(s string) (res string) {
 	return
 }
 
-// Create an options string suitable for passing to the mount helper.
-func (c *MountConfig) toOptionsString() string {
+func mapToOptionsString(opts map[string]string) string {
 	var components []string
-	for k, v := range c.toMap() {
+	for k, v := range opts {
 		k = escapeOptionsKey(k)
 
 		component := k
@@ -239,3 +238,8 @@ func (c *MountConfig) toOptionsString() string {
 
 	return strings.Join(components, ",")
 }
+
+// Create an options string suitable for passing to the mount helper.
+func (c *MountConfig) toOptionsString() string {
+	return mapToOptionsString(c.toMap())
+}

mount_linux.go

@@ -2,29 +2,21 @@ package fuse
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"net"
 	"os"
 	"os/exec"
 	"syscall"
-)
 
-// 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) (dev *os.File, err error) {
-	// On linux, mounting is never delayed.
-	ready <- nil
+	"golang.org/x/sys/unix"
+)
 
+func fusermount(dir string, cfg *MountConfig) (*os.File, error) {
 	// Create a socket pair.
 	fds, err := syscall.Socketpair(syscall.AF_FILE, syscall.SOCK_STREAM, 0)
 	if err != nil {
-		err = fmt.Errorf("Socketpair: %v", err)
-		return
+		return nil, fmt.Errorf("Socketpair: %v", err)
 	}
 
 	// Wrap the sockets into os.File objects that we will pass off to fusermount.
@@ -51,23 +43,20 @@ func mount(
 	// Run the command.
 	err = cmd.Run()
 	if err != nil {
-		err = fmt.Errorf("running fusermount: %v\n\nstderr:\n%s", err, stderr.Bytes())
-		return
+		return nil, fmt.Errorf("running fusermount: %v\n\nstderr:\n%s", err, stderr.Bytes())
 	}
 
 	// Wrap the socket file in a connection.
 	c, err := net.FileConn(readFile)
 	if err != nil {
-		err = fmt.Errorf("FileConn: %v", err)
-		return
+		return nil, fmt.Errorf("FileConn: %v", err)
 	}
 	defer c.Close()
 
 	// We expect to have a Unix domain socket.
 	uc, ok := c.(*net.UnixConn)
 	if !ok {
-		err = fmt.Errorf("Expected UnixConn, got %T", c)
-		return
+		return nil, fmt.Errorf("Expected UnixConn, got %T", c)
 	}
 
 	// Read a message.
@@ -75,21 +64,18 @@ func mount(
 	oob := make([]byte, 32) // expect 24 bytes
 	_, oobn, _, _, err := uc.ReadMsgUnix(buf, oob)
 	if err != nil {
-		err = fmt.Errorf("ReadMsgUnix: %v", err)
-		return
+		return nil, fmt.Errorf("ReadMsgUnix: %v", err)
 	}
 
 	// Parse the message.
 	scms, err := syscall.ParseSocketControlMessage(oob[:oobn])
 	if err != nil {
-		err = fmt.Errorf("ParseSocketControlMessage: %v", err)
-		return
+		return nil, fmt.Errorf("ParseSocketControlMessage: %v", err)
 	}
 
 	// We expect one message.
 	if len(scms) != 1 {
-		err = fmt.Errorf("expected 1 SocketControlMessage; got scms = %#v", scms)
-		return
+		return nil, fmt.Errorf("expected 1 SocketControlMessage; got scms = %#v", scms)
 	}
 
 	scm := scms[0]
@@ -97,17 +83,98 @@ func mount(
 	// Pull out the FD returned by fusermount
 	gotFds, err := syscall.ParseUnixRights(&scm)
 	if err != nil {
-		err = fmt.Errorf("syscall.ParseUnixRights: %v", err)
-		return
+		return nil, fmt.Errorf("syscall.ParseUnixRights: %v", err)
 	}
 
 	if len(gotFds) != 1 {
-		err = fmt.Errorf("wanted 1 fd; got %#v", gotFds)
-		return
+		return nil, fmt.Errorf("wanted 1 fd; got %#v", gotFds)
 	}
 
 	// Turn the FD into an os.File.
-	dev = os.NewFile(uintptr(gotFds[0]), "/dev/fuse")
+	return os.NewFile(uintptr(gotFds[0]), "/dev/fuse"), 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":      enableFunc(unix.MS_RDONLY),
+	"ro":      disableFunc(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)")
 
-	return
+func directmount(dir string, cfg *MountConfig) (*os.File, error) {
+	dev, err := os.OpenFile("/dev/fuse", os.O_RDWR, 0644)
+	if err != nil {
+		return nil, errFallback
+	}
+	// 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)
+	}
+	delete(opts, "fsname") // handled via fstype mount(2) parameter
+	data += "," + mapToOptionsString(opts)
+	if err := unix.Mount(
+		cfg.FSName, // source
+		dir,        // target
+		"fuse",     // fstype
+		mountflag,  // mountflag
+		data,       // data
+	); err != nil {
+		if err == syscall.EPERM {
+			return nil, errFallback
+
+		}
+		return nil, err
+	}
+	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
+
+	// 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 {
+		return fusermount(dir, cfg)
+	}
+	return dev, err
 }