func forkAndMountat(dirfd uintptr, source, target, fstype, flags, data uintptr, pipefd int) (pid uintptr, errno syscall.Errno) {
	beforeFork()
	pid, _, errno = syscall.RawSyscall6(syscall.SYS_CLONE, uintptr(syscall.SIGCHLD)|syscall.CLONE_FILES, 0, 0, 0, 0, 0)
	if errno != 0 || pid != 0 {
		afterFork()
		return
	}
	afterForkInChild()
	_, _, errno = syscall.RawSyscall(syscall.SYS_FCHDIR, dirfd, 0, 0)
	if errno != 0 {
		goto childerr
	}
	_, _, errno = syscall.RawSyscall6(syscall.SYS_MOUNT, source, target, fstype, flags, data, 0)
childerr:
	_, _, errno = syscall.RawSyscall(syscall.SYS_WRITE, uintptr(pipefd), uintptr(unsafe.Pointer(&errno)), unsafe.Sizeof(errno))
	syscall.RawSyscall(syscall.SYS_EXIT, uintptr(errno), 0, 0)
	panic("unreachable")
}
forkAndMountat will fork thread, change working dir and mount.

precondition: the runtime OS thread must be locked.
package sys
import (
	"runtime"
	"syscall"
	"unsafe"
	"github.com/containerd/containerd/log"
	"github.com/pkg/errors"
	"golang.org/x/sys/unix"
)
func FMountat(dirfd uintptr, source, target, fstype string, flags uintptr, data string) error {
	var (
		sourceP, targetP, fstypeP, dataP *byte
		pid                              uintptr
		err                              error
		errno, status                    syscall.Errno
	)
	sourceP, err = syscall.BytePtrFromString(source)
	if err != nil {
		return err
	}
	targetP, err = syscall.BytePtrFromString(target)
	if err != nil {
		return err
	}
	fstypeP, err = syscall.BytePtrFromString(fstype)
	if err != nil {
		return err
	}
	if data != "" {
		dataP, err = syscall.BytePtrFromString(data)
		if err != nil {
			return err
		}
	}
	runtime.LockOSThread()
	defer runtime.UnlockOSThread()
	var pipefds [2]int
	if err := syscall.Pipe2(pipefds[:], syscall.O_CLOEXEC); err != nil {
		return errors.Wrap(err, "failed to open pipe")
	}
	defer func() {
		syscall.Close(pipefds[0])
		syscall.Close(pipefds[1])
	}()
	pid, errno = forkAndMountat(dirfd,
		uintptr(unsafe.Pointer(sourceP)),
		uintptr(unsafe.Pointer(targetP)),
		uintptr(unsafe.Pointer(fstypeP)),
		flags,
		uintptr(unsafe.Pointer(dataP)),
		pipefds[1],
	)
	if errno != 0 {
		return errors.Wrap(errno, "failed to fork thread")
	}
	defer func() {
		_, err := unix.Wait4(int(pid), nil, 0, nil)
		for err == syscall.EINTR {
			_, err = unix.Wait4(int(pid), nil, 0, nil)
		}
		if err != nil {
			log.L.WithError(err).Debugf("failed to find pid=%d process", pid)
		}
	}()
	_, _, errno = syscall.RawSyscall(syscall.SYS_READ,
		uintptr(pipefds[0]),
		uintptr(unsafe.Pointer(&status)),
		unsafe.Sizeof(status))
	if errno != 0 {
		return errors.Wrap(errno, "failed to read pipe")
	}
	if status != 0 {
		return errors.Wrap(status, "failed to mount")
	}
	return nil
}