Properly set the controlling console

For FreeBSD it's required to become session leader of the process group
before a console can be set as controlling, furthermore it's required
to call TIOCSTTY on the console FD to actually set it as controlling.

cmd/runj-entrypoint/main.go

@@ -31,6 +31,7 @@ import (
 	"fmt"
 	"os"
 	"strconv"
+	"syscall"
 
 	"github.com/containerd/console"
 	"golang.org/x/sys/unix"
@@ -121,6 +122,14 @@ func dupStdio(slavePath string) error {
 			Err:  err,
 		}
 	}
+
+	if _, err := syscall.Setsid(); err != nil {
+		return err
+	}
+
+	if ret, _, err := unix.Syscall(unix.SYS_IOCTL, uintptr(fd), unix.TIOCSCTTY, uintptr(0)); ret != 0 || err != 0 {
+		return fmt.Errorf("Unable to set controlling tty")
+	}
 	for _, i := range []int{0, 1, 2} {
 		if err := unix.Dup2(fd, i); err != nil {
 			return err