container: ptrace protection via Yama LSM

This is only a nice to have feature as the init process has no additional privileges and the monitor process was never reachable anyway.

Closes #4.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/init.go

@@ -55,6 +55,11 @@ func Init(prepare func(prefix string), setVerbose func(verbose bool)) {
 		log.Fatal("this process must run as pid 1")
 	}
 
+	if err := SetPtracer(0); err != nil {
+		msg.Verbosef("cannot enable ptrace protection via Yama LSM: %v", err)
+		// not fatal: this program has no additional privileges at initial program start
+	}
+
 	var (
 		params      initParams
 		closeSetup  func() error

container/syscall.go

@@ -9,6 +9,14 @@ const (
 	SUID_DUMP_USER
 )
 
+func SetPtracer(pid uintptr) error {
+	_, _, errno := syscall.Syscall(syscall.SYS_PRCTL, syscall.PR_SET_PTRACER, pid, 0)
+	if errno == 0 {
+		return nil
+	}
+	return errno
+}
+
 func SetDumpable(dumpable uintptr) error {
 	// linux/sched/coredump.h
 	if _, _, errno := syscall.Syscall(syscall.SYS_PRCTL, syscall.PR_SET_DUMPABLE, dumpable, 0); errno != 0 {