ntp: set real-time clock (hwclock) if present

cmd/ntp/ntp.go

@@ -4,29 +4,50 @@ package main
 import (
 	"log"
 	"math/rand"
+	"os"
 	"syscall"
 	"time"
 
 	"github.com/beevik/ntp"
 )
 
-func set() error {
+func set(rtc *os.File) error {
 	r, err := ntp.Query("0.gokrazy.pool.ntp.org")
 	if err != nil {
 		return err
 	}
 
 	tv := syscall.NsecToTimeval(r.Time.UnixNano())
-	return syscall.Settimeofday(&tv)
+	if err := syscall.Settimeofday(&tv); err != nil {
+		return err
+	}
+
+	if rtc == nil {
+		return nil
+	}
+	return setRTC(rtc, r.Time.UTC())
 }
 
 func main() {
 	log.SetFlags(log.LstdFlags | log.Lshortfile)
 
-	mustDropPrivileges()
+	var rtc *os.File
+	var err error
+	if os.Getenv("NTP_PRIVILEGES_DROPPED") == "1" {
+		if os.Getenv("NTP_RTC") == "1" {
+			rtc = os.NewFile(3, "/dev/rtc0")
+		}
+	} else {
+		rtc, err = os.Open("/dev/rtc0")
+		if err != nil && !os.IsNotExist(err) {
+			log.Fatal(err)
+		}
+	}
+
+	mustDropPrivileges(rtc)
 
 	for {
-		if err := set(); err != nil {
+		if err := set(rtc); err != nil {
 			log.Fatalf("setting time failed: %v", err)
 		}
 		time.Sleep(1*time.Hour + time.Duration(rand.Int63n(250))*time.Millisecond)

cmd/ntp/privdrop.go

@@ -46,7 +46,7 @@ func getCaps() (caps, error) {
 // mustDropPrivileges executes the program in a child process, dropping root
 // privileges, but retaining the CAP_SYS_TIME capability to change the system
 // clock.
-func mustDropPrivileges() {
+func mustDropPrivileges(rtc *os.File) {
 	if os.Getenv("NTP_PRIVILEGES_DROPPED") == "1" {
 		return
 	}
@@ -71,6 +71,10 @@ func mustDropPrivileges() {
 
 	cmd := exec.Command(os.Args[0])
 	cmd.Env = append(os.Environ(), "NTP_PRIVILEGES_DROPPED=1")
+	if rtc != nil {
+		cmd.Env = append(cmd.Env, "NTP_RTC=1")
+		cmd.ExtraFiles = []*os.File{rtc}
+	}
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	cmd.SysProcAttr = &syscall.SysProcAttr{

cmd/ntp/privdrop_dummy.go

@@ -2,7 +2,9 @@
 
 package main
 
+import "os"
+
 // mustDropPrivileges (on Go < 1.9) does nothing, as the AmbientCaps field in
 // syscall.SysProcAttr was only introduced in Go 1.9.
-func mustDropPrivileges() {
+func mustDropPrivileges(*os.File) {
 }

cmd/ntp/rtc.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+	"log"
+	"os"
+	"time"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+func setRTC(rtc *os.File, now time.Time) error {
+	c := unix.RTCTime{
+		Sec:  int32(now.Second()),
+		Min:  int32(now.Minute()),
+		Hour: int32(now.Hour()),
+		Mday: int32(now.Day()),
+		Mon:  int32(now.Month() - 1),
+		Year: int32(now.Year() - 1900),
+	}
+	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, rtc.Fd(), unix.RTC_SET_TIME, uintptr(unsafe.Pointer(&c))); errno != 0 {
+		return errno
+	}
+	log.Printf("RTC set to %+v", c)
+	return nil
+}