From 6c059494af363f8b17a40a9a3c07ae36992c3bca Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <michael@stapelberg.de>
Date: Wed, 13 Jun 2018 23:51:31 +0200
Subject: [PATCH] ntp: set real-time clock (hwclock) if present

---
 cmd/ntp/ntp.go            | 29 +++++++++++++++++++++++++----
 cmd/ntp/privdrop.go       |  6 +++++-
 cmd/ntp/privdrop_dummy.go |  4 +++-
 cmd/ntp/rtc.go            | 26 ++++++++++++++++++++++++++
 4 files changed, 59 insertions(+), 6 deletions(-)
 create mode 100644 cmd/ntp/rtc.go

diff --git a/cmd/ntp/ntp.go b/cmd/ntp/ntp.go
index f3246bc..4eaa0cb 100644
--- a/cmd/ntp/ntp.go
+++ b/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)
diff --git a/cmd/ntp/privdrop.go b/cmd/ntp/privdrop.go
index 117ac70..9b35c12 100644
--- a/cmd/ntp/privdrop.go
+++ b/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{
diff --git a/cmd/ntp/privdrop_dummy.go b/cmd/ntp/privdrop_dummy.go
index c05d6c1..8267ea1 100644
--- a/cmd/ntp/privdrop_dummy.go
+++ b/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) {
 }
diff --git a/cmd/ntp/rtc.go b/cmd/ntp/rtc.go
new file mode 100644
index 0000000..7e1842d
--- /dev/null
+++ b/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
+}