sys_netbsd: allow running without root privileges

On NetBSD programs with write access to /dev/clockctl can adjust or set
the system clock without the root privileges. Add a function to drop the
privileges and check if the process has write access to the device to
get a more descriptive error message when the chrony uid/gid doesn't
match the owner of the device.

configure

@@ -215,6 +215,7 @@ feat_rtc=1
 try_rtc=0
 feat_droproot=1
 try_libcap=0
+try_clockctl=0
 readline_lib=""
 readline_inc=""
 ncurses_lib=""
@@ -405,6 +406,7 @@ case $SYSTEM in
     NetBSD-* )
         EXTRA_OBJECTS="sys_netbsd.o"
         EXTRA_LIBS="-lkvm"
+        try_clockctl=1
         add_def NETBSD
         echo "Configuring for $SYSTEM"
     ;;
@@ -590,6 +592,12 @@ then
     EXTRA_LIBS="$EXTRA_LIBS -lcap"
 fi
 
+if [ $feat_droproot = "1" ] && [ $try_clockctl = "1" ] && \
+  test_code '<sys/clockctl.h>' 'sys/clockctl.h' '' '' ''
+then
+  add_def FEAT_PRIVDROP
+fi
+
 if [ $feat_rtc = "1" ] && [ $try_rtc = "1" ] && \
   test_code '<linux/rtc.h>' 'sys/ioctl.h linux/rtc.h' '' '' \
     'ioctl(1, RTC_UIE_ON&RTC_UIE_OFF&RTC_RD_TIME&RTC_SET_TIME, 0&RTC_UF);'

sys.c

@@ -113,6 +113,8 @@ void SYS_DropRoot(uid_t uid, gid_t gid)
 {
 #if defined(LINUX) && defined (FEAT_PRIVDROP)
   SYS_Linux_DropRoot(uid, gid);
+#elif defined(NETBSD) && defined(FEAT_PRIVDROP)
+  SYS_NetBSD_DropRoot(uid, gid);
 #else
   LOG_FATAL(LOGF_Sys, "dropping root privileges not supported");
 #endif

sys_netbsd.c

@@ -324,5 +324,29 @@ SYS_NetBSD_Finalise(void)
 
 /* ================================================== */
 
+#ifdef FEAT_PRIVDROP
+void
+SYS_NetBSD_DropRoot(uid_t uid, gid_t gid)
+{
+  int fd;
+
+  if (setgroups(0, NULL))
+    LOG_FATAL(LOGF_SysNetBSD, "setgroups() failed : %s", strerror(errno));
+
+  if (setgid(gid))
+    LOG_FATAL(LOGF_SysNetBSD, "setgid(%d) failed : %s", gid, strerror(errno));
+
+  if (setuid(uid))
+    LOG_FATAL(LOGF_SysNetBSD, "setuid(%d) failed : %s", uid, strerror(errno));
+
+  DEBUG_LOG(LOGF_SysNetBSD, "Root dropped to uid %d gid %d", uid, gid);
+
+  /* Check if we have write access to /dev/clockctl */
+  fd = open("/dev/clockctl", O_WRONLY);
+  if (fd < 0)
+    LOG_FATAL(LOGF_SysNetBSD, "Can't write to /dev/clockctl");
+  close(fd);
+}
+#endif
 
 #endif /* NETBSD */

sys_netbsd.h

@@ -32,4 +32,6 @@ void SYS_NetBSD_Initialise(void);
 
 void SYS_NetBSD_Finalise(void);
 
+void SYS_NetBSD_DropRoot(uid_t uid, gid_t gid);
+
 #endif