Fix #72: loss of raw kernel log messages to console

This patch adds a command line flag `-l` to keep kernel logs to console.
A feature requested by embedded Linux users which often navigate issues
by console output.

With properly configured kernel logging, e.g., `quiet`, only error and
above in severity is logged by the kernel directly to the console.  So
for most users this would be a useful behavior.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

man/syslogd.8

@@ -38,7 +38,7 @@
 .Nd log systems messages
 .Sh SYNOPSIS
 .Nm
-.Op Fl ?468AcdFHKknsTtv
+.Op Fl ?468AcdFHKklnsTtv
 .Op Fl a Ar addr[/len][:port]
 .Op Fl a Ar name[:port]
 .Op Fl b Ar addr[:port]
@@ -292,6 +292,22 @@ Usually the
 .Dq kern
 facility is reserved for messages read directly from
 .Pa /dev/kmsg .
+.It Fl l
+Keep kernel console logging.  By default
+.Nm
+call
+.Xr klogctl 2
+to disable the kernel's logging to console after having opened
+.Pa /dev/kmsg .
+With this option the kernel's log level can be adjusted using
+.Xr sysctl 8 ,
+or the kernel command line, to suit your logging needs to the console.
+.Pp
+Please note, this does not affect logging of kernel messages, see
+.Fl K ,
+only what the kernel logs to
+.Pa /dev/console .
+Also, this is only applicable to Linux.
 .It Fl m Ar interval
 Select the number of minutes between
 .Dq mark

src/syslogd.c

@@ -154,6 +154,7 @@ static int	  RemoteHostname;	  /* Log remote hostname from the message */
 static int	  KernLog = 1;		  /* Track kernel logs by default */
 static int	  KeepKernFac;		  /* Keep remotely logged kernel facility */
 static int	  KeepKernTime;		  /* Keep kernel timestamp, evern after initial read */
+static int	  KeepKernConsole;	  /* Keep kernel logging to console */
 
 static off_t	  RotateSz = 0;		  /* Max file size (bytes) before rotating, disabled by default */
 static int	  RotateCnt = 5;	  /* Max number (count) of log files to keep, set with -c <NUM> */
@@ -363,6 +364,9 @@ int usage(int code)
 	       "  -H        Use hostname from message instead of address for remote messages\n"
 	       "  -K        Disable kernel logging, useful in container use-cases\n"
 	       "  -k        Allow logging with facility 'kernel', otherwise remapped to 'user'\n"
+#ifdef __linux__
+	       "  -l        Keep kernel logging to console, use sysctl to adjust kernel.printk\n"
+#endif
 	       "  -m MINS   Interval between MARK messages, 0 to disable, default: 20 min\n"
 	       "  -n        Disable DNS query for every request\n"
 	       "  -P FILE   File to store the process ID, default: %s\n"
@@ -397,7 +401,7 @@ int main(int argc, char *argv[])
 	char *ptr;
 	int ch;
 
-	while ((ch = getopt(argc, argv, "468Aa:b:C:cdHFf:Kkm:nP:p:r:sTtv?")) != EOF) {
+	while ((ch = getopt(argc, argv, "468Aa:b:C:cdHFf:Kklm:nP:p:r:sTtv?")) != EOF) {
 		switch ((char)ch) {
 		case '4':
 			family = PF_INET;
@@ -464,6 +468,10 @@ int main(int argc, char *argv[])
 			KeepKernFac = 1;
 			break;
 
+		case 'l':
+			KeepKernConsole = 1;
+			break;
+
 		case 'm': /* mark interval */
 			MarkInterval = atoi(optarg) * 60;
 			break;

src/syslogd.h

@@ -169,8 +169,8 @@
 #define SYSLOG_ACTION_CONSOLE_ON  7
 
 #ifdef __linux__
-#define kern_console_off() klogctl(SYSLOG_ACTION_CONSOLE_OFF, NULL, 0)
-#define kern_console_on()  klogctl(SYSLOG_ACTION_CONSOLE_ON, NULL, 0)
+#define kern_console_off() if (!KeepKernConsole) klogctl(SYSLOG_ACTION_CONSOLE_OFF, NULL, 0)
+#define kern_console_on()  if (!KeepKernConsole) klogctl(SYSLOG_ACTION_CONSOLE_ON, NULL, 0)
 #else
 #define kern_console_off() do { } while (0)
 #define kern_console_on()  do { } while (0)