klogd: make it work on non-linux systems

The klogctl() interface allows changing the console loglevel, but is
Linux-specific. The more portable method of reading from _PATH_KLOG is
added as an alternative.

Adapted from the Debian kFreeBSD patch at:
http://svn.debian.org/viewsvn/d-i/people/slackydeb/kfreebsd/busybox/1.14/debian/klogd.diff

Signed-off-by: Jeremie Koenig <jk@jk.fr.eu.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

sysklogd/Config.src

@@ -109,7 +109,6 @@ config FEATURE_LOGREAD_REDUCED_LOCKING
 config KLOGD
 	bool "klogd"
 	default y
-	depends on PLATFORM_LINUX
 	help
 	  klogd is a utility which intercepts and logs all
 	  messages from the Linux kernel and sends the messages
@@ -117,6 +116,22 @@ config KLOGD
 	  you wish to record the messages produced by the kernel,
 	  you should enable this option.
 
+config FEATURE_KLOGD_KLOGCTL
+	bool "Use the klogctl() interface"
+	default y
+	depends on KLOGD && PLATFORM_LINUX
+	help
+	  The klogd applet supports two interfaces for reading
+	  kernel messages. Linux provides the klogctl() interface
+	  which allows reading messages from the kernel ring buffer
+	  independently from the file system.
+
+	  If you answer 'N' here, klogd will use the more portable
+	  approach of reading them from /proc or a device node.
+	  However, this method requires the file to be available.
+
+	  If in doubt, say 'Y'.
+
 config LOGGER
 	bool "logger"
 	default y

sysklogd/klogd.c

@@ -4,7 +4,7 @@
  *
  * Copyright (C) 2001 by Gennady Feldman <gfeldman@gena01.com>.
  * Changes: Made this a standalone busybox module which uses standalone
- *					syslog() client interface.
+ * syslog() client interface.
  *
  * Copyright (C) 1999-2004 by Erik Andersen <andersen@codepoet.org>
  *
@@ -19,18 +19,93 @@
 
 #include "libbb.h"
 #include <syslog.h>
-#include <sys/klog.h>
 
-static void klogd_signal(int sig)
+
+/* The Linux-specific klogctl(3) interface does not rely on the filesystem and
+ * allows us to change the console loglevel. Alternatively, we read the
+ * messages from _PATH_KLOG. */
+
+#if ENABLE_FEATURE_KLOGD_KLOGCTL
+
+# include <sys/klog.h>
+
+static void klogd_open(void)
+{
+	/* "Open the log. Currently a NOP" */
+	klogctl(1, NULL, 0);
+}
+
+static void klogd_setloglevel(int lvl)
+{
+	/* "printk() prints a message on the console only if it has a loglevel
+	 * less than console_loglevel". Here we set console_loglevel = lvl. */
+	klogctl(8, NULL, lvl);
+}
+
+static int klogd_read(char *bufp, int len)
+{
+	return klogctl(2, bufp, len);
+}
+# define READ_ERROR "klogctl(2) error"
+
+static void klogd_close(void)
 {
 	/* FYI: cmd 7 is equivalent to setting console_loglevel to 7
 	 * via klogctl(8, NULL, 7). */
 	klogctl(7, NULL, 0); /* "7 -- Enable printk's to console" */
 	klogctl(0, NULL, 0); /* "0 -- Close the log. Currently a NOP" */
-	syslog(LOG_NOTICE, "klogd: exiting");
-	kill_myself_with_sig(sig);
 }
 
+#else
+
+# include <paths.h>
+# ifndef _PATH_KLOG
+#  ifdef __GNU__
+#   define _PATH_KLOG "/dev/klog"
+#  else
+#   error "your system's _PATH_KLOG is unknown"
+#  endif
+# endif
+# define PATH_PRINTK "/proc/sys/kernel/printk"
+
+enum { klogfd = 3 };
+
+static void klogd_open(void)
+{
+	int fd = xopen(_PATH_KLOG, O_RDONLY);
+	xmove_fd(fd, klogfd);
+}
+
+static void klogd_setloglevel(int lvl)
+{
+	FILE *fp = fopen_or_warn(PATH_PRINTK, "w");
+	if (fp) {
+		/* This changes only first value:
+		 * "messages with a higher priority than this
+		 * [that is, with numerically lower value]
+		 * will be printed to the console".
+		 * The other three values in this pseudo-file aren't changed.
+		 */
+		fprintf(fp, "%u\n", lvl);
+		fclose(fp);
+	}
+}
+
+static int klogd_read(char *bufp, int len)
+{
+	return read(klogfd, bufp, len);
+}
+# define READ_ERROR "read error"
+
+static void klogd_close(void)
+{
+	klogd_setloglevel(7);
+	if (ENABLE_FEATURE_CLEAN_UP)
+		close(klogfd);
+}
+
+#endif
+
 #define log_buffer bb_common_bufsiz1
 enum {
 	KLOGD_LOGBUF_SIZE = sizeof(log_buffer),
@@ -38,6 +113,19 @@ enum {
 	OPT_FOREGROUND = (1 << 1),
 };
 
+/* TODO: glibc openlog(LOG_KERN) reverts to LOG_USER instead,
+ * because that's how they interpret word "default"
+ * in the openlog() manpage:
+ *      LOG_USER (default)
+ *              generic user-level messages
+ * and the fact that LOG_KERN is a constant 0.
+ * glibc interprets it as "0 in openlog() call means 'use default'".
+ * I think it means "if openlog wasn't called before syslog() is called,
+ * use default".
+ * Convincing glibc maintainers otherwise is, as usual, nearly impossible.
+ * Should we open-code syslog() here to use correct facility?
+ */
+
 int klogd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int klogd_main(int argc UNUSED_PARAM, char **argv)
 {
@@ -55,34 +143,34 @@ int klogd_main(int argc UNUSED_PARAM, char **argv)
 		bb_daemonize_or_rexec(DAEMON_CHDIR_ROOT, argv);
 	}
 
+	logmode = LOGMODE_SYSLOG;
+
+	/* klogd_open() before openlog(), since it might use fixed fd 3,
+	 * and openlog() also may use the same fd 3 if we swap them:
+	 */
+	klogd_open();
 	openlog("kernel", 0, LOG_KERN);
 
-	bb_signals(BB_FATAL_SIGS, klogd_signal);
-	signal(SIGHUP, SIG_IGN);
-
-	/* "Open the log. Currently a NOP" */
-	klogctl(1, NULL, 0);
-
-	/* "printk() prints a message on the console only if it has a loglevel
-	 * less than console_loglevel". Here we set console_loglevel = i. */
 	if (i)
-		klogctl(8, NULL, i);
+		klogd_setloglevel(i);
+
+	bb_signals(BB_FATAL_SIGS, record_signo);
+	signal(SIGHUP, SIG_IGN);
 
 	syslog(LOG_NOTICE, "klogd started: %s", bb_banner);
 
-	while (1) {
+	while (!bb_got_signal) {
 		int n;
 		int priority;
 		char *start;
 
 		/* "2 -- Read from the log." */
 		start = log_buffer + used;
-		n = klogctl(2, start, KLOGD_LOGBUF_SIZE-1 - used);
+		n = klogd_read(start, KLOGD_LOGBUF_SIZE-1 - used);
 		if (n < 0) {
 			if (errno == EINTR)
 				continue;
-			syslog(LOG_ERR, "klogd: error %d in klogctl(2): %m",
+			bb_perror_msg(READ_ERROR);
-					errno);
 			break;
 		}
 		start[n] = '\0';
@@ -131,5 +219,9 @@ int klogd_main(int argc UNUSED_PARAM, char **argv)
 		}
 	}
 
+	klogd_close();
+	syslog(LOG_NOTICE, "klogd: exiting");
+	if (bb_got_signal)
+		kill_myself_with_sig(bb_got_signal);
 	return EXIT_FAILURE;
 }