Write backtrace to /var/lib/bitlbee/crash.log on SIGSEGV

Async-signal-safe code is very restricted (nothing that may call malloc
indirectly), so this code tries its best to show meaningful stuff, but
the output is still fairly raw. The contents of the log file are:

- BITLBEE_VERSION, BITLBEE_CONFIGURE_ARGS
- Backtrace as generated by backtrace()/backtrace_symbols_fd()
- A small help text explaining how to get more useful symbol names
- Memory maps (/proc/self/maps), which also mentions loaded plugins

The backtrace() function is a GNU extension, /proc/ is a linux thing.
Non-glibc platforms (such as musl) won't show anything, non-linux
platforms will skip the memory maps when /proc/self/maps fails to open.

I'd like to include timestamps, but I can't find a safe way to format
them. Even turning raw unix timestamps to strings is hard. Fun stuff.

I used the config directory because it's the only place we can be sure
we can write to. The filename is hardcoded for the same reason there are
no timestamps.

1 files changed, 76 insertions, 5 deletions

diff --git a/unix.c b/unix.c
index 2569c28e..f224c597 100644
--- a/unix.c
+++ b/unix.c
@@ -45,6 +45,10 @@
 #include "otr.h"
 #endif
 
+#ifdef HAVE_BACKTRACE
+#include <execinfo.h>
+#endif
+
 global_t global;        /* Against global namespace pollution */
 
 static struct {
@@ -62,6 +66,9 @@ int main(int argc, char *argv[])
 	int i = 0;
 	char *old_cwd = NULL;
 	struct sigaction sig, old;
+#ifdef HAVE_BACKTRACE
+	void *unused[1];
+#endif
 
 	/* Required to make iconv to ASCII//TRANSLIT work. This makes BitlBee
 	   system-locale-sensitive. :-( */
@@ -175,6 +182,13 @@ int main(int argc, char *argv[])
 	sigaction(SIGINT, &sig, &old);
 	sigaction(SIGTERM, &sig, &old);
 
+#ifdef HAVE_BACKTRACE
+	/* As per the backtrace(3) man page, call this outside of the signal
+	 * handler once to ensure any dynamic libraries are loaded in an
+	 * async-signal-safe environment to prevent deadlocks */
+	backtrace(unused, 1);
+#endif
+
 	if (!getuid() || !geteuid()) {
 		log_message(LOGLVL_WARNING, "BitlBee is running with root privileges. Why?");
 	}
@@ -291,25 +305,82 @@ static void sighandler_shutdown(int signal)
 	unused = write(shutdown_pipe.fd[1], "", 1);
 }
 
+#ifdef HAVE_BACKTRACE
+/* Writes a backtrace to (usually) /var/lib/bitlbee/crash.log
+ * No malloc allowed means not a lot can be written to that file */
+static void sighandler_crash_backtrace()
+{
+	int fd, mapsfd;
+	int size;
+	void *trace[128];
+	const char message[] = "## " PACKAGE " crashed\n"
+		"## Version: " BITLBEE_VERSION "\n"
+		"## Configure args: " BITLBEE_CONFIGURE_ARGS "\n"
+		"##\n"
+		"## Backtrace:\n\n";
+	const char message2[] = "\n"
+		"## Hint: To get details on addresses use\n"
+		"##   addr2line -e <binary> <address>\n"
+		"## or\n"
+		"##   gdb <binary> -ex 'l *<address>' -ex q\n"
+		"## where <binary> is a filename from above and <address> is the part between (...)\n"
+		"##\n\n";
+	const char message3[] = "\n## End of memory maps. See above for the backtrace\n\n";
+
+	fd = open(CRASHFILE, O_WRONLY | O_APPEND | O_CREAT, 0600);
+
+	if (fd == -1 || write(fd, message, sizeof(message) - 1) == -1) {
+		return;
+	}
+
+	size = backtrace(trace, 128);
+	backtrace_symbols_fd(trace, size, fd);
+
+	(void) write(fd, message2, sizeof(message2) - 1);
+
+	/* a bit too linux-specific, so fail gracefully */
+	mapsfd = open("/proc/self/maps", O_RDONLY, 0);
+
+	if (mapsfd != -1) {
+		char buf[4096] = {0};
+		ssize_t bytes;
+
+		while ((bytes = read(mapsfd, buf, sizeof(buf))) > 0) {
+			(void) write(fd, buf, bytes);
+		}
+		(void) close(mapsfd);
+		(void) write(fd, message3, sizeof(message3) - 1);
+	}
+
+	(void) close(fd);
+}
+#endif
+
 /* Signal handler for SIGSEGV
  * A desperate attempt to tell the user that everything is wrong in the world.
  * Avoids using irc_abort() because it has several unsafe calls to malloc */
 static void sighandler_crash(int signal)
 {
 	GSList *l;
-	int unused G_GNUC_UNUSED;
-	const char *message = "ERROR :BitlBee crashed! (SIGSEGV received)\r\n";
-	int len = strlen(message);
+	const char message[] = "ERROR :BitlBee crashed! (SIGSEGV received)\r\n"
+#ifdef HAVE_BACKTRACE
+		"ERROR :Writing backtrace to " CRASHFILE "\r\n"
+#endif
+		"ERROR :This is a bug either in BitlBee or a plugin, ask us on IRC if unsure\r\n";
 
 	for (l = irc_connection_list; l; l = l->next) {
 		irc_t *irc = l->data;
 		sock_make_blocking(irc->fd);
 		if (irc->sendbuffer) {
-			unused = write(irc->fd, irc->sendbuffer, strlen(irc->sendbuffer));
+			(void) write(irc->fd, irc->sendbuffer, strlen(irc->sendbuffer));
 		}
-		unused = write(irc->fd, message, len);
+		(void) write(irc->fd, message, sizeof(message) - 1);
 	}
 
+#ifdef HAVE_BACKTRACE
+	sighandler_crash_backtrace();
+#endif
+
 	raise(signal);
 }