Trap segfaults caused by service callbacks in separate process context

This patch traps segfaults caused by 3rd party service callbacks in a
separate process context.  Effectively preventing a single programming
mistake from taking down the entire system.  Previously it was up to the
callback coder to write error free code so that PID 1 did not crash.

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

sig.c

@@ -49,7 +49,7 @@
 
 static int   stopped = 0;
 static uev_t sigint_watcher, sigpwr_watcher;
-static uev_t sigchld_watcher;
+static uev_t sigchld_watcher, sigsegv_watcher;
 static uev_t sigstop_watcher, sigtstp_watcher, sigcont_watcher;
 
 
@@ -103,6 +103,15 @@ static void sigchld_cb(uev_ctx_t *UNUSED(ctx), uev_t *UNUSED(w), void *UNUSED(ar
 	svc_monitor(waitpid(-1, NULL, WNOHANG));
 }
 
+/*
+ * SIGSEGV: mostly if service callbacks segfault
+ */
+static void sigsegv_cb(uev_ctx_t *UNUSED(ctx), uev_t *UNUSED(w), void *UNUSED(arg), int UNUSED(events))
+{
+	_e("PID %d caused a segfault!\n", getpid());
+	exit(-1);
+}
+
 /*
  * SIGSTOP/SIGTSTP: Paused by user or netflash
  */
@@ -204,6 +213,9 @@ void sig_setup(uev_ctx_t *ctx)
 	/* After initial bootstrap of Finit we call the service monitor to reap children */
 	uev_signal_init(ctx, &sigchld_watcher, sigchld_cb, NULL, SIGCHLD);
 
+	/* Trap SIGSEGV in case service callbacks crash */
+	uev_signal_init(ctx, &sigsegv_watcher, sigsegv_cb, NULL, SIGSEGV);
+
 	/* Stopping init is a bit tricky. */
 	uev_signal_init(ctx, &sigstop_watcher, sigstop_cb, NULL, SIGSTOP);
 	uev_signal_init(ctx, &sigtstp_watcher, sigstop_cb, NULL, SIGTSTP);

svc.c

@@ -395,8 +395,39 @@ svc_cmd_t svc_enabled(svc_t *svc, int event, void *arg)
 		return SVC_STOP;
 
 	/* Is there a service plugin registered? */
-	if (svc->cb)
+	if (svc->cb) {
-		return svc->cb(svc, event, arg);
+		int   status;
+		pid_t pid;
+
+		/* Let callback run in separate process so it doesn't crash PID 1 */
+		pid = fork();
+		if (-1 == pid) {
+			_pe("Failed in %s callback", svc->cmd);
+			return SVC_STOP;
+		}
+
+		if (!pid) {
+			status = svc->cb(svc, event, arg);
+			exit(status);
+		}
+
+		if (waitpid(pid, &status, 0) == -1) {
+			_pe("Failed reading status from %s callback", svc->cmd);
+			return SVC_STOP;
+		}
+
+		/* Callback normally exits here. */
+		if (WIFEXITED(status))
+			return WEXITSTATUS(status);
+
+		/* Check for SEGFAULT or other error ... */
+		if (WCOREDUMP(status))
+			_e("Callback to %s crashed!\n", svc->cmd);
+		else
+			_e("Callback to %s did not exit normally!\n", svc->cmd);
+
+		return SVC_STOP;
+	}
 
 	/* No service plugin, default to start, since listed in finit.conf */
 	return SVC_START;