catatoinit: rewrite LISTEN_PID to the child process

In order to get systemd fd passing let's rewrite the LISTEN_PID
environment variable if it is pointing to catatonit init process.

Closes: openSUSE#13

Signed-off-by: Giuseppe Scrivano <[email protected]>

catatonit.c

@@ -21,6 +21,7 @@
 #include <fcntl.h>
 #include <signal.h>
 #include <stdarg.h>
+#include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
@@ -193,20 +194,46 @@ static int make_foreground(sigset_t *sigmask)
 	return 0;
 }
 
+/*
+ * If the LISTEN_PID environment variable is set to "1", rewrite it to
+ * point to the child process.
+ */
+static void rewrite_listen_pid_env(int parent_pid)
+{
+	char *listen_pid;
+	char pid_str[32];
+
+	sprintf(pid_str, "%d", parent_pid);
+
+	listen_pid = getenv("LISTEN_PID");
+	if (listen_pid != NULL && strcmp (listen_pid, pid_str) == 0) {
+		int r;
+
+		sprintf(pid_str, "%d", getpid());
+
+		r = setenv("LISTEN_PID", pid_str, 1);
+		if (r < 0)
+			warn("could not overwrite env variable LISTEN_PID: %m");
+	}
+}
+
 /*
  * Spawn a child process with the given arguments and signal map and make it a
  * faux-pid1 by placing it in the foreground. This is the main process which
  * catatonit is going to be managing throughout its life.
  */
 static int spawn_pid1(char *file, char **argv, sigset_t *sigmask)
 {
+	pid_t parent_pid = getpid();
 	pid_t child = fork();
 	if (child != 0) {
 		if (child < 0)
 			error("failed to fork child: %m");
 		return child;
 	}
 
+        rewrite_listen_pid_env(parent_pid);
+
 	/*
 	 * We are now in the child. Set up our sigmask, put ourselves in the
 	 * foreground, and then finally exec (with the environment inherited).