Limit the number of simultanious children

Make sure minidlna cannot fork more than 5 children, to avoid becoming a
fork bomb.

Reported-by: Rinat Ibragimov <ibragimovrinat@mail.ru>

Makefile.am

@@ -6,7 +6,7 @@ sbin_PROGRAMS = minidlnad
 check_PROGRAMS = testupnpdescgen
 minidlnad_SOURCES = minidlna.c upnphttp.c upnpdescgen.c upnpsoap.c \
 			upnpreplyparse.c minixml.c clients.c \
-			getifaddr.c daemonize.c upnpglobalvars.c \
+			getifaddr.c process.c upnpglobalvars.c \
 			options.c minissdp.c uuid.c upnpevents.c \
 			sql.c utils.c metadata.c scanner.c inotify.c \
 			tivo_utils.c tivo_beacon.c tivo_commands.c \

minidlna.c

@@ -86,7 +86,7 @@
 #include "utils.h"
 #include "minissdp.h"
 #include "minidlnatypes.h"
-#include "daemonize.h"
+#include "process.h"
 #include "upnpevents.h"
 #include "scanner.h"
 #include "inotify.h"
@@ -155,14 +155,6 @@ sigterm(int sig)
 	quitting = 1;
 }
 
-static void
-sigchld(int sig)
-{
-	if (!scanning)
-		signal(SIGCHLD, SIG_IGN);
-	waitpid(-1, NULL, WNOHANG);
-}
-
 static void
 sighup(int sig)
 {
@@ -369,9 +361,9 @@ rescan:
 #if USE_FORK
 		scanning = 1;
 		sqlite3_close(db);
-		*scanner_pid = fork();
+		*scanner_pid = process_fork();
 		open_db(&db);
-		if (!(*scanner_pid)) /* child (scanner) process */
+		if (*scanner_pid == 0) /* child (scanner) process */
 		{
 			start_scanner();
 			sqlite3_close(db);
@@ -379,6 +371,10 @@ rescan:
 			freeoptions();
 			exit(EXIT_SUCCESS);
 		}
+		else if (*scanner_pid < 0)
+		{
+			start_scanner();
+		}
 #else
 		start_scanner();
 #endif
@@ -884,7 +880,7 @@ init(int argc, char **argv)
 	}
 	else
 	{
-		pid = daemonize();
+		pid = process_daemonize();
 		#ifdef READYNAS
 		unlink("/ramfs/.upnp-av_scan");
 		path = "/var/log/upnp-av.log";
@@ -897,7 +893,7 @@ init(int argc, char **argv)
 	}
 	log_init(path, log_level);
 
-	if (checkforrunning(pidfilename) < 0)
+	if (process_check_if_running(pidfilename) < 0)
 	{
 		DPRINTF(E_ERROR, L_GENERAL, "MiniDLNA is already running. EXITING.\n");
 		return 1;
@@ -923,6 +919,9 @@ init(int argc, char **argv)
 		DPRINTF(E_FATAL, L_GENERAL, "Failed to set %s handler. EXITING.\n", "SIGPIPE");
 	if (signal(SIGHUP, &sighup) == SIG_ERR)
 		DPRINTF(E_FATAL, L_GENERAL, "Failed to set %s handler. EXITING.\n", "SIGHUP");
+	sa.sa_handler = process_handle_child_termination;
+	if (sigaction(SIGCHLD, &sa, NULL))
+		DPRINTF(E_FATAL, L_GENERAL, "Failed to set %s handler. EXITING.\n", "SIGCHLD");
 
 	if (writepidfile(pidfilename, pid, uid) != 0)
 		pidfilename = NULL;
@@ -989,7 +988,6 @@ main(int argc, char **argv)
 			ret = -1;
 	}
 	check_db(db, ret, &scanner_pid);
-	signal(SIGCHLD, &sigchld);
 #ifdef HAVE_INOTIFY
 	if( GETFLAG(INOTIFY_MASK) )
 	{

process.c

@@ -1,7 +1,8 @@
-/* MiniUPnP project
+/* Process handling
- * http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/
+ *
+ * Copyright © 2006, Thomas Bernard
+ * Copyright © 2013, Benoît Knecht <benoit.knecht@fsfe.org>
  *
- * Copyright (c) 2006, Thomas Bernard
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -35,48 +36,74 @@
 #include <errno.h>
 #include <string.h>
 #include <signal.h>
+#include <sys/wait.h>
 
-#include "daemonize.h"
+#include "process.h"
 #include "config.h"
 #include "log.h"
 
+static const int max_number_of_children = 5;
+static int number_of_children = 0;
+
+pid_t
+process_fork(void)
+{
+	if (number_of_children >= max_number_of_children)
+	{
+		errno = EAGAIN;
+		return -1;
+	}
+
+	pid_t pid = fork();
+	if (pid > 0)
+		++number_of_children;
+	return pid;
+}
+
+void
+process_handle_child_termination(int signal)
+{
+	waitpid(-1, NULL, WNOHANG);
+	--number_of_children;
+}
+
 int
-daemonize(void)
+process_daemonize(void)
 {
 	int pid;
 #ifndef USE_DAEMON
 	int i;
 
-	switch(fork())
+	switch(process_fork())
 	{
-	/* fork error */
+		/* fork error */
-	case -1:
+		case -1:
-		perror("fork()");
+			perror("fork()");
-		exit(1);
-	
-	/* child process */
-	case 0:
-		/* obtain a new process group */
-		if( (pid = setsid()) < 0)
-		{
-			perror("setsid()");
 			exit(1);
-		}
 
-		/* close all descriptors */
+		/* child process */
-		for (i=getdtablesize();i>=0;--i) close(i);		
+		case 0:
+		/* obtain a new process group */
+			if( (pid = setsid()) < 0)
+			{
+				perror("setsid()");
+				exit(1);
+			}
 
-		i = open("/dev/null",O_RDWR); /* open stdin */
+			/* close all descriptors */
-		dup(i); /* stdout */
+			for (i=getdtablesize();i>=0;--i) close(i);		
-		dup(i); /* stderr */
 
-		umask(027);
+			i = open("/dev/null",O_RDWR); /* open stdin */
-		chdir("/");
+			dup(i); /* stdout */
+			dup(i); /* stderr */
 
-		break;
+			umask(027);
-	/* parent process */
+			chdir("/");
-	default:
+
-		exit(0);
+			break;
+		/* parent process */
+		default:
+			exit(0);
 	}
 #else
 	if( daemon(0, 0) < 0 )
@@ -87,7 +114,7 @@ daemonize(void)
 }
 
 int
-checkforrunning(const char * fname)
+process_check_if_running(const char *fname)
 {
 	char buffer[64];
 	int pidfile;
@@ -98,9 +125,9 @@ checkforrunning(const char * fname)
 
 	if( (pidfile = open(fname, O_RDONLY)) < 0)
 		return 0;
-	
+
 	memset(buffer, 0, 64);
-	
+
 	if(read(pidfile, buffer, 63))
 	{
 		if( (pid = atol(buffer)) > 0)
@@ -112,9 +139,8 @@ checkforrunning(const char * fname)
 			}
 		}
 	}
-	
+
 	close(pidfile);
-	
+
 	return 0;
 }
-

process.h

@@ -1,7 +1,7 @@
-/* MiniUPnP project
+/* Process handling
- * http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/
+ *
+ * Copyright © 2013, Benoît Knecht <benoit.knecht@fsfe.org>
  *
- * Copyright (c) 2006, Thomas Bernard
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -26,24 +26,41 @@
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  * POSSIBILITY OF SUCH DAMAGE.
  */
-#ifndef __DAEMONIZE_H__
+#ifndef __PROCESS_H__
-#define __DAEMONIZE_H__
+#define __PROCESS_H__
 
-#include "config.h"
+#include <unistd.h>
 
-/* daemonize()
+/**
- * "fork" to background, detach from terminal, etc... 
+ * Fork a new child (just like fork()) but keep track of how many childs are
- * returns: pid of the daemon, exits upon failure */
+ * already running, and refuse fo fork if there are too many.
-int
+ * @return -1 if it couldn't fork, 0 in the child process, the pid of the
-daemonize(void);
+ *         child process in the parent process.
+ */
+pid_t process_fork(void);
 
-/* checkforrunning()
+/**
- * check for another instance running
+ * Handler to be called upon receiving SIGCHLD. This signal is received by the
- * returns: 0 only instance
+ * parent process when a child terminates, and this handler updates the number
- *          -1 invalid filename
+ * of running childs accordingly.
- *          -2 another instance running  */
+ * @param signal The signal number.
-int
+ */
-checkforrunning(const char * fname);
+void process_handle_child_termination(int signal);
 
-#endif
+/**
+ * Daemonize the current process by forking itself and redirecting standard
+ * input, standard output and standard error to /dev/null.
+ * @return The pid of the process.
+ */ 
+int process_daemonize(void);
 
+/**
+ * Check if the process corresponding to the pid found in the pid file is
+ * running.
+ * @param fname The path to the pid file.
+ * @return 0 if no other instance is running, -1 if the file name is invalid,
+ *         -2 if another instance is running.
+ */
+int process_check_if_running(const char *fname);
+
+#endif // __PROCESS_H__

upnphttp.c

@@ -78,6 +78,7 @@
 #include "tivo_utils.h"
 #include "tivo_commands.h"
 #include "clients.h"
+#include "process.h"
 
 #include "sendfile.h"
 
@@ -1574,8 +1575,8 @@ SendResp_resizedimg(struct upnphttp * h, char * object)
 
 #if USE_FORK
 	pid_t newpid = 0;
-	newpid = fork();
+	newpid = process_fork();
-	if( newpid )
+	if( newpid > 0 )
 	{
 		CloseSocket_upnphttp(h);
 		goto resized_error;
@@ -1716,7 +1717,7 @@ SendResp_resizedimg(struct upnphttp * h, char * object)
 resized_error:
 	sqlite3_free_table(result);
 #if USE_FORK
-	if( !newpid )
+	if( newpid == 0 )
 		_exit(0);
 #endif
 }
@@ -1809,8 +1810,8 @@ SendResp_dlnafile(struct upnphttp *h, char *object)
 		sqlite3_free_table(result);
 	}
 #if USE_FORK
-	newpid = fork();
+	newpid = process_fork();
-	if( newpid )
+	if( newpid > 0 )
 	{
 		CloseSocket_upnphttp(h);
 		goto error;
@@ -1951,7 +1952,7 @@ SendResp_dlnafile(struct upnphttp *h, char *object)
 	CloseSocket_upnphttp(h);
 error:
 #if USE_FORK
-	if( !newpid )
+	if( newpid == 0 )
 		_exit(0);
 #endif
 	return;