[PATCH v2] xtables: Add locking to prevent concurrent instances

[Phil Oester <kernel@linuxace.com>]

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There have been numerous complaints and bug reports over the years when admins
attempt to run more than one instance of iptables simultaneously.  Currently
open bug reports which are related:

325: Parallel execution of the iptables is impossible
758: Retry iptables command on transient failure
764: Doing -Z twice in parallel breaks counters
822: iptables shows negative or other bad packet/byte counts

As Patrick notes in 325:  "Since this has been a problem people keep running
into, I'd suggest to simply add some locking to iptables to catch the most
common case."

I started looking into alternatives to add locking, and of course the most
common/obvious solution is to use a pidfile.  But this has various downsides,
such as if the application is terminated abnormally and the pidfile isn't
cleaned up.  And this also requires a writable filesystem.  Using a UNIX domain
socket file (e.g. in /var/run) has similar issues.

Starting in 2.2, Linux added support for abstract sockets.  These sockets
require no filesystem, and automatically disappear once the application
terminates.  This is the locking solution I chose to implement in xtables-multi.
As an added bonus, since each network namespace has its own socket pool, an
iptables instance running in one namespace will not lock out an iptables
instance running in another namespace.  A filesystem approach would have
to recognize and handle multiple network namespaces.

Changes from v1:
- Addressed Patrick's comments - locking attempts will be made indefinitely
  until successful.

Phil

Signed-off-by: Phil Oester <kernel@linuxace.com>

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diff --git a/iptables/xtables-multi.c b/iptables/xtables-multi.c
index 8014d5f..5a57375 100644
--- a/iptables/xtables-multi.c
+++ b/iptables/xtables-multi.c
@@ -1,8 +1,12 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
 #include "xshared.h"
 
+#include "xtables.h"
 #include "xtables-multi.h"
 
 #ifdef ENABLE_IPV4
@@ -35,7 +39,31 @@ static const struct subcommand multi_subcommands[] = {
 	{NULL},
 };
 
+#define XTMSOCKET_NAME "xtables_multi"
+#define XTMSOCKET_LEN 14
+
 int main(int argc, char **argv)
 {
+	int i = 0, ret, xtm_socket;
+	struct sockaddr_un xtm_addr;
+
+	memset(&xtm_addr, 0, sizeof(xtm_addr));
+	xtm_addr.sun_family = AF_UNIX;
+	strcpy(xtm_addr.sun_path+1, XTMSOCKET_NAME);
+	xtm_socket = socket(AF_UNIX, SOCK_STREAM, 0);
+	/* If we can't even create a socket, just revert to prior (lockless) behavior */
+	if (xtm_socket < 0)
+		return subcmd_main(argc, argv, multi_subcommands);
+
+	while (1) {
+		ret = bind(xtm_socket, (struct sockaddr*)&xtm_addr,
+			   offsetof(struct sockaddr_un, sun_path)+XTMSOCKET_LEN);
+		if (ret == 0)
+			break;
+		if (++i % 5 == 0)
+			fprintf(stderr, "Waiting for lock, standby...\n");
+		sleep(1);
+	}
+
 	return subcmd_main(argc, argv, multi_subcommands);
 }