ip[6]tables: Add locking to prevent concurrent instances

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 ip[6]tables.
As an added bonus, since each network namespace has its own socket pool, an
ip[6]tables instance running in one namespace will not lock out an ip[6]tables
instance running in another namespace.  A filesystem approach would have
to recognize and handle multiple network namespaces.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 16 insertions, 1 deletions

diff --git a/iptables/ip6tables.c b/iptables/ip6tables.c
index c8d34e2d..eededee1 100644
--- a/iptables/ip6tables.c
+++ b/iptables/ip6tables.c
@@ -102,6 +102,7 @@ static struct option original_opts[] = {
 	{.name = "numeric",       .has_arg = 0, .val = 'n'},
 	{.name = "out-interface", .has_arg = 1, .val = 'o'},
 	{.name = "verbose",       .has_arg = 0, .val = 'v'},
+	{.name = "wait",          .has_arg = 0, .val = 'w'},
 	{.name = "exact",         .has_arg = 0, .val = 'x'},
 	{.name = "version",       .has_arg = 0, .val = 'V'},
 	{.name = "help",          .has_arg = 2, .val = 'h'},
@@ -257,6 +258,7 @@ exit_printhelp(const struct xtables_rule_match *matches)
 "				network interface name ([+] for wildcard)\n"
 "  --table	-t table	table to manipulate (default: `filter')\n"
 "  --verbose	-v		verbose mode\n"
+"  --wait	-w		wait for the xtables lock\n"
 "  --line-numbers		print line numbers when listing\n"
 "  --exact	-x		expand numbers (display exact values)\n"
 /*"[!] --fragment	-f		match second or further fragments only\n"*/
@@ -1293,6 +1295,7 @@ int do_command6(int argc, char *argv[], char **table, struct xtc_handle **handle
 	struct in6_addr *smasks = NULL, *dmasks = NULL;
 
 	int verbose = 0;
+	bool wait = false;
 	const char *chain = NULL;
 	const char *shostnetworkmask = NULL, *dhostnetworkmask = NULL;
 	const char *policy = NULL, *newname = NULL;
@@ -1328,7 +1331,7 @@ int do_command6(int argc, char *argv[], char **table, struct xtc_handle **handle
 
 	opts = xt_params->orig_opts;
 	while ((cs.c = getopt_long(argc, argv,
-	   "-:A:C:D:R:I:L::S::M:F::Z::N:X::E:P:Vh::o:p:s:d:j:i:bvnt:m:xc:g:46",
+	   "-:A:C:D:R:I:L::S::M:F::Z::N:X::E:P:Vh::o:p:s:d:j:i:bvwnt:m:xc:g:46",
 					   opts, NULL)) != -1) {
 		switch (cs.c) {
 			/*
@@ -1573,6 +1576,10 @@ int do_command6(int argc, char *argv[], char **table, struct xtc_handle **handle
 			verbose++;
 			break;
 
+		case 'w':
+			wait = true;
+			break;
+
 		case 'm':
 			command_match(&cs);
 			break;
@@ -1724,6 +1731,14 @@ int do_command6(int argc, char *argv[], char **table, struct xtc_handle **handle
 			   "chain name `%s' too long (must be under %u chars)",
 			   chain, XT_EXTENSION_MAXNAMELEN);
 
+	/* Attempt to acquire the xtables lock */
+	if (!xtables_lock(wait)) {
+		fprintf(stderr, "Another app is currently holding the xtables lock. "
+			"Perhaps you want to use the -w option?\n");
+		xtables_free_opts(1);
+		exit(RESOURCE_PROBLEM);
+	}
+
 	/* only allocate handle if we weren't called with a handle */
 	if (!*handle)
 		*handle = ip6tc_init(*table);