[PATCH v4] xtables: Add locking to prevent concurrent instances

[Phil Oester <kernel@linuxace.com>]

[-- Attachment #1: Type: text/plain, Size: 1779 bytes --]

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.

Phil

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

---

v2: Addressed Patrick's comments - locking attempts will be made indefinitely until successful
v3: Update warning message to more closely resemble locking output from yum
v4: Move lock to ip[6]tables only.  Add -w option to wait for lock - default is to only try once then exit


[-- Attachment #2: patch-xtables-lock --]
[-- Type: text/plain, Size: 8812 bytes --]

diff --git a/iptables/ip6tables.8.in b/iptables/ip6tables.8.in
index 8634854..f9d3db1 100644
--- a/iptables/ip6tables.8.in
+++ b/iptables/ip6tables.8.in
@@ -363,6 +363,13 @@ For appending, insertion, deletion and replacement, this causes
 detailed information on the rule or rules to be printed. \fB\-v\fP may be
 specified multiple times to possibly emit more detailed debug statements.
 .TP
+\fB\-w\fP, \fB\-\-wait\fP
+Wait for the xtables lock.
+To prevent multiple instances of the program from running concurrently,
+an attempt will be made to obtain an exclusive lock at launch.  By default,
+the program will exit if the lock cannot be obtained.  This option will
+make the program wait until the exclusive lock can be obtained.
+.TP
 \fB\-n\fP, \fB\-\-numeric\fP
 Numeric output.
 IP addresses and port numbers will be printed in numeric format.
diff --git a/iptables/ip6tables.c b/iptables/ip6tables.c
index c8d34e2..3877d2f 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(1);
+	}
+
 	/* only allocate handle if we weren't called with a handle */
 	if (!*handle)
 		*handle = ip6tc_init(*table);
diff --git a/iptables/iptables.8.in b/iptables/iptables.8.in
index 9643705..cec204f 100644
--- a/iptables/iptables.8.in
+++ b/iptables/iptables.8.in
@@ -351,6 +351,13 @@ For appending, insertion, deletion and replacement, this causes
 detailed information on the rule or rules to be printed. \fB\-v\fP may be
 specified multiple times to possibly emit more detailed debug statements.
 .TP
+\fB\-w\fP, \fB\-\-wait\fP
+Wait for the xtables lock.
+To prevent multiple instances of the program from running concurrently,
+an attempt will be made to obtain an exclusive lock at launch.  By default,
+the program will exit if the lock cannot be obtained.  This option will
+make the program wait until the exclusive lock can be obtained.
+.TP
 \fB\-n\fP, \fB\-\-numeric\fP
 Numeric output.
 IP addresses and port numbers will be printed in numeric format.
diff --git a/iptables/iptables.c b/iptables/iptables.c
index 79fa37b..787c145 100644
--- a/iptables/iptables.c
+++ b/iptables/iptables.c
@@ -99,6 +99,7 @@ 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 = "fragments",     .has_arg = 0, .val = 'f'},
 	{.name = "version",       .has_arg = 0, .val = 'V'},
@@ -251,6 +252,7 @@ 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"
@@ -1289,6 +1291,7 @@ int do_command4(int argc, char *argv[], char **table, struct xtc_handle **handle
 	struct in_addr *daddrs = 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;
@@ -1324,7 +1327,7 @@ int do_command4(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:fbvnt: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:fbvwnt:m:xc:g:46",
 					   opts, NULL)) != -1) {
 		switch (cs.c) {
 			/*
@@ -1567,6 +1570,10 @@ int do_command4(int argc, char *argv[], char **table, struct xtc_handle **handle
 			verbose++;
 			break;
 
+		case 'w':
+			wait = true;
+			break;
+
 		case 'm':
 			command_match(&cs);
 			break;
@@ -1721,6 +1728,14 @@ int do_command4(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(1);
+	}
+
 	/* only allocate handle if we weren't called with a handle */
 	if (!*handle)
 		*handle = iptc_init(*table);
diff --git a/iptables/xshared.c b/iptables/xshared.c
index e61c28c..48f93fc 100644
--- a/iptables/xshared.c
+++ b/iptables/xshared.c
@@ -6,9 +6,15 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
 #include <xtables.h>
 #include "xshared.h"
 
+#define XT_SOCKET_NAME "xtables"
+#define XT_SOCKET_LEN 8
+
 /*
  * Print out any special helps. A user might like to be able to add a --help
  * to the commandline, and see expected results. So we call help for all
@@ -236,3 +242,31 @@ void xs_init_match(struct xtables_match *match)
 	if (match->init != NULL)
 		match->init(match->m);
 }
+
+bool xtables_lock(bool wait)
+{
+	int i = 0, ret, xt_socket;
+	struct sockaddr_un xt_addr;
+
+	memset(&xt_addr, 0, sizeof(xt_addr));
+	xt_addr.sun_family = AF_UNIX;
+	strcpy(xt_addr.sun_path+1, XT_SOCKET_NAME);
+	xt_socket = socket(AF_UNIX, SOCK_STREAM, 0);
+	/* If we can't even create a socket, fall back to prior (lockless) behavior */
+	if (xt_socket < 0)
+		return true;
+
+	while (1) {
+		ret = bind(xt_socket, (struct sockaddr*)&xt_addr,
+			   offsetof(struct sockaddr_un, sun_path)+XT_SOCKET_LEN);
+		if (ret == 0)
+			return true;
+		else if (wait == false)
+			return false;
+		if (++i % 2 == 0)
+			fprintf(stderr, "Another app is currently holding the xtables lock; "
+				"waiting for it to exit...\n");
+		sleep(1);
+	}
+
+}
diff --git a/iptables/xshared.h b/iptables/xshared.h
index b804aaf..1e2b9b8 100644
--- a/iptables/xshared.h
+++ b/iptables/xshared.h
@@ -2,6 +2,7 @@
 #define IPTABLES_XSHARED_H 1
 
 #include <limits.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <netinet/in.h>
 #include <net/if.h>
@@ -83,6 +84,7 @@ extern struct xtables_match *load_proto(struct iptables_command_state *);
 extern int subcmd_main(int, char **, const struct subcommand *);
 extern void xs_init_target(struct xtables_target *);
 extern void xs_init_match(struct xtables_match *);
+extern bool xtables_lock(bool wait);
 
 extern const struct xtables_afinfo *afinfo;