iptables-restore: support acquiring the lock.

Currently, ip[6]tables-restore does not perform any locking, so it
is not safe to use concurrently with ip[6]tables.

This patch makes ip[6]tables-restore wait for the lock if -w
was specified. Arguments to -w and -W are supported in the same
was as they are in ip[6]tables.

The lock is not acquired on startup. Instead, it is acquired when
a new table handle is created (on encountering '*') and released
when the table is committed (COMMIT). This makes it possible to
keep long-running iptables-restore processes in the background
(for example, reading commands from a pipe opened by a system
management daemon) and simultaneously run iptables commands.

If -w is not specified, then the command proceeds without taking
the lock.

Tested as follows:

1. Run iptables-restore -w, and check that iptables commands work
   with or without -w.
2. Type "*filter" into the iptables-restore input. Verify that
   a) ip[6]tables commands without -w fail with "another app is
      currently holding the xtables lock...".
   b) ip[6]tables commands with "-w 2" fail after 2 seconds.
   c) ip[6]tables commands with "-w" hang until "COMMIT" is
      typed into the iptables-restore window.
3. With the lock held by an ip6tables-restore process:
     strace -e flock /tmp/iptables/sbin/iptables-restore -w 1 -W 100000
   shows 11 calls to flock and fails.
4. Run an iptables-restore with -w and one without -w, and check:
   a) Type "*filter" in the first and then the second, and the
      second exits with an error.
   b) Type "*filter" in the second and "*filter" "-S" "COMMIT"
      into the first. The rules are listed only when the first
      copy sees "COMMIT".

Signed-off-by: Narayan Kamath <narayan@google.com>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 43 insertions, 12 deletions

diff --git a/iptables/ip6tables-restore.c b/iptables/ip6tables-restore.c
index dc0acb05..8a47f09c 100644
--- a/iptables/ip6tables-restore.c
+++ b/iptables/ip6tables-restore.c
@@ -15,6 +15,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include "ip6tables.h"
+#include "xshared.h"
 #include "xtables.h"
 #include "libiptc/libip6tc.h"
 #include "ip6tables-multi.h"
@@ -25,17 +26,23 @@
 #define DEBUGP(x, args...)
 #endif
 
-static int counters = 0, verbose = 0, noflush = 0;
+static int counters = 0, verbose = 0, noflush = 0, wait = 0;
+
+static struct timeval wait_interval = {
+	.tv_sec	= 1,
+};
 
 /* Keeping track of external matches and targets.  */
 static const struct option options[] = {
-	{.name = "counters", .has_arg = false, .val = 'c'},
-	{.name = "verbose",  .has_arg = false, .val = 'v'},
-	{.name = "test",     .has_arg = false, .val = 't'},
-	{.name = "help",     .has_arg = false, .val = 'h'},
-	{.name = "noflush",  .has_arg = false, .val = 'n'},
-	{.name = "modprobe", .has_arg = true,  .val = 'M'},
-	{.name = "table",    .has_arg = true,  .val = 'T'},
+	{.name = "counters",      .has_arg = 0, .val = 'c'},
+	{.name = "verbose",       .has_arg = 0, .val = 'v'},
+	{.name = "test",          .has_arg = 0, .val = 't'},
+	{.name = "help",          .has_arg = 0, .val = 'h'},
+	{.name = "noflush",       .has_arg = 0, .val = 'n'},
+	{.name = "modprobe",      .has_arg = 1, .val = 'M'},
+	{.name = "table",         .has_arg = 1, .val = 'T'},
+	{.name = "wait",          .has_arg = 2, .val = 'w'},
+	{.name = "wait-interval", .has_arg = 2, .val = 'W'},
 	{NULL},
 };
 
@@ -43,14 +50,16 @@ static void print_usage(const char *name, const char *version) __attribute__((no
 
 static void print_usage(const char *name, const char *version)
 {
-	fprintf(stderr, "Usage: %s [-c] [-v] [-t] [-h] [-n] [-T table] [-M command]\n"
+	fprintf(stderr, "Usage: %s [-c] [-v] [-t] [-h] [-n] [-w secs] [-W usecs] [-T table] [-M command]\n"
 			"	   [ --counters ]\n"
 			"	   [ --verbose ]\n"
 			"	   [ --test ]\n"
 			"	   [ --help ]\n"
 			"	   [ --noflush ]\n"
+			"	   [ --wait=<seconds>\n"
+			"	   [ --wait-interval=<usecs>\n"
 			"	   [ --table=<TABLE> ]\n"
-			"          [ --modprobe=<command> ]\n", name);
+			"	   [ --modprobe=<command> ]\n", name);
 
 	exit(1);
 }
@@ -181,7 +190,7 @@ int ip6tables_restore_main(int argc, char *argv[])
 {
 	struct xtc_handle *handle = NULL;
 	char buffer[10240];
-	int c;
+	int c, lock;
 	char curtable[XT_TABLE_MAXNAMELEN + 1];
 	FILE *in;
 	int in_table = 0, testing = 0;
@@ -189,6 +198,7 @@ int ip6tables_restore_main(int argc, char *argv[])
 	const struct xtc_ops *ops = &ip6tc_ops;
 
 	line = 0;
+	lock = XT_LOCK_NOT_ACQUIRED;
 
 	ip6tables_globals.program_name = "ip6tables-restore";
 	c = xtables_init_all(&ip6tables_globals, NFPROTO_IPV6);
@@ -203,7 +213,7 @@ int ip6tables_restore_main(int argc, char *argv[])
 	init_extensions6();
 #endif
 
-	while ((c = getopt_long(argc, argv, "bcvthnM:T:", options, NULL)) != -1) {
+	while ((c = getopt_long(argc, argv, "bcvthnwWM:T:", options, NULL)) != -1) {
 		switch (c) {
 			case 'b':
 				fprintf(stderr, "-b/--binary option is not implemented\n");
@@ -224,6 +234,12 @@ int ip6tables_restore_main(int argc, char *argv[])
 			case 'n':
 				noflush = 1;
 				break;
+			case 'w':
+				wait = parse_wait_time(argc, argv);
+				break;
+			case 'W':
+				parse_wait_interval(argc, argv, &wait_interval);
+				break;
 			case 'M':
 				xtables_modprobe_program = optarg;
 				break;
@@ -268,8 +284,23 @@ int ip6tables_restore_main(int argc, char *argv[])
 				DEBUGP("Not calling commit, testing\n");
 				ret = 1;
 			}
+
+			/* Done with the current table, release the lock. */
+			if (lock >= 0) {
+				xtables_unlock(lock);
+				lock = XT_LOCK_NOT_ACQUIRED;
+			}
+
 			in_table = 0;
 		} else if ((buffer[0] == '*') && (!in_table)) {
+			/* Acquire a lock before we create a new table handle */
+			lock = xtables_lock(wait, &wait_interval);
+			if (lock == XT_LOCK_BUSY) {
+				fprintf(stderr, "Another app is currently holding the xtables lock. "
+					"Perhaps you want to use the -w option?\n");
+				exit(RESOURCE_PROBLEM);
+			}
+
 			/* New table */
 			char *table;