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, 12 insertions, 6 deletions

diff --git a/iptables/xshared.c b/iptables/xshared.c
index 06147d18..3fbe3b1a 100644
--- a/iptables/xshared.c
+++ b/iptables/xshared.c
@@ -246,7 +246,7 @@ void xs_init_match(struct xtables_match *match)
 		match->init(match->m);
 }
 
-bool xtables_lock(int wait, struct timeval *wait_interval)
+int xtables_lock(int wait, struct timeval *wait_interval)
 {
 	struct timeval time_left, wait_time;
 	int fd, i = 0;
@@ -256,22 +256,22 @@ bool xtables_lock(int wait, struct timeval *wait_interval)
 
 	fd = open(XT_LOCK_NAME, O_CREAT, 0600);
 	if (fd < 0)
-		return true;
+		return XT_LOCK_UNSUPPORTED;
 
 	if (wait == -1) {
 		if (flock(fd, LOCK_EX) == 0)
-			return true;
+			return fd;
 
 		fprintf(stderr, "Can't lock %s: %s\n", XT_LOCK_NAME,
 			strerror(errno));
-		return false;
+		return XT_LOCK_BUSY;
 	}
 
 	while (1) {
 		if (flock(fd, LOCK_EX | LOCK_NB) == 0)
-			return true;
+			return fd;
 		else if (timercmp(&time_left, wait_interval, <))
-			return false;
+			return XT_LOCK_BUSY;
 
 		if (++i % 10 == 0) {
 			fprintf(stderr, "Another app is currently holding the xtables lock; "
@@ -285,6 +285,12 @@ bool xtables_lock(int wait, struct timeval *wait_interval)
 	}
 }
 
+void xtables_unlock(int lock)
+{
+	if (lock >= 0)
+		close(lock);
+}
+
 int parse_wait_time(int argc, char *argv[])
 {
 	int wait = -1;