quota2: add a no-change mode

This commit adds an option to xt_quota2 called "no-change". The
effect of this option, when used, is that it will skip incrementing
or decrementing the quota counter, effectively providing a quota test
only.

The reason for implementing this is so that I could have a rule check
if quota is available for a rule in the PREROUTING tables, without
actually decrementing the amount of available quota. I only wanted to
decrement the amount of available quota in the FORWARD rule.
Otherwise, the first packet of every connection would be counted
twice.

extensions/libxt_quota2.c

@@ -21,10 +21,12 @@ enum {
 	FL_NAME      = 1 << 1,
 	FL_GROW      = 1 << 2,
 	FL_PACKET    = 1 << 3,
+	FL_NO_CHANGE = 1 << 4,
 };
 
 static const struct option quota_mt2_opts[] = {
 	{.name = "grow",      .has_arg = false, .val = 'g'},
+	{.name = "no-change", .has_arg = false, .val = 'c'},
 	{.name = "name",      .has_arg = true,  .val = 'n'},
 	{.name = "quota",     .has_arg = true,  .val = 'q'},
 	{.name = "packets",   .has_arg = false, .val = 'p'},
@@ -36,6 +38,7 @@ static void quota_mt2_help(void)
 	printf(
 	"quota match options:\n"
 	"    --grow           provide an increasing counter\n"
+	"    --no-change      never change counter/quota value for matching packets\n"
 	"    --name name      name for the file in sysfs\n"
 	"[!] --quota quota    initial quota (bytes or packets)\n"
 	"    --packets        count packets instead of bytes\n"
@@ -56,6 +59,12 @@ quota_mt2_parse(int c, char **argv, int invert, unsigned int *flags,
 		info->flags |= XT_QUOTA_GROW;
 		*flags |= FL_GROW;
 		return true;
+	case 'c': /* no-change */
+		xtables_param_act(XTF_ONLY_ONCE, "quota", "--no-change", *flags & FL_NO_CHANGE);
+		xtables_param_act(XTF_NO_INVERT, "quota", "--no-change", invert);
+		info->flags |= XT_QUOTA_NO_CHANGE;
+		*flags |= FL_NO_CHANGE;
+		return true;
 	case 'n':
 		/* zero termination done on behalf of the kernel module */
 		xtables_param_act(XTF_ONLY_ONCE, "quota", "--name", *flags & FL_NAME);
@@ -92,6 +101,8 @@ quota_mt2_save(const void *ip, const struct xt_entry_match *match)
 		printf("! ");
 	if (q->flags & XT_QUOTA_GROW)
 		printf("--grow ");
+	if (q->flags & XT_QUOTA_NO_CHANGE)
+		printf("--no-change ");
 	if (q->flags & XT_QUOTA_PACKET)
 		printf("--packets ");
 	if (*q->name != '\0')
@@ -117,6 +128,8 @@ static void quota_mt2_print(const void *ip, const struct xt_entry_match *match,
 		printf("packets ");
 	else
 		printf("bytes ");
+	if (q->flags & XT_QUOTA_NO_CHANGE)
+		printf("(no-change mode) ");
 }
 
 static struct xtables_match quota_mt2_reg = {

extensions/libxt_quota2.man

@@ -10,6 +10,12 @@ the match will return false, just like the original "quota" match. In growing
 \fB\-\-grow\fP
 Count upwards instead of downwards.
 .TP
+\fB\-\-no\-change\fP
+Makes it so the counter or quota amount is never changed by packets matching
+this rule. This is only really useful in "quota" mode, as it will allow you to
+use complex prerouting rules in association with the quota system, without
+counting a packet twice.
+.TP
 \fB\-\-name\fP \fIname\fP
 Assign the counter a specific name. This option must be present, as an empty
 name is not allowed. Names starting with a dot or names containing a slash are

extensions/xt_quota2.Kconfig

@@ -5,4 +5,5 @@ config NETFILTER_XT_MATCH_QUOTA2
 	This option adds the "quota2" match which is an advanced form of
 	xt_quota that also allows counting upwards, and where the counter can
 	be set through procfs. This allows for simple interfacing of
-	accounting information.
+	accounting information. It also allows for a test mode without changing
+	the quota value.

extensions/xt_quota2.c

@@ -199,11 +199,18 @@ quota_mt2(const struct sk_buff *skb, const struct xt_match_param *par)
 
 	spin_lock_bh(&e->lock);
 	if (q->flags & XT_QUOTA_GROW) {
+		/*
+		 * While no_change is pointless in "grow" mode, we will
+		 * implement it here simply to have a consistent behavior.
+		 */
+		if (!(q->flags & XT_QUOTA_NO_CHANGE)) {
 			e->quota += (q->flags & XT_QUOTA_PACKET) ? 1 : skb->len;
 			q->quota = e->quota;
+		}
 		ret = true;
 	} else {
 		if (e->quota >= skb->len) {
+			if (!(q->flags & XT_QUOTA_NO_CHANGE))
 				e->quota -= (q->flags & XT_QUOTA_PACKET) ? 1 : skb->len;
 			ret = !ret;
 		} else {

extensions/xt_quota2.h

@@ -5,7 +5,8 @@ enum xt_quota_flags {
 	XT_QUOTA_INVERT    = 1 << 0,
 	XT_QUOTA_GROW      = 1 << 1,
 	XT_QUOTA_PACKET    = 1 << 2,
-	XT_QUOTA_MASK   = 0x7,
+	XT_QUOTA_NO_CHANGE = 1 << 3,
+	XT_QUOTA_MASK      = 0x0F,
 };
 
 struct xt_quota_counter;