Subject: Re: [PATCH net-2.6.26] netlink: make socket filters work on netlink

From: Patrick McHardy <[email-masked]>
Date: Wed, 26 Mar 2008 21:19:56 +0100

Stephen Hemminger wrote:
> Make socket filters work for netlink unicast and notifications.
> This is useful for applications like Zebra that get overrun with
> messages that are then ignored.
> 
> Note: netlink messages are in host byte order, but packet filter
> state machine operations are done as network byte order.

Do you have an example for a filter for this? I have a similar
patch that adds a new filter instruction for parsing netlink
attributes, which seemed necessary for getting at nested
attributes without too much trouble.

Attached for reference together with a libnl testing
patch for ctnetlink.

diff --git a/include/linux/filter.h b/include/linux/filter.h
index ddfa037..0e39016 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -136,7 +136,8 @@ static inline unsigned int sk_filter_len(struct sk_filter *fp)
 #define SKF_AD_PROTOCOL 0
 #define SKF_AD_PKTTYPE 	4
 #define SKF_AD_IFINDEX 	8
-#define SKF_AD_MAX 	12
+#define SKF_AD_NLATTR	12
+#define SKF_AD_MAX 	16
 #define SKF_NET_OFF   (-0x100000)
 #define SKF_LL_OFF    (-0x200000)
 
diff --git a/net/core/filter.c b/net/core/filter.c
index e0a0694..20ed056 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -27,6 +27,7 @@
 #include <linux/if_packet.h>
 #include <net/ip.h>
 #include <net/protocol.h>
+#include <net/netlink.h>
 #include <linux/skbuff.h>
 #include <net/sock.h>
 #include <linux/errno.h>
@@ -268,6 +269,22 @@ load_b:
 		case SKF_AD_IFINDEX:
 			A = skb->dev->ifindex;
 			continue;
+		case SKF_AD_NLATTR: {
+			struct nlattr *nla;
+
+			if (skb_is_nonlinear(skb))
+				return 0;
+			if (A > skb->len - sizeof(struct nlattr))
+				return 0;
+
+			nla = nla_find((struct nlattr *)&skb->data[A],
+				       skb->len - A, X);
+			if (nla)
+				A = (void *)nla - (void *)skb->data;
+			else
+				A = 0;
+			continue;
+		}
 		default:
 			return 0;
 		}
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 524e826..6f68f2b 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -919,6 +919,17 @@ static inline int netlink_broadcast_deliver(struct sock *sk,
 					    struct sk_buff *skb)
 {
 	struct netlink_sock *nlk = nlk_sk(sk);
+	struct sk_filter *filter;
+	unsigned int len = skb->len;
+
+	rcu_read_lock_bh();
+	filter = rcu_dereference(sk->sk_filter);
+	if (filter)
+		len = sk_run_filter(skb, filter->insns, filter->len);
+	rcu_read_unlock_bh();
+
+	if (len == 0)
+		return 0;
 
 	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf &&
 	    !test_bit(0, &nlk->state)) {
diff --git a/src/nf-monitor.c b/src/nf-monitor.c
index 2bc58c9..fad5100 100644
--- a/src/nf-monitor.c
+++ b/src/nf-monitor.c
@@ -13,6 +13,9 @@
 
 #include "utils.h"
 #include <netlink/netfilter/nfnl.h>
+#include <linux/netfilter/nfnetlink_conntrack.h>
+#include <linux/netfilter/nf_conntrack_tcp.h>
+#include <linux/filter.h>
 
 static void obj_input(struct nl_object *obj, void *arg)
 {
@@ -34,6 +37,113 @@ static int event_input(struct nl_msg *msg, void *arg)
 	return NL_STOP;
 }
 
+#define SKF_AD_NLATTR	12
+
+static int sk_set_filter(int fd)
+{
+	struct sock_filter filter[] = {
+		{
+			/* A = sizeof(struct nlmsghdr) + sizeof(struct nfgenmsg) */
+			.code	= BPF_LD|BPF_IMM,
+			.k	= sizeof(struct nlmsghdr) + sizeof(struct nfgenmsg),
+		},
+		{
+			/* X = CTA_PROTOINFO */
+			.code	= BPF_LDX|BPF_IMM,
+			.k	= CTA_PROTOINFO,
+		},
+		{
+			/* A = netlink attribute offset */
+			.code	= BPF_LD|BPF_B|BPF_ABS,
+			.k	= SKF_AD_OFF + SKF_AD_NLATTR,
+		},
+		{
+			/* Reject if not found (A == 0) */
+			.code	= BPF_JMP|BPF_JEQ|BPF_K,
+			.k	= 0,
+			.jt	= 20 - 3 - 1,
+		},
+
+		{
+			/* A += sizeof(struct nlattr) */
+			.code	= BPF_ALU|BPF_ADD|BPF_K,
+			.k	= sizeof(struct nlattr),
+		},
+		{
+			/* X = CTA_PROTOINFO_TCP */
+			.code	= BPF_LDX|BPF_IMM,
+			.k	= CTA_PROTOINFO_TCP,
+		},
+		{
+			/* A = netlink attribute offset */
+			.code	= BPF_LD|BPF_B|BPF_ABS,
+			.k	= SKF_AD_OFF + SKF_AD_NLATTR,
+		},
+		{
+			/* Reject if not found (A == 0) */
+			.code	= BPF_JMP|BPF_JEQ|BPF_K,
+			.k	= 0,
+			.jt	= 20 - 7 - 1,
+		},
+
+		{
+			/* A += sizeof(struct nlattr) */
+			.code	= BPF_ALU|BPF_ADD|BPF_K,
+			.k	= sizeof(struct nlattr),
+		},
+		{
+			/* X = CTA_PROTOINFO_TCP_STATE */
+			.code	= BPF_LDX|BPF_IMM,
+			.k	= CTA_PROTOINFO_TCP_STATE,
+		},
+		{
+			/* A = netlink attribute offset */
+			.code	= BPF_LD|BPF_B|BPF_ABS,
+			.k	= SKF_AD_OFF + SKF_AD_NLATTR,
+		},
+		{
+			/* Reject if not found (A == 0) */
+			.code	= BPF_JMP|BPF_JEQ|BPF_K,
+			.k	= 0,
+			.jt	= 20 - 11 - 1,
+		},
+
+		{
+			/* X = A */
+			.code	= BPF_MISC|BPF_TAX,
+		},
+		{
+			/* A = skb->data[X + k] */
+			.code	= BPF_LD|BPF_B|BPF_IND,
+			.k	= sizeof(struct nlattr),
+		},
+		{
+			/* Reject if A != TCA_CONNTRACK_ESTABLISHED */
+			.code	= BPF_JMP|BPF_JEQ|BPF_K,
+			.k	= TCP_CONNTRACK_ESTABLISHED,
+			.jf	= 20 - 14 - 1,
+		},
+
+		{
+			/* Accept */
+			.code	= BPF_RET|BPF_K,
+			.k	= 1,
+		},
+		[20]	= {
+			/* Reject */
+			.code	= BPF_RET|BPF_K,
+			.k	= 0,
+		},
+	};
+	struct sock_fprog fprog = {
+		.len		= sizeof(filter) / sizeof(filter[0]),
+		.filter		= filter,
+	};
+
+	return setsockopt(fd, SOL_SOCKET, SO_ATTACH_FILTER,
+			  &fprog, sizeof(fprog));
+}
+
 int main(int argc, char *argv[])
 {
 	struct nl_handle *nlh;
@@ -92,6 +202,11 @@ int main(int argc, char *argv[])
 			fprintf(stderr, "Warning: Unknown group: %s\n", argv[idx]);
 	}
 
+	if (sk_set_filter(nl_socket_get_fd(nlh)) < 0) {
+		perror("setsockopt(SO_ATTACH_FILTER)");
+		goto errout;
+	}
+
 	while (1) {
 		fd_set rfds;
 		int fd, retval;