From 4738c1db1593687713869fa69e733eebc7b0d6d8 Mon Sep 17 00:00:00 2001
From: Patrick McHardy <kaber@trash.net>
Date: Thu, 10 Apr 2008 02:02:28 -0700
Subject: [SKFILTER]: Add SKF_ADF_NLATTR instruction

SKF_ADF_NLATTR searches for a netlink attribute, which avoids manually
parsing and walking attributes. It takes the offset at which to start
searching in the 'A' register and the attribute type in the 'X' register
and returns the offset in the 'A' register. When the attribute is not
found it returns zero.

A top-level attribute can be located using a filter like this
(example for nfnetlink, using struct nfgenmsg):

	...
	{
		/* A = offset of first attribute */
		.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
	},
	{
		/* Exit if not found */
		.code   = BPF_JMP | BPF_JEQ | BPF_K,
		.k	= 0,
		.jt	= <error>
	},
	...

A nested attribute below the CTA_PROTOINFO attribute would then
be parsed like this:

	...
	{
		/* 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
	},
	...

The data of an attribute can be loaded into 'A' like this:

	...
	{
		/* X = A (attribute offset) */
		.code	= BPF_MISC | BPF_TAX,
	},
	{
		/* A = skb->data[X + k] */
		.code 	= BPF_LD | BPF_B | BPF_IND,
		.k	= sizeof(struct nlattr),
	},
	...

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 include/linux/filter.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

(limited to 'include')

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 673e5677ebcc..b6ea9aa9e853 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -121,7 +121,8 @@ struct sock_fprog	/* Required for SO_ATTACH_FILTER. */
 #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)
 
-- 
cgit v1.2.3-59-g8ed1b