Commit 4738c1db authored Apr 10, 2008 by Patrick McHardy Committed by David S. Miller Apr 10, 2008

[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>

parent 3cccd607

include/linux/filter.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -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)

net/core/filter.c

+17 −0

Original line number	Diff line number	Diff line
		@@ -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>
		@@ -303,6 +304,22 @@ 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;
		}

Admin message