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[SKFILTER]: Add SKF_ADF_NLATTR instruction

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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>
This commit is contained in:
Patrick McHardy 2008-04-10 02:02:28 -07:00 committed by David S. Miller
parent 3cccd60784
commit 4738c1db15
2 changed files with 19 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/filter.h
View File

@ -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)

17
net/core/filter.c
View File

@ -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;
}