filter: add SKF_AD_NLATTR_NEST to look for nested attributes

SKF_AD_NLATTR allows us to find the first matching attribute in a
stream of netlink attributes from one offset to the end of the
netlink message. This is not suitable to look for a specific
matching inside a set of nested attributes.

For example, in ctnetlink messages, if we look for the CTA_V6_SRC
attribute in a message that talks about an IPv4 connection,
SKF_AD_NLATTR returns the offset of CTA_STATUS which has the same
value of CTA_V6_SRC but outside the nest. To differenciate
CTA_STATUS and CTA_V6_SRC, we would have to make assumptions on the
size of the attribute and the usual offset, resulting in horrible
BSF code.

This patch adds SKF_AD_NLATTR_NEST, which is a variant of
SKF_AD_NLATTR, that looks for an attribute inside the limits of
a nested attributes, but not further.

This patch validates that we have enough room to look for the
nested attributes - based on a suggestion from Patrick McHardy.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/filter.h b/include/linux/filter.h
index b6ea9aa9e85..1354aaf6abb 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -122,7 +122,8 @@ struct sock_fprog	/* Required for SO_ATTACH_FILTER. */
 #define SKF_AD_PKTTYPE 	4
 #define SKF_AD_IFINDEX 	8
 #define SKF_AD_NLATTR	12
-#define SKF_AD_MAX 	16
+#define SKF_AD_NLATTR_NEST	16
+#define SKF_AD_MAX	20
 #define SKF_NET_OFF   (-0x100000)
 #define SKF_LL_OFF    (-0x200000)