[PATCH] rtnetlink: Call nlmsg_parse() with correct header length

From: Michael Riesch
Date: Mon Apr 08 2013 - 11:53:16 EST

Signed-off-by: Michael Riesch <michael.riesch@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Benc <jbenc@xxxxxxxxxx>
Cc: "Theodore Ts'o" <tytso@xxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx

I encountered a netlink kernel warning when running avahi 0.6.31 on my system
with kernel v3.4.35 (it appears several times):

netlink: 12 bytes leftover after parsing attributes.

Searching the web showed that commit "115c9b81928360d769a76c632bae62d15206a94a
rtnetlink: Fix problem with buffer allocation" introduced this behaviour[1].

Now I - knowing nothing about netlink whatsoever - assume that the nlmsg_parse
function is called with the wrong header length. In user space the request
message consists out of the message header (struct nlmsghdr, 16 bytes) and an
ifinfomsg (struct ifinfomsg, 16 bytes). After that, request attributes could
follow. nlmsg_parse checks for this attributes after a given header length. In
rtnl_get_link() this header length is sizeof(struct ifinfomsg), but in
rtnl_calcit() as well as in rntl_dump_ifinfo() the header length is
sizeof(struct rtgenmsg), which is 1 byte.

With this patch I got rid of these warnings. However, I do not know whether
this is the correct solution, so I am looking forward to your comments.
Regards, Michael

[1] http://lists.infradead.org/pipermail/libnl/2012-April/000515.html

net/core/rtnetlink.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 900fc61..ebf6ace 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -1065,7 +1065,7 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
cb->seq = net->dev_base_seq;

- if (nlmsg_parse(cb->nlh, sizeof(struct rtgenmsg), tb, IFLA_MAX,
+ if (nlmsg_parse(cb->nlh, sizeof(struct ifinfomsg), tb, IFLA_MAX,
ifla_policy) >= 0) {

if (tb[IFLA_EXT_MASK])
@@ -1909,7 +1909,7 @@ static u16 rtnl_calcit(struct sk_buff *skb, struct nlmsghdr *nlh)
u32 ext_filter_mask = 0;
u16 min_ifinfo_dump_size = 0;

- if (nlmsg_parse(nlh, sizeof(struct rtgenmsg), tb, IFLA_MAX,
+ if (nlmsg_parse(nlh, sizeof(struct ifinfomsg), tb, IFLA_MAX,
ifla_policy) >= 0) {
if (tb[IFLA_EXT_MASK])
ext_filter_mask = nla_get_u32(tb[IFLA_EXT_MASK]);

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