[PATCH][RFC]: ingress socket filter by mark

[jamal <hadi@cyberus.ca>]

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Maciej forced me to dig into this ;->

at the socket level if a packet arrives with a different mark than
what we bind to, drop it. I have tested this patch and it drops a packet
with mismatching mark.

There are several approaches - and i think the patch suggestion i have
made here maybe too strict. I assume that if someone binds to a mark,
they want to not only send packets with that mark but receive
only if that mark is set. 
A looser check would be something along the line accept as well if mark
is not set i.e
if (sk->sk_mark && skb->mark && sk->sk_mark != skb->mark)

Alternatively i could add one bit in the socket flags and have it so
that check is made only if app has been explicit:
if (sock_flag(sk, SOCK_CHK_SOMARK) && sk->sk_mark != skb->mark) drop

Another approach  is to set sock filter from app. I dont like this
approach because it will be the least usable from app level and would be
the least simple from kernel level.

cheers,
jamal

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diff --git a/net/core/filter.c b/net/core/filter.c
index d1d779c..6fcf577 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -85,6 +85,9 @@ int sk_filter(struct sock *sk, struct sk_buff *skb)
 	if (err)
 		return err;
 
+	if (sk->sk_mark && sk->sk_mark != skb->mark)
+		return -EPERM;
+
 	rcu_read_lock_bh();
 	filter = rcu_dereference(sk->sk_filter);
 	if (filter) {