bpf: Set skb redirect and from_ingress info in __bpf_tx_skb

There are some use-cases where it is desirable to use bpf_redirect()
in combination with ifb device, which currently is not supported, for
example, around filtering inbound traffic with BPF to then push it to
ifb which holds the qdisc for shaping in contrast to doing that on the
egress device.

Toke mentions the following case related to OpenWrt:

   Because there's not always a single egress on the other side. These are
   mainly home routers, which tend to have one or more WiFi devices bridged
   to one or more ethernet ports on the LAN side, and a single upstream WAN
   port. And the objective is to control the total amount of traffic going
   over the WAN link (in both directions), to deal with bufferbloat in the
   ISP network (which is sadly still all too prevalent).

   In this setup, the traffic can be split arbitrarily between the links
   on the LAN side, and the only "single bottleneck" is the WAN link. So we
   install both egress and ingress shapers on this, configured to something
   like 95-98% of the true link bandwidth, thus moving the queues into the
   qdisc layer in the router. It's usually necessary to set the ingress
   bandwidth shaper a bit lower than the egress due to being "downstream"
   of the bottleneck link, but it does work surprisingly well.

   We usually use something like a matchall filter to put all ingress
   traffic on the ifb, so doing the redirect from BPF has not been an
   immediate requirement thus far. However, it does seem a bit odd that
   this is not possible, and we do have a BPF-based filter that layers on
   top of this kind of setup, which currently uses u32 as the ingress
   filter and so it could presumably be improved to use BPF instead if
   that was available.

Reported-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reported-by: Yafang Shao <laoar.shao@gmail.com>
Reported-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://git.openwrt.org/?p=project/qosify.git;a=blob;f=README
Link: https://lore.kernel.org/bpf/875y9yzbuy.fsf@toke.dk
Link: https://lore.kernel.org/r/8cebc8b2b6e967e10cbafe2ffd6795050e74accd.1681739137.git.daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/skbuff.h

@@ -5041,6 +5041,15 @@ static inline void skb_reset_redirect(struct sk_buff *skb)
 	skb->redirected = 0;
 }
 
+static inline void skb_set_redirected_noclear(struct sk_buff *skb,
+					      bool from_ingress)
+{
+	skb->redirected = 1;
+#ifdef CONFIG_NET_REDIRECT
+	skb->from_ingress = from_ingress;
+#endif
+}
+
 static inline bool skb_csum_is_sctp(struct sk_buff *skb)
 {
 	return skb->csum_not_inet;

net/core/filter.c

@@ -2122,6 +2122,7 @@ static inline int __bpf_tx_skb(struct net_device *dev, struct sk_buff *skb)
 	}
 
 	skb->dev = dev;
+	skb_set_redirected_noclear(skb, skb_at_tc_ingress(skb));
 	skb_clear_tstamp(skb);
 
 	dev_xmit_recursion_inc();