netdev - [PATCH bpf-next v3] bpf_fib_lookup: always return target ifindex

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Date:   Fri,  9 Oct 2020 20:42:34 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     daniel@...earbox.net, ast@...com
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        David Ahern <dsahern@...il.com>
Subject: [PATCH bpf-next v3] bpf_fib_lookup: always return target ifindex

The bpf_fib_lookup() helper performs a neighbour lookup for the destination
IP and returns BPF_FIB_LKUP_NO_NEIGH if this fails, with the expectation
that the BPF program will pass the packet up the stack in this case.
However, with the addition of bpf_redirect_neigh() that can be used instead
to perform the neighbour lookup, at the cost of a bit of duplicated work.

For that we still need the target ifindex, and since bpf_fib_lookup()
already has that at the time it performs the neighbour lookup, there is
really no reason why it can't just return it in any case. So let's just
always return the ifindex if the FIB lookup itself succeeds.

Cc: David Ahern <dsahern@...il.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
v3:
- Get rid of the flag again, to be revisited later (David)

v2:
- Add flag (Daniel)
- Remove misleading code example from commit message (David)

net/core/filter.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 5da44b11e1ec..a0c30f3ea7ca 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5196,7 +5196,6 @@ static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params,
 	memcpy(params->smac, dev->dev_addr, ETH_ALEN);
 	params->h_vlan_TCI = 0;
 	params->h_vlan_proto = 0;
-	params->ifindex = dev->ifindex;
 
 	return 0;
 }
@@ -5293,6 +5292,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	dev = nhc->nhc_dev;
 
 	params->rt_metric = res.fi->fib_priority;
+	params->ifindex = dev->ifindex;
 
 	/* xdp and cls_bpf programs are run in RCU-bh so
 	 * rcu_read_lock_bh is not needed here
@@ -5418,6 +5418,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 
 	dev = res.nh->fib_nh_dev;
 	params->rt_metric = res.f6i->fib6_metric;
+	params->ifindex = dev->ifindex;
 
 	/* xdp and cls_bpf programs are run in RCU-bh so rcu_read_lock_bh is
 	 * not needed here.
-- 
2.28.0