[PATCH bpf-next V3 2/6] bpf: bpf_fib_lookup return MTU value as output when looked up

Jesper Dangaard Brouer
2020-10-08T14:09:23Z

The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED. The BPF-prog
don't know the MTU value that caused this rejection.

If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
need to know this MTU value for the ICMP packet.

Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
value as output via a union with 'tot_len' as this is the value used for
the MTU lookup.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
include/uapi/linux/bpf.h | 11 +++++++++--
net/core/filter.c | 17 ++++++++++++-----
tools/include/uapi/linux/bpf.h | 11 +++++++++--
3 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index d83561e8cd2c..4a46a1de6d16 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2216,6 +2216,9 @@ union bpf_attr {
* * > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
* packet is not forwarded or needs assist from full stack
*
+ * If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
+ * was exceeded and result params->mtu contains the MTU.
+ *
* long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
* Description
* Add an entry to, or update a sockhash *map* referencing sockets.
@@ -4844,9 +4847,13 @@ struct bpf_fib_lookup {
__be16 sport;
__be16 dport;

- /* total length of packet from network header - used for MTU check */
- __u16 tot_len;
+ union { /* used for MTU check */
+ /* input to lookup */
+ __u16 tot_len; /* total length of packet from network hdr */

+ /* output: MTU value (if requested check_mtu) */
+ __u16 mtu;
+ };
/* input: L3 device index for lookup
* output: device index from FIB lookup
*/
diff --git a/net/core/filter.c b/net/core/filter.c
index ddc1f9ba89d1..da74d6ddc4d7 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5186,13 +5186,14 @@ static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = {
#if IS_ENABLED(CONFIG_INET) || IS_ENABLED(CONFIG_IPV6)
static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params,
const struct neighbour *neigh,
- const struct net_device *dev)
+ const struct net_device *dev, u32 mtu)
{
memcpy(params->dmac, neigh->ha, ETH_ALEN);
memcpy(params->smac, dev->dev_addr, ETH_ALEN);
params->h_vlan_TCI = 0;
params->h_vlan_proto = 0;
params->ifindex = dev->ifindex;
+ params->mtu = mtu;

return 0;
}
@@ -5276,8 +5277,10 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,

if (check_mtu) {
mtu = ip_mtu_from_fib_result(&res, params->ipv4_dst);
- if (params->tot_len > mtu)
+ if (params->tot_len > mtu) {
+ params->mtu = mtu; /* union with tot_len */
return BPF_FIB_LKUP_RET_FRAG_NEEDED;
+ }
}

nhc = res.nhc;
@@ -5310,7 +5313,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
if (!neigh)
return BPF_FIB_LKUP_RET_NO_NEIGH;

- return bpf_fib_set_fwd_params(params, neigh, dev);
+ return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
}
#endif

@@ -5402,8 +5405,10 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,

if (check_mtu) {
mtu = ipv6_stub->ip6_mtu_from_fib6(&res, dst, src);
- if (params->tot_len > mtu)
+ if (params->tot_len > mtu) {
+ params->mtu = mtu; /* union with tot_len */
return BPF_FIB_LKUP_RET_FRAG_NEEDED;
+ }
}

if (res.nh->fib_nh_lws)
@@ -5422,7 +5427,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
if (!neigh)
return BPF_FIB_LKUP_RET_NO_NEIGH;

- return bpf_fib_set_fwd_params(params, neigh, dev);
+ return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
}
#endif

@@ -5491,6 +5496,8 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
dev = dev_get_by_index_rcu(net, params->ifindex);
if (!is_skb_forwardable(dev, skb))
rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
+
+ params->mtu = dev->mtu; /* union with tot_len */
}

return rc;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index d83561e8cd2c..4a46a1de6d16 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2216,6 +2216,9 @@ union bpf_attr {
* * > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
* packet is not forwarded or needs assist from full stack
*
+ * If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
+ * was exceeded and result params->mtu contains the MTU.
+ *
* long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
* Description
* Add an entry to, or update a sockhash *map* referencing sockets.
@@ -4844,9 +4847,13 @@ struct bpf_fib_lookup {
__be16 sport;
__be16 dport;

- /* total length of packet from network header - used for MTU check */
- __u16 tot_len;
+ union { /* used for MTU check */
+ /* input to lookup */
+ __u16 tot_len; /* total length of packet from network hdr */

+ /* output: MTU value (if requested check_mtu) */
+ __u16 mtu;
+ };
/* input: L3 device index for lookup
* output: device index from FIB lookup
*/