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Date: Wed, 2 Dec 2020 23:23:51 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Jesper Dangaard Brouer <brouer@...hat.com>, bpf@...r.kernel.org
Cc: netdev@...r.kernel.org, Daniel Borkmann <borkmann@...earbox.net>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
John Fastabend <john.fastabend@...il.com>,
Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com,
colrack@...il.com
Subject: Re: [PATCH bpf-next V8 3/8] bpf: bpf_fib_lookup return MTU value as
output when looked up
On 11/27/20 7:06 PM, Jesper Dangaard Brouer wrote:
> 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.
>
> V5:
> - Fixed uninit value spotted by Dan Carpenter.
> - Name struct output member mtu_result
>
> Reported-by: kernel test robot <lkp@...el.com>
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
> include/uapi/linux/bpf.h | 11 +++++++++--
> net/core/filter.c | 22 +++++++++++++++-------
> tools/include/uapi/linux/bpf.h | 11 +++++++++--
> 3 files changed, 33 insertions(+), 11 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index c3458ec1f30a..848398bd5a54 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2220,6 +2220,9 @@ 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 output params->mtu_result 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.
> @@ -4961,9 +4964,13 @@ 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; /* L3 length from network hdr (iph->tot_len) */
>
> + /* output: MTU value */
> + __u16 mtu_result;
> + };
> /* 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 84d77c425fbe..25b137ffdced 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5265,12 +5265,14 @@ 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;
> + if (mtu)
> + params->mtu_result = mtu; /* union with tot_len */
>
> return 0;
> }
> @@ -5286,8 +5288,8 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
> struct net_device *dev;
> struct fib_result res;
> struct flowi4 fl4;
> + u32 mtu = 0;
> int err;
> - u32 mtu;
>
> dev = dev_get_by_index_rcu(net, params->ifindex);
> if (unlikely(!dev))
> @@ -5354,8 +5356,10 @@ 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_result = mtu; /* union with tot_len */
> return BPF_FIB_LKUP_RET_FRAG_NEEDED;
> + }
> }
>
> nhc = res.nhc;
> @@ -5389,7 +5393,7 @@ 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
>
> @@ -5406,7 +5410,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
> struct flowi6 fl6;
> int strict = 0;
> int oif, err;
> - u32 mtu;
> + u32 mtu = 0;
>
> /* link local addresses are never forwarded */
> if (rt6_need_strict(dst) || rt6_need_strict(src))
> @@ -5481,8 +5485,10 @@ 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_result = mtu; /* union with tot_len */
> return BPF_FIB_LKUP_RET_FRAG_NEEDED;
And with mentioned approach from [0] we also only ever need to set the above,
but we don't need to drag it anywhere else via bpf_fib_set_fwd_params() with
the extra zero check on mtu. So this should simplify this one as well.
[0] https://lore.kernel.org/bpf/6f9cac4e-a231-ff8d-43a5-828995ca5ec7@iogearbox.net/
> + }
> }
>
> if (res.nh->fib_nh_lws)
> @@ -5502,7 +5508,7 @@ 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);
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