netdev - RE: [PATCH bpf-next V5 2/5] bpf: bpf_fib_lookup return MTU value as output when looked up

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Date:   Fri, 30 Oct 2020 12:40:21 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, bpf@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, 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
Subject: RE: [PATCH bpf-next V5 2/5] bpf: bpf_fib_lookup return MTU value as
 output when looked up

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 e6ceac3f7d62..01b2b17c645a 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2219,6 +2219,9 @@ 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.
> + *

Do we need to hide this behind a flag? It seems otherwise you might confuse
users. I imagine on error we could reuse the params arg, but now we changed
the tot_len value underneath them?

>   * 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.
> @@ -4872,9 +4875,13 @@ struct bpf_fib_lookup {
>  	__be16	sport;
>  	__be16	dport;