netdev - Re: [PATCH bpf-next V1 3/6] bpf: add BPF-helper for reading MTU from net

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Date:   Tue, 6 Oct 2020 18:33:02 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     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>, brouer@...hat.com
Subject: Re: [PATCH bpf-next V1 3/6] bpf: add BPF-helper for reading MTU
 from net_device via ifindex

On Tue, 06 Oct 2020 18:03:01 +0200
Jesper Dangaard Brouer <brouer@...hat.com> wrote:

> FIXME: add description.

Ups, I will obviously send a V2.

I still want feedback on whether I should implement another BPF-helper
as sketched below:

> FIXME: IMHO we can create a better BPF-helper named bpf_mtu_check()
> instead of bpf_mtu_lookup(), because a flag can be used for requesting
> GRO segment size checking.  The ret value of bpf_mtu_check() says
> if MTU was violoated, but also return MTU via pointer arg to allow
> BPF-progs to do own logic.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>  include/uapi/linux/bpf.h |   13 +++++++++++
>  net/core/filter.c        |   56 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 69 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 50ce65e37b16..29b335cb96ef 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3718,6 +3718,18 @@ union bpf_attr {
>   *		never return NULL.
>   *	Return
>   *		A pointer pointing to the kernel percpu variable on this cpu.
> + *
> + * int bpf_mtu_lookup(void *ctx, u32 ifindex, u64 flags)
> + *	Description
> + *		Lookup MTU of net device based on ifindex.  The Linux kernel
> + *		route table can configure MTUs on a more specific per route
> + *		level, which is not provided by this helper. For route level
> + *		MTU checks use the **bpf_fib_lookup**\ () helper.
> + *
> + *		*ctx* is either **struct xdp_md** for XDP programs or
> + *		**struct sk_buff** tc cls_act programs.
> + *	Return
> + *		On success, MTU size is returned. On error, a negative value.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -3875,6 +3887,7 @@ union bpf_attr {
>  	FN(redirect_neigh),		\
>  	FN(bpf_per_cpu_ptr),            \
>  	FN(bpf_this_cpu_ptr),		\
> +	FN(mtu_lookup),			\
>  	/* */
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/net/core/filter.c b/net/core/filter.c
> index d84723f347c0..49ae3b80027b 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5512,6 +5512,58 @@ static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {
>  	.arg4_type	= ARG_ANYTHING,
>  };
>  
> +static int bpf_mtu_lookup(struct net *netns, u32 ifindex, u64 flags)
> +{
> +	struct net_device *dev;
> +
> +	// XXX: Do we even need flags?
> +	// Flag idea: get ctx dev->mtu for XDP_TX or redir out-same-dev
> +	if (flags)
> +		return -EINVAL;
> +
> +	dev = dev_get_by_index_rcu(netns, ifindex);
> +	if (!dev)
> +		return -ENODEV;
> +
> +	return dev->mtu;
> +}
> +
> +BPF_CALL_3(bpf_skb_mtu_lookup, struct sk_buff *, skb,
> +	   u32, ifindex, u64, flags)
> +{
> +	struct net *netns = dev_net(skb->dev);
> +
> +	return bpf_mtu_lookup(netns, ifindex, flags);
> +}
> +
> +BPF_CALL_3(bpf_xdp_mtu_lookup, struct xdp_buff *, xdp,
> +	   u32, ifindex, u64, flags)
> +{
> +	struct net *netns = dev_net(xdp->rxq->dev);
> +	// XXX: Handle if this runs in devmap prog (then is rxq invalid?)
> +
> +	return bpf_mtu_lookup(netns, ifindex, flags);
> +}
> +
> +static const struct bpf_func_proto bpf_skb_mtu_lookup_proto = {
> +	.func		= bpf_skb_mtu_lookup,
> +	.gpl_only	= true,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type      = ARG_PTR_TO_CTX,
> +	.arg2_type      = ARG_ANYTHING,
> +	.arg3_type      = ARG_ANYTHING,
> +};
> +
> +static const struct bpf_func_proto bpf_xdp_mtu_lookup_proto = {
> +	.func		= bpf_xdp_mtu_lookup,
> +	.gpl_only	= true,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type      = ARG_PTR_TO_CTX,
> +	.arg2_type      = ARG_ANYTHING,
> +	.arg3_type      = ARG_ANYTHING,
> +};
> +
> +
>  #if IS_ENABLED(CONFIG_IPV6_SEG6_BPF)
>  static int bpf_push_seg6_encap(struct sk_buff *skb, u32 type, void *hdr, u32 len)
>  {
> @@ -7075,6 +7127,8 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_get_socket_uid_proto;
>  	case BPF_FUNC_fib_lookup:
>  		return &bpf_skb_fib_lookup_proto;
> +	case BPF_FUNC_mtu_lookup:
> +		return &bpf_skb_mtu_lookup_proto;
>  	case BPF_FUNC_sk_fullsock:
>  		return &bpf_sk_fullsock_proto;
>  	case BPF_FUNC_sk_storage_get:
> @@ -7144,6 +7198,8 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_xdp_adjust_tail_proto;
>  	case BPF_FUNC_fib_lookup:
>  		return &bpf_xdp_fib_lookup_proto;
> +	case BPF_FUNC_mtu_lookup:
> +		return &bpf_xdp_mtu_lookup_proto;
>  #ifdef CONFIG_INET
>  	case BPF_FUNC_sk_lookup_udp:
>  		return &bpf_xdp_sk_lookup_udp_proto;
> 
> 



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer