netdev - Re: [RFC PATCH 14/16] gtp: add support for flow based tunneling

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Date:   Sun, 24 Jan 2021 15:27:47 +0100
From:   Jonas Bonn <jonas@...rbonn.se>
To:     laforge@...monks.org, netdev@...r.kernel.org, pbshelar@...com,
        kuba@...nel.org
Cc:     pablo@...filter.org
Subject: Re: [RFC PATCH 14/16] gtp: add support for flow based tunneling

Hi Pravin,

A couple more comments around the GTP_METADATA bits:

On 23/01/2021 20:59, Jonas Bonn wrote:
> From: Pravin B Shelar <pbshelar@...com>
> 
> This patch adds support for flow based tunneling, allowing to send and
> receive GTP tunneled packets via the (lightweight) tunnel metadata
> mechanism.  This would allow integration with OVS and eBPF using flow
> based tunneling APIs.
> 
> The mechanism used here is to get the required GTP tunnel parameters
> from the tunnel metadata instead of looking up a pre-configured PDP
> context.  The tunnel metadata contains the necessary information for
> creating the GTP header.
> 
> Signed-off-by: Jonas Bonn <jonas@...rbonn.se>
> ---
>   drivers/net/gtp.c                  | 160 +++++++++++++++++++++++++----
>   include/uapi/linux/gtp.h           |  12 +++
>   include/uapi/linux/if_tunnel.h     |   1 +
>   tools/include/uapi/linux/if_link.h |   1 +
>   4 files changed, 156 insertions(+), 18 deletions(-)
> 

<...>

>   
> +static int gtp_set_tun_dst(struct pdp_ctx *pctx, struct sk_buff *skb,
> +			   unsigned int hdrlen)
> +{
> +	struct metadata_dst *tun_dst;
> +	struct gtp1_header *gtp1;
> +	int opts_len = 0;
> +	__be64 tid;
> +
> +	gtp1 = (struct gtp1_header *)(skb->data + sizeof(struct udphdr));
> +
> +	tid = key32_to_tunnel_id(gtp1->tid);
> +
> +	if (unlikely(gtp1->flags & GTP1_F_MASK))
> +		opts_len = sizeof(struct gtpu_metadata);

So if there are GTP flags sets, you're saying that this no longer a 
T-PDU but something else.  That's wrong... the flags indicate the 
presence of extensions to the GTP header itself.

> +
> +	tun_dst = udp_tun_rx_dst(skb,
> +			pctx->sk->sk_family, TUNNEL_KEY, tid, opts_len);
> +	if (!tun_dst) {
> +		netdev_dbg(pctx->dev, "Failed to allocate tun_dst");
> +		goto err;
> +	}
> +
> +	netdev_dbg(pctx->dev, "attaching metadata_dst to skb, gtp ver %d hdrlen %d\n",
> +		   pctx->gtp_version, hdrlen);
> +	if (unlikely(opts_len)) {
> +		struct gtpu_metadata *opts;
> +
> +		opts = ip_tunnel_info_opts(&tun_dst->u.tun_info);
> +		opts->ver = GTP_METADATA_V1;
> +		opts->flags = gtp1->flags;
> +		opts->type = gtp1->type;
> +		netdev_dbg(pctx->dev, "recved control pkt: flag %x type: %d\n",
> +			   opts->flags, opts->type);
> +		tun_dst->u.tun_info.key.tun_flags |= TUNNEL_GTPU_OPT;
> +		tun_dst->u.tun_info.options_len = opts_len;
> +		skb->protocol = htons(0xffff);         /* Unknown */

It might be relevant to set protocol to unknown for 'end markers' (i.e. 
gtp1->type == 0xfe), but not for everything that happens to have flag 
bits set.  'flags' and 'type' are independent of each other and need to 
handled as such.

/Jonas