Re: [PATCH v2 4/8] udp: forward external source mac address through tap interface

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 12 Jun 2025 00:21:48 -0400
Jon Maloy <jmaloy@redhat.com> wrote:

> We forward the incoming mac address through the tap interface when
> receiving incoming packets from network local hosts. Packets from
> the local host are excepted from this rule, and are still forwarded
> with the default passt/pasta mac address as source.
> 
> This is a part of the solution to bug #120
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  udp.c | 29 +++++++++++++----------------
>  1 file changed, 13 insertions(+), 16 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index 65a52e0..ae8fbaf 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -133,11 +133,8 @@ static int udp_splice_init[IP_VERSIONS][NUM_PORTS];
>  /* UDP header and data for inbound messages */
>  static struct udp_payload_t udp_payload[UDP_MAX_FRAMES];
>  
> -/* Ethernet header for IPv4 frames */
> -static struct ethhdr udp4_eth_hdr;
> -
> -/* Ethernet header for IPv6 frames */
> -static struct ethhdr udp6_eth_hdr;
> +/* Ethernet headers for IPv4 and IPv6 frames */
> +static struct ethhdr udp_eth_hdr[UDP_MAX_FRAMES];

An alternative could be to keep two separated sets of headers. It
avoids setting eh->h_proto every time. I'm not sure if it's worth it,
perhaps a quick throughput test would be a good idea, just to be sure
it's fine (I don't expect any substantial regression).

>  
>  /**
>   * struct udp_meta_t - Pre-cooked headers for UDP packets
> @@ -214,8 +211,10 @@ void udp_portmap_clear(void)
>   */
>  void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s)
>  {
> -	eth_update_mac(&udp4_eth_hdr, eth_d, eth_s);
> -	eth_update_mac(&udp6_eth_hdr, eth_d, eth_s);
> +	int i;
> +
> +	for (i = 0; i < UDP_MAX_FRAMES; i++)
> +		eth_update_mac(&udp_eth_hdr[i], eth_d, eth_s);
>  }
>  
>  /**
> @@ -238,6 +237,7 @@ static void udp_iov_init_one(const struct ctx *c, size_t i)
>  
>  	*siov = IOV_OF_LVALUE(payload->data);
>  
> +	tiov[UDP_IOV_ETH] = IOV_OF_LVALUE(udp_eth_hdr[i]);
>  	tiov[UDP_IOV_TAP] = tap_hdr_iov(c, &meta->taph);
>  	tiov[UDP_IOV_PAYLOAD].iov_base = payload;
>  
> @@ -253,9 +253,6 @@ static void udp_iov_init(const struct ctx *c)
>  {
>  	size_t i;
>  
> -	udp4_eth_hdr.h_proto = htons_constant(ETH_P_IP);
> -	udp6_eth_hdr.h_proto = htons_constant(ETH_P_IPV6);
> -
>  	for (i = 0; i < UDP_MAX_FRAMES; i++)
>  		udp_iov_init_one(c, i);
>  }
> @@ -362,21 +359,21 @@ static void udp_tap_prepare(const struct mmsghdr *mmh,
>  	struct iovec (*tap_iov)[UDP_NUM_IOVS] = &udp_l2_iov[idx];
>  	struct udp_payload_t *bp = &udp_payload[idx];
>  	struct udp_meta_t *bm = &udp_meta[idx];
> +	struct ethhdr *eh = (*tap_iov)[UDP_IOV_ETH].iov_base;

Nit: this could be moved after the declaration of tap_iov, so that we
keep initialisers from longest to shortest.

>  	size_t l4len;
>  
> +	eth_update_mac(eh, 0, toside->mac);
>  	if (!inany_v4(&toside->eaddr) || !inany_v4(&toside->oaddr)) {
>  		l4len = udp_update_hdr6(&bm->ip6h, bp, toside,
>  					mmh[idx].msg_len, no_udp_csum);
> -		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip6h) +
> -			       sizeof(udp6_eth_hdr));
> -		(*tap_iov)[UDP_IOV_ETH] = IOV_OF_LVALUE(udp6_eth_hdr);
> +		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip6h) + ETH_HLEN);
> +		eh->h_proto = htons_constant(ETH_P_IPV6);
>  		(*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip6h);
>  	} else {
>  		l4len = udp_update_hdr4(&bm->ip4h, bp, toside,
>  					mmh[idx].msg_len, no_udp_csum);
> -		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip4h) +
> -			       sizeof(udp4_eth_hdr));
> -		(*tap_iov)[UDP_IOV_ETH] = IOV_OF_LVALUE(udp4_eth_hdr);
> +		tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip4h) + ETH_HLEN);
> +		eh->h_proto = htons_constant(ETH_P_IP);
>  		(*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip4h);
>  	}
>  	(*tap_iov)[UDP_IOV_PAYLOAD].iov_len = l4len;

-- 
Stefano