Re: [PATCH] netpoll: support sending over raw IP interfaces

[Ratheesh Kannoth]

On 2024-03-13 at 18:16:13, Mark Cilissen (mark@xxxxxxxxxx) wrote:
> Currently, netpoll only supports interfaces with an ethernet-compatible
> link layer. Certain interfaces like SLIP do not have a link layer
> on the network interface level at all and expect raw IP packets,
> and could benefit from being supported by netpoll.
>
> This commit adds support for such interfaces by using the network device's
> `hard_header_len` field as an indication that no link layer is present.
> If that is the case we simply skip adding the ethernet header, causing
> a raw IP packet to be sent over the interface. This has been confirmed
> to add netconsole support to at least SLIP and WireGuard interfaces.
>
> Signed-off-by: Mark Cilissen <mark@xxxxxxxxxx>
> ---
>  Documentation/networking/netconsole.rst |  3 ++-
>  net/core/netpoll.c                      | 30 ++++++++++++++++---------
>  2 files changed, 22 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst
> index d55c2a22ec7a..434ce0366027 100644
> --- a/Documentation/networking/netconsole.rst
> +++ b/Documentation/networking/netconsole.rst
> @@ -327,4 +327,5 @@ enable the logging of even the most critical kernel bugs. It works
>  from IRQ contexts as well, and does not enable interrupts while
>  sending packets. Due to these unique needs, configuration cannot
>  be more automatic, and some fundamental limitations will remain:
> -only IP networks, UDP packets and ethernet devices are supported.
> +only UDP packets over IP networks, over ethernet links if a
> +hardware layer is required, are supported.
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index 543007f159f9..0299fb71b456 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -399,7 +399,7 @@ EXPORT_SYMBOL(netpoll_send_skb);
>
>  void netpoll_send_udp(struct netpoll *np, const char *msg, int len)
>  {
> -	int total_len, ip_len, udp_len;
> +	int total_len, ip_len, udp_len, link_len;
>  	struct sk_buff *skb;
>  	struct udphdr *udph;
>  	struct iphdr *iph;
> @@ -416,7 +416,10 @@ void netpoll_send_udp(struct netpoll *np, const char *msg, int len)
>  	else
>  		ip_len = udp_len + sizeof(*iph);
>
> -	total_len = ip_len + LL_RESERVED_SPACE(np->dev);
> +	/* if there's a hardware header assume ethernet, else raw IP */
Taking an assumption based on dev's lower layer does not look to be good.
why not transmit packet from skb_network_header() in your driver (by making
changes in your driver)

> +	eth = NULL;
> +	link_len = np->dev->hard_header_len ? LL_RESERVED_SPACE(np->dev) : 0;
> +	total_len = ip_len + link_len;
>
>  	skb = find_skb(np, total_len + np->dev->needed_tailroom,
>  		       total_len - len);
> @@ -458,9 +461,11 @@ void netpoll_send_udp(struct netpoll *np, const char *msg, int len)
>  		ip6h->saddr = np->local_ip.in6;
>  		ip6h->daddr = np->remote_ip.in6;
>
> -		eth = skb_push(skb, ETH_HLEN);
> -		skb_reset_mac_header(skb);
> -		skb->protocol = eth->h_proto = htons(ETH_P_IPV6);
> +		skb->protocol = htons(ETH_P_IPV6);
> +		if (link_len) {
> +			eth = skb_push(skb, ETH_HLEN);
> +			skb_reset_mac_header(skb);
> +		}
>  	} else {
>  		udph->check = 0;
>  		udph->check = csum_tcpudp_magic(np->local_ip.ip,
> @@ -487,13 +492,18 @@ void netpoll_send_udp(struct netpoll *np, const char *msg, int len)
>  		put_unaligned(np->remote_ip.ip, &(iph->daddr));
>  		iph->check    = ip_fast_csum((unsigned char *)iph, iph->ihl);
>
> -		eth = skb_push(skb, ETH_HLEN);
> -		skb_reset_mac_header(skb);
> -		skb->protocol = eth->h_proto = htons(ETH_P_IP);
> +		skb->protocol = htons(ETH_P_IP);
> +		if (link_len) {
> +			eth = skb_push(skb, ETH_HLEN);
> +			skb_reset_mac_header(skb);
> +		}
>  	}
>
> -	ether_addr_copy(eth->h_source, np->dev->dev_addr);
> -	ether_addr_copy(eth->h_dest, np->remote_mac);
> +	if (eth) {
> +		eth->h_proto = skb->protocol;
> +		ether_addr_copy(eth->h_source, np->dev->dev_addr);
> +		ether_addr_copy(eth->h_dest, np->remote_mac);
> +	}
>
>  	skb->dev = np->dev;
>
> --
> 2.43.0
>