Re: [PATCH libnftnl 5/6] src: add trace infrastructure support

[Patrick McHardy <kaber@trash.net>]

On 24.11, Florian Westphal wrote:
> +static int
> +print_th(const struct nftnl_trace *t, char *buf, size_t size)
> +{
> +	uint8_t proto = nftnl_trace_get_u8(t, NFTNL_TRACE_TRANSPORT_PROTO);
> +	int ret, len = size, offset = 0;
> +	const struct udphdr *uh;
> +	uint32_t plen;
> +
> +	ret = snprintf(buf+offset, len, " protocol %u", proto);
> +
> +	SNPRINTF_BUFFER_SIZE(ret, size, len, offset);
> +
> +	switch (proto) {
> +	case IPPROTO_DCCP:
> +	case IPPROTO_SCTP:
> +	case IPPROTO_TCP:
> +	case IPPROTO_UDP:
> +	case IPPROTO_UDPLITE:
> +		break;
> +	default:
> +		return 0;
> +	}
> +
> +	/* warning: only sport/dport are valid */
> +	uh = nftnl_trace_get_data(t, NFTNL_TRACE_TRANSPORT_HEADER, &plen);
> +	if (!uh)
> +		return 0;
> +
> +	ret = snprintf(buf+offset, len, " sport %"PRIu16 " dport %"PRIu16,
> +			ntohs(uh->uh_sport), ntohs(uh->uh_dport));
> +
> +	SNPRINTF_BUFFER_SIZE(ret, size, len, offset);
> +	return offset;
> +}
> + ...

Regarding this header decoding, a further upside of using nft would be that
we can already fully decode all known headers based on our protocol descriptions
in a generic fashion.

Basically we just splice off data from the beginning and feed it to payload
expression decoding. That should result in a full decode of every protocol
we support. Every improvement we will make there will automatically apply to
decoding, like (I talked to Pablo about this for a couple of times) supporing
encapsulated headers, like GRE/IPIP/... + inner headers.

Hope I can convince you of the upsides :)