Re: [PATCH bpf-next v2 11/13] selftests/bpf: add network helpers to generate udp checksums

On 11/15/24 16:54, Stanislav Fomichev wrote:

On 11/14, Alexis Lothoré (eBPF Foundation) wrote:

...

+static inline __sum16 build_udp_v4_csum(const struct iphdr *iph, __u8 l4_proto,

• 			__u16 l4_len, const void *l4_start,
  

• 			int num_words)
  

+{

• unsigned long pseudo_sum;
• int num_u16 = sizeof(iph->saddr); /* halfwords: twice byte len */
• pseudo_sum = add_csum_hword((void *)&iph->saddr, num_u16);
• pseudo_sum += htons(l4_proto);
• pseudo_sum += l4_len;
• pseudo_sum += add_csum_hword(l4_start, num_words);
• return csum_fold(pseudo_sum);

I was expecting to see a call to csum_tcpudp_magic here. And csum_ipv6_magic down below. These build pseudo header csum, so no need to manually do it again.

I initially tried to fit csum_tcpudp_magic here and did not manage to make a valid UDP checksum, but after more attempts, it looks like I had a misunderstanding this checksum computation. I am now able to used csum_tcpudp_magic in build_udp_v4_csum, it will be fixed in the next revision :)

Thanks,

Alexis