# \[PATCH net 2/3\] selftests: net: tuntap: fix UDP checksum length in build\_udp\_packet\_csum() - From: Eric Dumazet \ - Date: 2026-11-03 20:31:29 - List: netdev - Page: https://ratatoskr.run/netdev/2026/11/17855922 - Thread: https://ratatoskr.run/netdev/2026/11/17855924/t.md ```text build_udp_packet_csum() passes udph->len, which is in network byte order, as the number of bytes to checksum. On little-endian hosts this reads up to 64KB past the UDP header and computes a wrong checksum. Existing callers did not crash only because they use packet buffers of MAX_VNET_TUNNEL_PACKET_SZ bytes. Use ntohs(udph->len), as tap.c does in its own build_udp_packet(). Fixes: a942fcd72e97 ("selftest: tun: Introduce tuntap_helpers.h header for TUN/TAP testing") Signed-off-by: Eric Dumazet --- tools/testing/selftests/net/tuntap_helpers.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/testing/selftests/net/tuntap_helpers.h b/tools/testing/selftests/net/tuntap_helpers.h index d6c0437136ec..ac9b2da57a6d 100644 --- a/tools/testing/selftests/net/tuntap_helpers.h +++ b/tools/testing/selftests/net/tuntap_helpers.h @@ -335,7 +335,7 @@ static inline void build_udp_packet_csum(uint8_t *buf, int family, sum += htons(IPPROTO_UDP) + udph->len; if (!csum_off) - sum += add_csum(buf, udph->len); + sum += add_csum(buf, ntohs(udph->len)); udph->check = finish_ip_csum(sum); } -- 2.53.0 ```