[PATCH net 2/3] selftests: net: tuntap: fix UDP checksum length in build_udp_packet_csum()

[Eric Dumazet <edumazet@kernel.org>]

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 <edumazet@kernel.org>
---
 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