selftests/net: skip partial checksum packets in csum test

Detect packets with ip_summed CHECKSUM_PARTIAL and skip these. These
should not exist, as the test sends individual packets between two
hosts. But if (HW) GRO is on, with randomized content sometimes
subsequent packets can be coalesced.

In this case the GSO packet checksum is converted to a pseudo checksum
in anticipation of sending out as TSO/USO. So the field will not match
the expected value.

Do not count these as test errors.
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240501193156.3627344-1-willemdebruijn.kernel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/csum.c

@@ -682,7 +682,7 @@ static int recv_verify_packet_ipv6(void *nh, int len)
 }
 
 /* return whether auxdata includes TP_STATUS_CSUM_VALID */
-static bool recv_verify_packet_csum(struct msghdr *msg)
+static uint32_t recv_get_packet_csum_status(struct msghdr *msg)
 {
 	struct tpacket_auxdata *aux = NULL;
 	struct cmsghdr *cm;
@@ -706,7 +706,7 @@ static bool recv_verify_packet_csum(struct msghdr *msg)
 	if (!aux)
 		error(1, 0, "cmsg: no auxdata");
 
-	return aux->tp_status & TP_STATUS_CSUM_VALID;
+	return aux->tp_status;
 }
 
 static int recv_packet(int fd)
@@ -716,6 +716,7 @@ static int recv_packet(int fd)
 	char ctrl[CMSG_SPACE(sizeof(struct tpacket_auxdata))];
 	struct pkt *buf = (void *)_buf;
 	struct msghdr msg = {0};
+	uint32_t tp_status;
 	struct iovec iov;
 	int len, ret;
 
@@ -737,6 +738,17 @@ static int recv_packet(int fd)
 		if (len == -1)
 			error(1, errno, "recv p");
 
+		tp_status = recv_get_packet_csum_status(&msg);
+
+		/* GRO might coalesce randomized packets. Such GSO packets are
+		 * then reinitialized for csum offload (CHECKSUM_PARTIAL), with
+		 * a pseudo csum. Do not try to validate these checksums.
+		 */
+		if (tp_status & TP_STATUS_CSUMNOTREADY) {
+			fprintf(stderr, "cmsg: GSO packet has partial csum: skip\n");
+			continue;
+		}
+
 		if (cfg_family == PF_INET6)
 			ret = recv_verify_packet_ipv6(buf, len);
 		else
@@ -753,7 +765,7 @@ static int recv_packet(int fd)
 		 * Do not fail if kernel does not validate a good csum:
 		 * Absence of validation does not imply invalid.
 		 */
-		if (recv_verify_packet_csum(&msg) && cfg_bad_csum) {
+		if (tp_status & TP_STATUS_CSUM_VALID && cfg_bad_csum) {
 			fprintf(stderr, "cmsg: expected bad csum, pf_packet returns valid\n");
 			bad_validations++;
 		}