[PATCH v2 bpf] selftests/bpf: Fix csum_partial() dropping trailing byte on odd length

[Madhav Khosla <madhav.khoslaa@gmail.com>]

csum_partial() computes num_u16 = len >> 1 and only sums that many
16-bit words, so the last byte of an odd-length buffer never gets
added to the checksum. RFC 1071 says it should be padded with a zero
byte and summed as one more word, not dropped.

This backs build_ip_csum(), build_udp_v4_csum() and
build_udp_v6_csum(), used by flow_dissector_classification.c and
xdp_metadata.c to hand-build packets. No current caller builds an
odd-length payload, so nothing fails today, but a future one would
get a silently wrong checksum.

Also bump flow_dissector_classification's TEST_PACKET_LEN from 100 to
99 so this actually gets exercised instead of staying latent.

Without the fix and with TEST_PACKET_LEN=99, flow_dissector_classification
fails under vmtest.sh:

    test_flow_dissector_classification:FAIL:test third port unexpected
    test third port: actual 0 != expected 10
    #137/6   flow_dissector_classification/ipv6:FAIL
    #137     flow_dissector_classification:FAIL
    Summary: 1/0 PASSED, 0 SKIPPED, 1/6 FAILED

Kernel just drops the packet over a bad checksum. With the fix, both
flow_dissector_classification and xdp_metadata pass.

v1 -> v2:
- comment style: opening /* on its own line, per BPF selftests style
- Retarget the Fixes tag to bcc00987bc56. commit f4504af68575
  ("selftests/bpf: move ip checksum helper to network helpers") moved
  the helper, but sizeof(iphdr) is always a multiple of 32 bit words /
  4 Bytes (iph->ihl counts in 4-byte words), so the odd-length path
  was never reachable through build_ip_csum(). csum_partial() first
  gets called with a length that isn't guaranteed even in
  bcc00987bc56, via build_udp_v4_csum()/build_udp_v6_csum().
- TEST_PACKET_LEN 100 -> 99 so an existing test catches this instead
  of the bug staying unexercised

Fixes: bcc00987bc56 ("selftests/bpf: add network helpers to generate udp checksums")
Link: https://lore.kernel.org/bpf/DLG5TIIHIMC2.2YCCY0UX86YQF@bootlin.com/T/#t
Signed-off-by: Madhav Khosla <madhav.khoslaa@gmail.com>
---
 tools/testing/selftests/bpf/network_helpers.h     | 15 +++++++++++++--
 .../prog_tests/flow_dissector_classification.c    |  2 +-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/network_helpers.h b/tools/testing/selftests/bpf/network_helpers.h
index 75133119c04a..878c9fc5c37c 100644
--- a/tools/testing/selftests/bpf/network_helpers.h
+++ b/tools/testing/selftests/bpf/network_helpers.h
@@ -129,12 +129,23 @@ static __u16 csum_fold(__u32 csum)
 
 static __wsum csum_partial(const void *buf, int len, __wsum sum)
 {
-	__u16 *p = (__u16 *)buf;
+	const __u8 *p = buf;
 	int num_u16 = len >> 1;
 	int i;
 
 	for (i = 0; i < num_u16; i++)
-		sum += p[i];
+		sum += ((const __u16 *)p)[i];
+
+	/*
+	 * RFC 1071: an odd-length buffer's trailing byte is paired with
+	 * a zero pad byte to form the final 16-bit word.
+	 */
+	if (len & 1) {
+		__u16 tail = 0;
+
+		__builtin_memcpy(&tail, p + len - 1, 1);
+		sum += tail;
+	}
 
 	return sum;
 }
diff --git a/tools/testing/selftests/bpf/prog_tests/flow_dissector_classification.c b/tools/testing/selftests/bpf/prog_tests/flow_dissector_classification.c
index 80b153d3ddec..421dfa6c4ea3 100644
--- a/tools/testing/selftests/bpf/prog_tests/flow_dissector_classification.c
+++ b/tools/testing/selftests/bpf/prog_tests/flow_dissector_classification.c
@@ -27,7 +27,7 @@
 #define TEST_NAME_MAX_LEN (32 + SUBTEST_NAME_MAX_LEN)
 #define MAX_SOURCE_PORTS 3
 #define TEST_PACKETS_COUNT 10
-#define TEST_PACKET_LEN 100
+#define TEST_PACKET_LEN 99
 #define TEST_PACKET_PATTERN 'a'
 #define TEST_IPV4 "192.168.0.1/32"
 #define TEST_IPV6 "100::a/128"
-- 
2.55.0