[PATCH bpf 2/2] selftests/bpf: Cover test_run csum validation after bpf_skb_change_tail()

[Maxim Skokov <skokovmaksimevg@gmail.com>]

Add BPF_F_TEST_SKB_CHECKSUM_COMPLETE tests for tc programs that move the
skb out of CHECKSUM_COMPLETE with bpf_skb_change_tail():

- change_tail_trim trims the last 4 bytes of pkt_v4. They hold the
  non-zero tcp.urg_ptr, so the stale skb->csum no longer matches the
  packet.
- change_tail_grow_write appends 4 zero bytes, which on their own do
  not change the sum, and then writes a byte with BPF_F_RECOMPUTE_CSUM.
  That write can no longer update skb->csum.

Both return -EBADMSG without the preceding test_run fix and pass with
it.

Also add store_no_recompute as a negative test. It writes a byte
without BPF_F_RECOMPUTE_CSUM, so the skb stays CHECKSUM_COMPLETE with a
stale skb->csum, and test_run must keep returning -EBADMSG. This checks
that the validation still runs for CHECKSUM_COMPLETE skbs. The passing
case, a CHECKSUM_COMPLETE skb whose checksum is updated correctly, is
already covered by test_skb_pkt_end.

Assisted-by: LLM
Signed-off-by: Maxim Skokov <skokovmaksimevg@gmail.com>
---
 .../bpf/prog_tests/skb_csum_complete.c        | 39 ++++++++++++++++
 .../selftests/bpf/progs/skb_csum_complete.c   | 44 +++++++++++++++++++
 2 files changed, 83 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/skb_csum_complete.c
 create mode 100644 tools/testing/selftests/bpf/progs/skb_csum_complete.c

diff --git a/tools/testing/selftests/bpf/prog_tests/skb_csum_complete.c b/tools/testing/selftests/bpf/prog_tests/skb_csum_complete.c
new file mode 100644
index 0000000000000..919f24dbacd3d
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/skb_csum_complete.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <network_helpers.h>
+#include "skb_csum_complete.skel.h"
+
+static void run(struct bpf_program *prog, int expected_err)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts,
+		.data_in = &pkt_v4,
+		.data_size_in = sizeof(pkt_v4),
+		.repeat = 1,
+		.flags = BPF_F_TEST_SKB_CHECKSUM_COMPLETE,
+	);
+	int err;
+
+	err = bpf_prog_test_run_opts(bpf_program__fd(prog), &topts);
+	if (!ASSERT_EQ(err, expected_err, "test_run"))
+		return;
+	if (!expected_err)
+		ASSERT_EQ(topts.retval, 0, "retval");
+}
+
+void test_skb_csum_complete(void)
+{
+	struct skb_csum_complete *skel;
+
+	skel = skb_csum_complete__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	if (test__start_subtest("change_tail_trim"))
+		run(skel->progs.change_tail_trim, 0);
+	if (test__start_subtest("change_tail_grow_write"))
+		run(skel->progs.change_tail_grow_write, 0);
+	if (test__start_subtest("store_no_recompute"))
+		run(skel->progs.store_no_recompute, -EBADMSG);
+
+	skb_csum_complete__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/skb_csum_complete.c b/tools/testing/selftests/bpf/progs/skb_csum_complete.c
new file mode 100644
index 0000000000000..8e8d53530899a
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/skb_csum_complete.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("tc")
+int change_tail_trim(struct __sk_buff *skb)
+{
+	/* Drops CHECKSUM_COMPLETE. skb->csum still covers the trimmed bytes,
+	 * which hold the non-zero tcp.urg_ptr of pkt_v4, so it no longer
+	 * matches the packet.
+	 */
+	if (bpf_skb_change_tail(skb, skb->len - 4, 0))
+		return 1;
+	return 0;
+}
+
+SEC("tc")
+int change_tail_grow_write(struct __sk_buff *skb)
+{
+	__u8 val = 0xab;
+
+	/* Drops CHECKSUM_COMPLETE. The new bytes are zero and do not change
+	 * the sum, but the write below can no longer update skb->csum.
+	 */
+	if (bpf_skb_change_tail(skb, skb->len + 4, 0))
+		return 1;
+	if (bpf_skb_store_bytes(skb, skb->len - 1, &val, sizeof(val),
+				BPF_F_RECOMPUTE_CSUM))
+		return 2;
+	return 0;
+}
+
+SEC("tc")
+int store_no_recompute(struct __sk_buff *skb)
+{
+	__u8 val = 0xab;
+
+	/* skb stays CHECKSUM_COMPLETE, but skb->csum is not updated. */
+	if (bpf_skb_store_bytes(skb, skb->len - 1, &val, sizeof(val), 0))
+		return 1;
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.47.3