[RFC bpf-next v2 8/9] selftests/bpf: add flow dissector bpf_skb_load_bytes helper test

[Stanislav Fomichev <sdf@google.com>]

With the per-cpu skb, we want to make sure we don't trigger any
shinfo access. Add small test which tries to read the data past
the packet boundary.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 .../prog_tests/flow_dissector_load_bytes.c    | 50 +++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/flow_dissector_load_bytes.c

diff --git a/tools/testing/selftests/bpf/prog_tests/flow_dissector_load_bytes.c b/tools/testing/selftests/bpf/prog_tests/flow_dissector_load_bytes.c
new file mode 100644
index 000000000000..facf5734c915
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/flow_dissector_load_bytes.c
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+
+void test_flow_dissector_load_bytes(void)
+{
+	struct bpf_flow_keys flow_keys;
+	__u32 duration = 0, retval, size;
+	struct bpf_insn prog[] = {
+		// BPF_REG_1 - 1st argument: context
+		// BPF_REG_2 - 2nd argument: offset, start at last byte + 1
+		BPF_MOV64_IMM(BPF_REG_2, sizeof(pkt_v4)),
+		// BPF_REG_3 - 3rd argument: destination, reserve byte on stack
+		BPF_ALU64_REG(BPF_MOV, BPF_REG_3, BPF_REG_10),
+		BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, -1),
+		// BPF_REG_4 - 4th argument: copy one byte
+		BPF_MOV64_IMM(BPF_REG_4, 1),
+		// bpf_skb_load_bytes(ctx, sizeof(pkt_v4), ptr, 1)
+		BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
+			     BPF_FUNC_skb_load_bytes),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 2),
+		// if (ret == 0) return BPF_DROP (2)
+		BPF_MOV64_IMM(BPF_REG_0, BPF_DROP),
+		BPF_EXIT_INSN(),
+		// if (ret != 0) return BPF_OK (0)
+		BPF_MOV64_IMM(BPF_REG_0, BPF_OK),
+		BPF_EXIT_INSN(),
+	};
+	int fd, err;
+
+	/* make sure bpf_skb_load_bytes helper doesn't cause any
+	 * problems when used with the fake skb in the flow
+	 * dissector (try to read past the last byte)
+	 */
+	fd = bpf_load_program(BPF_PROG_TYPE_FLOW_DISSECTOR, prog,
+			      ARRAY_SIZE(prog), "GPL", 0, NULL, 0);
+	CHECK(fd < 0,
+	      "flow_dissector-bpf_skb_load_bytes-load",
+	      "fd %d errno %d\n",
+	      fd, errno);
+
+	err = bpf_prog_test_run(fd, 1, &pkt_v4, sizeof(pkt_v4),
+				&flow_keys, &size, &retval, &duration);
+	CHECK(size != sizeof(flow_keys) || err || retval != 1,
+	      "flow_dissector-bpf_skb_load_bytes",
+	      "err %d errno %d retval %d duration %d size %u/%lu\n",
+	      err, errno, retval, duration, size, sizeof(flow_keys));
+
+	if (fd >= -1)
+		close(fd);
+}
-- 
2.21.0.225.g810b269d1ac-goog