selftests/bpf: add flow dissector bpf_skb_load_bytes helper test

When flow dissector is called without skb, we want to make sure
bpf_skb_load_bytes invocations return error. Add small test which tries
to read single byte from a packet.

bpf_skb_load_bytes should always fail under BPF_PROG_TEST_RUN because
it was converted to the skb-less mode.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/prog_tests/flow_dissector_load_bytes.c

+// 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 first byte
+		BPF_MOV64_IMM(BPF_REG_2, 0),
+		// 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 is not allowed from skb-less context
+	 */
+	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/%zu\n",
+	      err, errno, retval, duration, size, sizeof(flow_keys));
+
+	if (fd >= -1)
+		close(fd);
+}