selftests/bpf: Test access to bpf map pointer

Add selftests to test access to map pointers from bpf program for all
map types except struct_ops (that one would need additional work).

verifier test focuses mostly on scenarios that must be rejected.

prog_tests test focuses on accessing multiple fields both scalar and a
nested struct from bpf program and verifies that those fields have
expected values.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/139a6a17f8016491e39347849b951525335c6eb4.1592600985.git.rdna@fb.com

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

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2020 Facebook
+
+#include <test_progs.h>
+#include <network_helpers.h>
+
+#include "map_ptr_kern.skel.h"
+
+void test_map_ptr(void)
+{
+	struct map_ptr_kern *skel;
+	__u32 duration = 0, retval;
+	char buf[128];
+	int err;
+
+	skel = map_ptr_kern__open_and_load();
+	if (CHECK(!skel, "skel_open_load", "open_load failed\n"))
+		return;
+
+	err = bpf_prog_test_run(bpf_program__fd(skel->progs.cg_skb), 1, &pkt_v4,
+				sizeof(pkt_v4), buf, NULL, &retval, NULL);
+
+	if (CHECK(err, "test_run", "err=%d errno=%d\n", err, errno))
+		goto cleanup;
+
+	if (CHECK(!retval, "retval", "retval=%d map_type=%u line=%u\n", retval,
+		  skel->bss->g_map_type, skel->bss->g_line))
+		goto cleanup;
+
+cleanup:
+	map_ptr_kern__destroy(skel);
+}