selftests/bpf: Add test validating data section to struct convertion layout

Add a simple selftests validating datasection-to-struct layour dumping. Global
variables are constructed in such a way as to cause both natural and
artificial padding (through custom alignment requirement).
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191214014341.3442258-17-andriin@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2019 Facebook */
+
+#include <test_progs.h>
+
+struct s {
+	int a;
+	long long b;
+} __attribute__((packed));
+
+#include "test_skeleton.skel.h"
+
+BPF_EMBED_OBJ(skeleton, "test_skeleton.o");
+
+void test_skeleton(void)
+{
+	int duration = 0, err;
+	struct test_skeleton* skel;
+	struct test_skeleton__bss *bss;
+
+	skel = test_skeleton__open_and_load(&skeleton_embed);
+	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
+		return;
+
+	bss = skel->bss;
+	bss->in1 = 1;
+	bss->in2 = 2;
+	bss->in3 = 3;
+	bss->in4 = 4;
+	bss->in5.a = 5;
+	bss->in5.b = 6;
+
+	err = test_skeleton__attach(skel);
+	if (CHECK(err, "skel_attach", "skeleton attach failed: %d\n", err))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	usleep(1);
+
+	CHECK(bss->out1 != 1, "res1", "got %d != exp %d\n", bss->out1, 1);
+	CHECK(bss->out2 != 2, "res2", "got %lld != exp %d\n", bss->out2, 2);
+	CHECK(bss->out3 != 3, "res3", "got %d != exp %d\n", (int)bss->out3, 3);
+	CHECK(bss->out4 != 4, "res4", "got %lld != exp %d\n", bss->out4, 4);
+	CHECK(bss->handler_out5.a != 5, "res5", "got %d != exp %d\n",
+	      bss->handler_out5.a, 5);
+	CHECK(bss->handler_out5.b != 6, "res6", "got %lld != exp %d\n",
+	      bss->handler_out5.b, 6);
+
+cleanup:
+	test_skeleton__destroy(skel);
+}

tools/testing/selftests/bpf/progs/test_skeleton.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2019 Facebook */
+
+#include <linux/bpf.h>
+#include "bpf_helpers.h"
+
+struct s {
+	int a;
+	long long b;
+} __attribute__((packed));
+
+int in1 = 0;
+long long in2 = 0;
+char in3 = '\0';
+long long in4 __attribute__((aligned(64))) = 0;
+struct s in5 = {};
+
+long long out2 = 0;
+char out3 = 0;
+long long out4 = 0;
+int out1 = 0;
+
+
+SEC("raw_tp/sys_enter")
+int handler(const void *ctx)
+{
+	static volatile struct s out5;
+
+	out1 = in1;
+	out2 = in2;
+	out3 = in3;
+	out4 = in4;
+	out5 = in5;
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";