selftests/bpf: Test BTF's handling of endianness

Add selftests juggling endianness back and forth to validate BTF's handling of
endianness convertions internally.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200929043046.1324350-4-andriin@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#define _GNU_SOURCE
+#include <string.h>
+#include <byteswap.h>
+#include <test_progs.h>
+#include <bpf/btf.h>
+
+static int duration = 0;
+
+void test_btf_endian() {
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+	enum btf_endianness endian = BTF_LITTLE_ENDIAN;
+#elif __BYTE_ORDER == __BIG_ENDIAN
+	enum btf_endianness endian = BTF_BIG_ENDIAN;
+#else
+#error "Unrecognized __BYTE_ORDER"
+#endif
+	enum btf_endianness swap_endian = 1 - endian;
+	struct btf *btf = NULL, *swap_btf = NULL;
+	const void *raw_data, *swap_raw_data;
+	const struct btf_type *t;
+	const struct btf_header *hdr;
+	__u32 raw_sz, swap_raw_sz;
+	int var_id;
+
+	/* Load BTF in native endianness */
+	btf = btf__parse_elf("btf_dump_test_case_syntax.o", NULL);
+	if (!ASSERT_OK_PTR(btf, "parse_native_btf"))
+		goto err_out;
+
+	ASSERT_EQ(btf__endianness(btf), endian, "endian");
+	btf__set_endianness(btf, swap_endian);
+	ASSERT_EQ(btf__endianness(btf), swap_endian, "endian");
+
+	/* Get raw BTF data in non-native endianness... */
+	raw_data = btf__get_raw_data(btf, &raw_sz);
+	if (!ASSERT_OK_PTR(raw_data, "raw_data_inverted"))
+		goto err_out;
+
+	/* ...and open it as a new BTF instance */
+	swap_btf = btf__new(raw_data, raw_sz);
+	if (!ASSERT_OK_PTR(swap_btf, "parse_swap_btf"))
+		goto err_out;
+
+	ASSERT_EQ(btf__endianness(swap_btf), swap_endian, "endian");
+	ASSERT_EQ(btf__get_nr_types(swap_btf), btf__get_nr_types(btf), "nr_types");
+
+	swap_raw_data = btf__get_raw_data(swap_btf, &swap_raw_sz);
+	if (!ASSERT_OK_PTR(swap_raw_data, "swap_raw_data"))
+		goto err_out;
+
+	/* both raw data should be identical (with non-native endianness) */
+	ASSERT_OK(memcmp(raw_data, swap_raw_data, raw_sz), "mem_identical");
+
+	/* make sure that at least BTF header data is really swapped */
+	hdr = swap_raw_data;
+	ASSERT_EQ(bswap_16(hdr->magic), BTF_MAGIC, "btf_magic_swapped");
+	ASSERT_EQ(raw_sz, swap_raw_sz, "raw_sizes");
+
+	/* swap it back to native endianness */
+	btf__set_endianness(swap_btf, endian);
+	swap_raw_data = btf__get_raw_data(swap_btf, &swap_raw_sz);
+	if (!ASSERT_OK_PTR(swap_raw_data, "swap_raw_data"))
+		goto err_out;
+
+	/* now header should have native BTF_MAGIC */
+	hdr = swap_raw_data;
+	ASSERT_EQ(hdr->magic, BTF_MAGIC, "btf_magic_native");
+	ASSERT_EQ(raw_sz, swap_raw_sz, "raw_sizes");
+
+	/* now modify original BTF */
+	var_id = btf__add_var(btf, "some_var", BTF_VAR_GLOBAL_ALLOCATED, 1);
+	CHECK(var_id <= 0, "var_id", "failed %d\n", var_id);
+
+	btf__free(swap_btf);
+	swap_btf = NULL;
+
+	btf__set_endianness(btf, swap_endian);
+	raw_data = btf__get_raw_data(btf, &raw_sz);
+	if (!ASSERT_OK_PTR(raw_data, "raw_data_inverted"))
+		goto err_out;
+
+	/* and re-open swapped raw data again */
+	swap_btf = btf__new(raw_data, raw_sz);
+	if (!ASSERT_OK_PTR(swap_btf, "parse_swap_btf"))
+		goto err_out;
+
+	ASSERT_EQ(btf__endianness(swap_btf), swap_endian, "endian");
+	ASSERT_EQ(btf__get_nr_types(swap_btf), btf__get_nr_types(btf), "nr_types");
+
+	/* the type should appear as if it was stored in native endianness */
+	t = btf__type_by_id(swap_btf, var_id);
+	ASSERT_STREQ(btf__str_by_offset(swap_btf, t->name_off), "some_var", "var_name");
+	ASSERT_EQ(btf_var(t)->linkage, BTF_VAR_GLOBAL_ALLOCATED, "var_linkage");
+	ASSERT_EQ(t->type, 1, "var_type");
+
+err_out:
+	btf__free(btf);
+	btf__free(swap_btf);
+}