selftests/bpf: Add byte swapping selftest

Add simple selftest validating byte swap built-ins and compile-time macros.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200630152125.3631920-3-andriin@fb.com

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

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+#define IN16 0x1234
+#define IN32 0x12345678U
+#define IN64 0x123456789abcdef0ULL
+
+__u16 in16 = 0;
+__u32 in32 = 0;
+__u64 in64 = 0;
+
+__u16 out16 = 0;
+__u32 out32 = 0;
+__u64 out64 = 0;
+
+__u16 const16 = 0;
+__u32 const32 = 0;
+__u64 const64 = 0;
+
+SEC("raw_tp/sys_enter")
+int sys_enter(const void *ctx)
+{
+	out16 = __builtin_bswap16(in16);
+	out32 = __builtin_bswap32(in32);
+	out64 = __builtin_bswap64(in64);
+	const16 = ___bpf_swab16(IN16);
+	const32 = ___bpf_swab32(IN32);
+	const64 = ___bpf_swab64(IN64);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";