bpf: Reliably preserve btf_trace_xxx types

btf_trace_xxx types, crucial for tp_btf BPF programs (raw tracepoint with
verifier-checked direct memory access), have to be preserved in kernel BTF to
allow verifier do its job and enforce type/memory safety. It was reported
([0]) that for kernels built with Clang current type-casting approach doesn't
preserve these types.

This patch fixes it by declaring an anonymous union for each registered
tracepoint, capturing both struct bpf_raw_event_map information, as well as
recording btf_trace_##call type reliably. Structurally, it's still the same
content as for a plain struct bpf_raw_event_map, so no other changes are
necessary.

  [0] https://github.com/iovisor/bcc/issues/2770#issuecomment-591007692

Fixes: e8c423fb31 ("bpf: Add typecast to raw_tracepoints to help BTF generation")
Reported-by: Wenbo Zhang <ethercflow@gmail.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200301081045.3491005-2-andriin@fb.com

include/trace/bpf_probe.h

@@ -75,13 +75,17 @@ static inline void bpf_test_probe_##call(void)				\
 	check_trace_callback_type_##call(__bpf_trace_##template);	\
 }									\
 typedef void (*btf_trace_##call)(void *__data, proto);			\
-static struct bpf_raw_event_map	__used					\
-	__attribute__((section("__bpf_raw_tp_map")))			\
-__bpf_trace_tp_map_##call = {						\
-	.tp		= &__tracepoint_##call,				\
-	.bpf_func	= (void *)(btf_trace_##call)__bpf_trace_##template,	\
-	.num_args	= COUNT_ARGS(args),				\
-	.writable_size	= size,						\
+static union {								\
+	struct bpf_raw_event_map event;					\
+	btf_trace_##call handler;					\
+} __bpf_trace_tp_map_##call __used					\
+__attribute__((section("__bpf_raw_tp_map"))) = {			\
+	.event = {							\
+		.tp		= &__tracepoint_##call,			\
+		.bpf_func	= __bpf_trace_##template,		\
+		.num_args	= COUNT_ARGS(args),			\
+		.writable_size	= size,					\
+	},								\
 };
 
 #define FIRST(x, ...) x