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: e8c423fb31fa ("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

diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
index b04c292..1ce3be6 100644 (file)
--- a/include/trace/bpf_probe.h
+++ b/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