[PATCH AUTOSEL 5.6 011/129] bpf: Reliably preserve btf_trace_xxx types

Sasha Levin
2020-04-15T11:38:29Z

From: Andrii Nakryiko <andriin@fb.com>

[ Upstream commit 441420a1f0b3031f228453697406c86f110e59d4 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/trace/bpf_probe.h | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
index b04c292709730..1ce3be63add1f 100644
--- 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
--
2.20.1