kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 137 insertions, 0 deletions

diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
new file mode 100644
index 000000000..d1de8f9aa
--- /dev/null
+++ b/include/trace/bpf_probe.h
@@ -0,0 +1,137 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#undef TRACE_SYSTEM_VAR
+
+#ifdef CONFIG_BPF_EVENTS
+
+#include "stages/stage6_event_callback.h"
+
+#undef __perf_count
+#define __perf_count(c)	(c)
+
+#undef __perf_task
+#define __perf_task(t)	(t)
+
+#include <linux/args.h>
+
+/* cast any integer, pointer, or small struct to u64 */
+#define UINTTYPE(size) \
+	__typeof__(__builtin_choose_expr(size == 1,  (u8)1, \
+		   __builtin_choose_expr(size == 2, (u16)2, \
+		   __builtin_choose_expr(size == 4, (u32)3, \
+		   __builtin_choose_expr(size == 8, (u64)4, \
+					 (void)5)))))
+#define __CAST_TO_U64(x) ({ \
+	typeof(x) __src = (x); \
+	UINTTYPE(sizeof(x)) __dst; \
+	memcpy(&__dst, &__src, sizeof(__dst)); \
+	(u64)__dst; })
+
+#define __CAST1(a,...) __CAST_TO_U64(a)
+#define __CAST2(a,...) __CAST_TO_U64(a), __CAST1(__VA_ARGS__)
+#define __CAST3(a,...) __CAST_TO_U64(a), __CAST2(__VA_ARGS__)
+#define __CAST4(a,...) __CAST_TO_U64(a), __CAST3(__VA_ARGS__)
+#define __CAST5(a,...) __CAST_TO_U64(a), __CAST4(__VA_ARGS__)
+#define __CAST6(a,...) __CAST_TO_U64(a), __CAST5(__VA_ARGS__)
+#define __CAST7(a,...) __CAST_TO_U64(a), __CAST6(__VA_ARGS__)
+#define __CAST8(a,...) __CAST_TO_U64(a), __CAST7(__VA_ARGS__)
+#define __CAST9(a,...) __CAST_TO_U64(a), __CAST8(__VA_ARGS__)
+#define __CAST10(a,...) __CAST_TO_U64(a), __CAST9(__VA_ARGS__)
+#define __CAST11(a,...) __CAST_TO_U64(a), __CAST10(__VA_ARGS__)
+#define __CAST12(a,...) __CAST_TO_U64(a), __CAST11(__VA_ARGS__)
+/* tracepoints with more than 12 arguments will hit build error */
+#define CAST_TO_U64(...) CONCATENATE(__CAST, COUNT_ARGS(__VA_ARGS__))(__VA_ARGS__)
+
+#define __BPF_DECLARE_TRACE(call, proto, args)				\
+static notrace void							\
+__bpf_trace_##call(void *__data, proto)					\
+{									\
+	CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args));	\
+}
+
+#undef DECLARE_EVENT_CLASS
+#define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
+	__BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args))
+
+#define __BPF_DECLARE_TRACE_SYSCALL(call, proto, args)			\
+static notrace void							\
+__bpf_trace_##call(void *__data, proto)					\
+{									\
+	might_fault();							\
+	CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args));	\
+}
+
+#undef DECLARE_EVENT_SYSCALL_CLASS
+#define DECLARE_EVENT_SYSCALL_CLASS(call, proto, args, tstruct, assign, print)	\
+	__BPF_DECLARE_TRACE_SYSCALL(call, PARAMS(proto), PARAMS(args))
+
+/*
+ * This part is compiled out, it is only here as a build time check
+ * to make sure that if the tracepoint handling changes, the
+ * bpf probe will fail to compile unless it too is updated.
+ */
+#define __DEFINE_EVENT(template, call, proto, args, size)		\
+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 union {								\
+	struct bpf_raw_event_map event;					\
+	btf_trace_##call handler;					\
+} __bpf_trace_tp_map_##call __used					\
+__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
+
+#define __CHECK_WRITABLE_BUF_SIZE(call, proto, args, size)		\
+static inline void bpf_test_buffer_##call(void)				\
+{									\
+	/* BUILD_BUG_ON() is ignored if the code is completely eliminated, but \
+	 * BUILD_BUG_ON_ZERO() uses a different mechanism that is not	\
+	 * dead-code-eliminated.					\
+	 */								\
+	FIRST(proto);							\
+	(void)BUILD_BUG_ON_ZERO(size != sizeof(*FIRST(args)));		\
+}
+
+#undef DEFINE_EVENT_WRITABLE
+#define DEFINE_EVENT_WRITABLE(template, call, proto, args, size) \
+	__CHECK_WRITABLE_BUF_SIZE(call, PARAMS(proto), PARAMS(args), size) \
+	__DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), size)
+
+#undef DEFINE_EVENT
+#define DEFINE_EVENT(template, call, proto, args)			\
+	__DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), 0)
+
+#undef DEFINE_EVENT_PRINT
+#define DEFINE_EVENT_PRINT(template, name, proto, args, print)	\
+	DEFINE_EVENT(template, name, PARAMS(proto), PARAMS(args))
+
+#undef DECLARE_TRACE
+#define DECLARE_TRACE(call, proto, args)				\
+	__BPF_DECLARE_TRACE(call##_tp, PARAMS(proto), PARAMS(args))		\
+	__DEFINE_EVENT(call##_tp, call##_tp, PARAMS(proto), PARAMS(args), 0)
+
+#undef DECLARE_TRACE_WRITABLE
+#define DECLARE_TRACE_WRITABLE(call, proto, args, size) \
+	__CHECK_WRITABLE_BUF_SIZE(call, PARAMS(proto), PARAMS(args), size) \
+	__BPF_DECLARE_TRACE(call##_tp, PARAMS(proto), PARAMS(args)) \
+	__DEFINE_EVENT(call##_tp, call##_tp, PARAMS(proto), PARAMS(args), size)
+
+#include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
+
+#undef DECLARE_TRACE_WRITABLE
+#undef DEFINE_EVENT_WRITABLE
+#undef __CHECK_WRITABLE_BUF_SIZE
+#undef __DEFINE_EVENT
+#undef FIRST
+
+#endif /* CONFIG_BPF_EVENTS */