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, 47 insertions, 0 deletions

diff --git a/drivers/perf/arm_brbe.h b/drivers/perf/arm_brbe.h
new file mode 100644
index 000000000..b7c7d8796
--- /dev/null
+++ b/drivers/perf/arm_brbe.h
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Branch Record Buffer Extension Helpers.
+ *
+ * Copyright (C) 2022-2025 ARM Limited
+ *
+ * Author: Anshuman Khandual <anshuman.khandual@arm.com>
+ */
+
+struct arm_pmu;
+struct perf_branch_stack;
+struct perf_event;
+
+#ifdef CONFIG_ARM64_BRBE
+void brbe_probe(struct arm_pmu *arm_pmu);
+unsigned int brbe_num_branch_records(const struct arm_pmu *armpmu);
+void brbe_invalidate(void);
+
+void brbe_enable(const struct arm_pmu *arm_pmu);
+void brbe_disable(void);
+
+bool brbe_branch_attr_valid(struct perf_event *event);
+void brbe_read_filtered_entries(struct perf_branch_stack *branch_stack,
+				const struct perf_event *event);
+#else
+static inline void brbe_probe(struct arm_pmu *arm_pmu) { }
+static inline unsigned int brbe_num_branch_records(const struct arm_pmu *armpmu)
+{
+	return 0;
+}
+
+static inline void brbe_invalidate(void) { }
+
+static inline void brbe_enable(const struct arm_pmu *arm_pmu) { };
+static inline void brbe_disable(void) { };
+
+static inline bool brbe_branch_attr_valid(struct perf_event *event)
+{
+	WARN_ON_ONCE(!has_branch_stack(event));
+	return false;
+}
+
+static void brbe_read_filtered_entries(struct perf_branch_stack *branch_stack,
+				       const struct perf_event *event)
+{
+}
+#endif