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, 85 insertions, 0 deletions

diff --git a/tools/perf/util/unwind.h b/tools/perf/util/unwind.h
new file mode 100644
index 000000000..a2d4af694
--- /dev/null
+++ b/tools/perf/util/unwind.h
@@ -0,0 +1,85 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __UNWIND_H
+#define __UNWIND_H
+
+#include <stdint.h>
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include "map_symbol.h"
+
+struct maps;
+struct option;
+struct perf_sample;
+struct thread;
+
+struct unwind_entry {
+	struct map_symbol ms;
+	u64		  ip;
+};
+
+typedef int (*unwind_entry_cb_t)(struct unwind_entry *entry, void *arg);
+
+int unwind__configure(const char *var, const char *value, void *cb);
+int unwind__option(const struct option *opt, const char *arg, int unset);
+
+/*
+ * When best_effort is set, don't report errors and fail silently. This could
+ * be expanded in the future to be more permissive about things other than
+ * error messages.
+ */
+int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
+			struct thread *thread,
+			struct perf_sample *data, int max_stack,
+			bool best_effort);
+
+#ifdef HAVE_LIBDW_SUPPORT
+int libdw__get_entries(unwind_entry_cb_t cb, void *arg,
+		       struct thread *thread,
+		       struct perf_sample *data, int max_stack,
+		       bool best_effort);
+#else
+#include "debug.h"
+static inline int libdw__get_entries(unwind_entry_cb_t cb __maybe_unused, void *arg __maybe_unused,
+				     struct thread *thread __maybe_unused,
+				     struct perf_sample *data __maybe_unused,
+				     int max_stack __maybe_unused,
+				     bool best_effort __maybe_unused)
+{
+	pr_warning_once("Error: libdw dwarf unwinding not built into perf\n");
+	return 0;
+}
+#endif
+
+#ifdef HAVE_LIBUNWIND_SUPPORT
+/* libunwind specific */
+int libunwind__get_entries(unwind_entry_cb_t cb, void *arg,
+			   struct thread *thread,
+			   struct perf_sample *data, int max_stack,
+			   bool best_effort);
+int unwind__prepare_access(struct maps *maps, uint16_t e_machine);
+void unwind__flush_access(struct maps *maps);
+void unwind__finish_access(struct maps *maps);
+#else
+#include "debug.h"
+static inline int libunwind__get_entries(unwind_entry_cb_t cb __maybe_unused,
+					 void *arg __maybe_unused,
+					 struct thread *thread __maybe_unused,
+					 struct perf_sample *data __maybe_unused,
+					 int max_stack __maybe_unused,
+					 bool best_effort __maybe_unused)
+{
+	pr_warning_once("Error: libunwind dwarf unwinding not built into perf\n");
+	return 0;
+}
+
+static inline int unwind__prepare_access(struct maps *maps __maybe_unused,
+					 uint16_t e_machine __maybe_unused)
+{
+	return 0;
+}
+
+static inline void unwind__flush_access(struct maps *maps __maybe_unused) {}
+static inline void unwind__finish_access(struct maps *maps __maybe_unused) {}
+#endif
+
+#endif /* __UNWIND_H */