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, 54 insertions, 0 deletions

diff --git a/tools/perf/util/iostat.c b/tools/perf/util/iostat.c
new file mode 100644
index 000000000..b770bd473
--- /dev/null
+++ b/tools/perf/util/iostat.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "util/iostat.h"
+#include "util/debug.h"
+
+enum iostat_mode_t iostat_mode = IOSTAT_NONE;
+
+__weak int iostat_prepare(struct evlist *evlist __maybe_unused,
+			  struct perf_stat_config *config __maybe_unused)
+{
+	return -1;
+}
+
+__weak int iostat_parse(const struct option *opt __maybe_unused,
+			 const char *str __maybe_unused,
+			 int unset __maybe_unused)
+{
+	pr_err("iostat mode is not supported on current platform\n");
+	return -1;
+}
+
+__weak void iostat_list(struct evlist *evlist __maybe_unused,
+		       struct perf_stat_config *config __maybe_unused)
+{
+}
+
+__weak void iostat_release(struct evlist *evlist __maybe_unused)
+{
+}
+
+__weak void iostat_print_header_prefix(struct perf_stat_config *config __maybe_unused)
+{
+}
+
+__weak void iostat_print_metric(struct perf_stat_config *config __maybe_unused,
+				struct evsel *evsel __maybe_unused,
+				struct perf_stat_output_ctx *out __maybe_unused)
+{
+}
+
+__weak void iostat_prefix(struct evlist *evlist __maybe_unused,
+			  struct perf_stat_config *config __maybe_unused,
+			  char *prefix __maybe_unused,
+			  struct timespec *ts __maybe_unused)
+{
+}
+
+__weak void iostat_print_counters(struct evlist *evlist __maybe_unused,
+				  struct perf_stat_config *config __maybe_unused,
+				  struct timespec *ts __maybe_unused,
+				  char *prefix __maybe_unused,
+				  iostat_print_counter_t print_cnt_cb __maybe_unused,
+				  void *arg __maybe_unused)
+{
+}