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, 118 insertions, 0 deletions

diff --git a/tools/perf/util/trace-event.c b/tools/perf/util/trace-event.c
new file mode 100644
index 000000000..6a8c66c64
--- /dev/null
+++ b/tools/perf/util/trace-event.c
@@ -0,0 +1,118 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <linux/kernel.h>
+#include <linux/err.h>
+#include <event-parse.h>
+#include <api/fs/tracing_path.h>
+#include <api/fs/fs.h>
+#include "trace-event.h"
+#include "machine.h"
+
+/*
+ * global trace_event object used by trace_event__tp_format
+ *
+ * TODO There's no cleanup call for this. Add some sort of
+ * __exit function support and call trace_event__cleanup
+ * there.
+ */
+static struct trace_event tevent;
+static bool tevent_initialized;
+
+int trace_event__init(struct trace_event *t)
+{
+	struct tep_handle *pevent = tep_alloc();
+
+	if (pevent) {
+		t->plugin_list = tep_load_plugins(pevent);
+		t->pevent  = pevent;
+	}
+
+	return pevent ? 0 : -1;
+}
+
+static int trace_event__init2(void)
+{
+	int be = tep_is_bigendian();
+	struct tep_handle *pevent;
+
+	if (trace_event__init(&tevent))
+		return -1;
+
+	pevent = tevent.pevent;
+	tep_set_flag(pevent, TEP_NSEC_OUTPUT);
+	tep_set_file_bigendian(pevent, be);
+	tep_set_local_bigendian(pevent, be);
+	tevent_initialized = true;
+	return 0;
+}
+
+int trace_event__register_resolver(struct machine *machine,
+				   tep_func_resolver_t *func)
+{
+	if (!tevent_initialized && trace_event__init2())
+		return -1;
+
+	return tep_set_function_resolver(tevent.pevent, func, machine);
+}
+
+void trace_event__cleanup(struct trace_event *t)
+{
+	tep_unload_plugins(t->plugin_list, t->pevent);
+	tep_free(t->pevent);
+}
+
+/*
+ * Returns pointer with encoded error via <linux/err.h> interface.
+ */
+static struct tep_event*
+tp_format(const char *sys, const char *name)
+{
+	char *tp_dir = get_events_file(sys);
+	struct tep_handle *pevent = tevent.pevent;
+	struct tep_event *event = NULL;
+	char path[PATH_MAX];
+	size_t size;
+	char *data;
+	int err;
+
+	if (!tp_dir)
+		return ERR_PTR(-errno);
+
+	scnprintf(path, PATH_MAX, "%s/%s/format", tp_dir, name);
+	put_events_file(tp_dir);
+
+	err = filename__read_str(path, &data, &size);
+	if (err)
+		return ERR_PTR(err);
+
+	tep_parse_format(pevent, &event, data, size, sys);
+
+	free(data);
+	return event;
+}
+
+/*
+ * Returns pointer with encoded error via <linux/err.h> interface.
+ */
+struct tep_event*
+trace_event__tp_format(const char *sys, const char *name)
+{
+	if (!tevent_initialized && trace_event__init2())
+		return ERR_PTR(-ENOMEM);
+
+	return tp_format(sys, name);
+}
+
+struct tep_event *trace_event__tp_format_id(int id)
+{
+	if (!tevent_initialized && trace_event__init2())
+		return ERR_PTR(-ENOMEM);
+
+	return tep_find_event(tevent.pevent, id);
+}