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, 104 insertions, 0 deletions

diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c
new file mode 100644
index 000000000..4ed4b1d55
--- /dev/null
+++ b/tools/perf/util/unwind.c
@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "debug.h"
+#include "symbol_conf.h"
+#include "unwind.h"
+#include <linux/string.h>
+#include <string.h>
+#include <stdlib.h>
+
+int unwind__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)
+{
+	int ret = 0;
+
+#if defined(HAVE_LIBDW_SUPPORT) || defined(HAVE_LIBUNWIND_SUPPORT)
+	if (symbol_conf.unwind_style[0] == UNWIND_STYLE_UNKNOWN) {
+		int i = 0;
+#ifdef HAVE_LIBDW_SUPPORT
+		symbol_conf.unwind_style[i++] = UNWIND_STYLE_LIBDW;
+#endif
+#ifdef HAVE_LIBUNWIND_SUPPORT
+		symbol_conf.unwind_style[i++] = UNWIND_STYLE_LIBUNWIND;
+#endif
+	}
+#endif //defined(HAVE_LIBDW_SUPPORT) || defined(HAVE_LIBUNWIND_SUPPORT)
+
+	for (size_t i = 0; i < ARRAY_SIZE(symbol_conf.unwind_style); i++) {
+		switch (symbol_conf.unwind_style[i]) {
+		case UNWIND_STYLE_LIBDW:
+			ret = libdw__get_entries(cb, arg, thread, data, max_stack, best_effort);
+			break;
+		case UNWIND_STYLE_LIBUNWIND:
+			ret = libunwind__get_entries(cb, arg, thread, data, max_stack, best_effort);
+			break;
+		case UNWIND_STYLE_UNKNOWN:
+		default:
+#if !defined(HAVE_LIBDW_SUPPORT) && !defined(HAVE_LIBUNWIND_SUPPORT)
+			pr_warning_once(
+				"Error: dwarf unwinding not supported, build perf with libdw or libunwind.\n");
+#endif
+			ret = 0;
+			break;
+		}
+		if (ret > 0) {
+			ret = 0;
+			break;
+		}
+		if (ret < 0)
+			break;
+	}
+	return ret;
+}
+
+int unwind__configure(const char *var, const char *value, void *cb __maybe_unused)
+{
+	static const char * const unwind_style_names[] = {
+		[UNWIND_STYLE_LIBDW] = "libdw",
+		[UNWIND_STYLE_LIBUNWIND] = "libunwind",
+		NULL
+	};
+	char *s, *p, *saveptr;
+	size_t i = 0;
+
+	if (strcmp(var, "unwind.style"))
+		return 0;
+
+	if (!value)
+		return -1;
+
+	s = strdup(value);
+	if (!s)
+		return -1;
+
+	memset(symbol_conf.unwind_style, 0, sizeof(symbol_conf.unwind_style));
+
+	p = strtok_r(s, ",", &saveptr);
+	while (p && i < ARRAY_SIZE(symbol_conf.unwind_style)) {
+		bool found = false;
+		char *q = strim(p);
+
+		for (size_t j = UNWIND_STYLE_LIBDW; j < MAX_UNWIND_STYLE; j++) {
+			if (!strcasecmp(q, unwind_style_names[j])) {
+				symbol_conf.unwind_style[i++] = j;
+				found = true;
+				break;
+			}
+		}
+		if (!found)
+			pr_warning("Unknown unwind style: %s\n", q);
+		p = strtok_r(NULL, ",", &saveptr);
+	}
+
+	free(s);
+	return 0;
+}
+
+int unwind__option(const struct option *opt __maybe_unused,
+		   const char *arg,
+		   int unset __maybe_unused)
+{
+	return unwind__configure("unwind.style", arg, NULL);
+}