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, 124 insertions, 0 deletions

diff --git a/tools/perf/util/parse-regs-options.c b/tools/perf/util/parse-regs-options.c
new file mode 100644
index 000000000..c93c2f0c8
--- /dev/null
+++ b/tools/perf/util/parse-regs-options.c
@@ -0,0 +1,124 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <stdbool.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+#include <stdio.h>
+#include "util/debug.h"
+#include <dwarf-regs.h>
+#include <subcmd/parse-options.h>
+#include "util/perf_regs.h"
+#include "util/parse-regs-options.h"
+
+static void list_perf_regs(FILE *fp, uint64_t mask)
+{
+	const char *last_name = NULL;
+
+	fprintf(fp, "available registers: ");
+	for (int reg = 0; reg < 64; reg++) {
+		const char *name;
+
+		if (((1ULL << reg) & mask) == 0)
+			continue;
+
+		name = perf_reg_name(reg, EM_HOST, EF_HOST);
+		if (name && (!last_name || strcmp(last_name, name)))
+			fprintf(fp, "%s%s", reg > 0 ? " " : "", name);
+		last_name = name;
+	}
+	fputc('\n', fp);
+}
+
+static uint64_t name_to_perf_reg_mask(const char *to_match, uint64_t mask)
+{
+	uint64_t reg_mask = 0;
+
+	for (int reg = 0; reg < 64; reg++) {
+		const char *name;
+
+		if (((1ULL << reg) & mask) == 0)
+			continue;
+
+		name = perf_reg_name(reg, EM_HOST, EF_HOST);
+		if (!name)
+			continue;
+
+		if (!strcasecmp(to_match, name))
+			reg_mask |= 1ULL << reg;
+	}
+	return reg_mask;
+}
+
+static int
+__parse_regs(const struct option *opt, const char *str, int unset, bool intr)
+{
+	uint64_t *mode = (uint64_t *)opt->value;
+	char *s, *os = NULL, *p;
+	int ret = -1;
+	uint64_t mask;
+
+	if (unset)
+		return 0;
+
+	/*
+	 * cannot set it twice
+	 */
+	if (*mode)
+		return -1;
+
+	mask = intr ? perf_intr_reg_mask(EM_HOST) : perf_user_reg_mask(EM_HOST);
+
+	/* str may be NULL in case no arg is passed to -I */
+	if (!str) {
+		*mode = mask;
+		return 0;
+	}
+
+	/* because str is read-only */
+	s = os = strdup(str);
+	if (!s)
+		return -1;
+
+	for (;;) {
+		uint64_t reg_mask;
+
+		p = strchr(s, ',');
+		if (p)
+			*p = '\0';
+
+		if (!strcmp(s, "?")) {
+			list_perf_regs(stderr, mask);
+			goto error;
+		}
+
+		reg_mask = name_to_perf_reg_mask(s, mask);
+		if (reg_mask == 0) {
+			ui__warning("Unknown register \"%s\", check man page or run \"perf record %s?\"\n",
+				s, intr ? "-I" : "--user-regs=");
+			goto error;
+		}
+		*mode |= reg_mask;
+
+		if (!p)
+			break;
+
+		s = p + 1;
+	}
+	ret = 0;
+
+error:
+	free(os);
+	return ret;
+}
+
+int
+parse_user_regs(const struct option *opt, const char *str, int unset)
+{
+	return __parse_regs(opt, str, unset, false);
+}
+
+int
+parse_intr_regs(const struct option *opt, const char *str, int unset)
+{
+	return __parse_regs(opt, str, unset, true);
+}