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, 229 insertions, 0 deletions

diff --git a/tools/perf/util/branch.c b/tools/perf/util/branch.c
new file mode 100644
index 000000000..3712be067
--- /dev/null
+++ b/tools/perf/util/branch.c
@@ -0,0 +1,229 @@
+#include "util/map_symbol.h"
+#include "util/branch.h"
+#include <linux/kernel.h>
+
+static bool cross_area(u64 addr1, u64 addr2, int size)
+{
+	u64 align1, align2;
+
+	align1 = addr1 & ~(size - 1);
+	align2 = addr2 & ~(size - 1);
+
+	return (align1 != align2) ? true : false;
+}
+
+#define AREA_4K		4096
+#define AREA_2M		(2 * 1024 * 1024)
+
+void branch_type_count(struct branch_type_stat *st, struct branch_flags *flags,
+		       u64 from, u64 to)
+{
+	if (flags->type == PERF_BR_UNKNOWN || from == 0)
+		return;
+
+	if (flags->type == PERF_BR_EXTEND_ABI)
+		st->new_counts[flags->new_type]++;
+	else
+		st->counts[flags->type]++;
+
+	if (flags->type == PERF_BR_COND) {
+		if (to > from)
+			st->cond_fwd++;
+		else
+			st->cond_bwd++;
+	}
+
+	if (cross_area(from, to, AREA_2M))
+		st->cross_2m++;
+	else if (cross_area(from, to, AREA_4K))
+		st->cross_4k++;
+}
+
+const char *branch_new_type_name(int new_type)
+{
+	const char *branch_new_names[PERF_BR_NEW_MAX] = {
+		"FAULT_ALGN",
+		"FAULT_DATA",
+		"FAULT_INST",
+/*
+ * TODO: This switch should happen on 'perf_session__env(session)->arch'
+ * instead, because an arm64 platform perf recording could be
+ * opened for analysis on other platforms as well.
+ */
+#ifdef __aarch64__
+		"ARM64_FIQ",
+		"ARM64_DEBUG_HALT",
+		"ARM64_DEBUG_EXIT",
+		"ARM64_DEBUG_INST",
+		"ARM64_DEBUG_DATA"
+#else
+		"ARCH_1",
+		"ARCH_2",
+		"ARCH_3",
+		"ARCH_4",
+		"ARCH_5"
+#endif
+	};
+
+	if (new_type >= 0 && new_type < PERF_BR_NEW_MAX)
+		return branch_new_names[new_type];
+
+	return NULL;
+}
+
+const char *branch_type_name(int type)
+{
+	const char *branch_names[PERF_BR_MAX] = {
+		"N/A",
+		"COND",
+		"UNCOND",
+		"IND",
+		"CALL",
+		"IND_CALL",
+		"RET",
+		"SYSCALL",
+		"SYSRET",
+		"COND_CALL",
+		"COND_RET",
+		"ERET",
+		"IRQ",
+		"SERROR",
+		"NO_TX",
+		"", // Needed for PERF_BR_EXTEND_ABI that ends up triggering some compiler warnings about NULL deref
+	};
+
+	if (type >= 0 && type < PERF_BR_MAX)
+		return branch_names[type];
+
+	return NULL;
+}
+
+const char *get_branch_type(struct branch_entry *e)
+{
+	if (e->flags.type == PERF_BR_UNKNOWN)
+		return "";
+
+	if (e->flags.type == PERF_BR_EXTEND_ABI)
+		return branch_new_type_name(e->flags.new_type);
+
+	return branch_type_name(e->flags.type);
+}
+
+void branch_type_stat_display(FILE *fp, const struct branch_type_stat *st)
+{
+	u64 total = 0;
+	int i;
+
+	for (i = 0; i < PERF_BR_MAX; i++)
+		total += st->counts[i];
+
+	if (total == 0)
+		return;
+
+	fprintf(fp, "\n#");
+	fprintf(fp, "\n# Branch Statistics:");
+	fprintf(fp, "\n#");
+
+	if (st->cond_fwd > 0) {
+		fprintf(fp, "\n%8s: %5.1f%%",
+			"COND_FWD",
+			100.0 * (double)st->cond_fwd / (double)total);
+	}
+
+	if (st->cond_bwd > 0) {
+		fprintf(fp, "\n%8s: %5.1f%%",
+			"COND_BWD",
+			100.0 * (double)st->cond_bwd / (double)total);
+	}
+
+	if (st->cross_4k > 0) {
+		fprintf(fp, "\n%8s: %5.1f%%",
+			"CROSS_4K",
+			100.0 * (double)st->cross_4k / (double)total);
+	}
+
+	if (st->cross_2m > 0) {
+		fprintf(fp, "\n%8s: %5.1f%%",
+			"CROSS_2M",
+			100.0 * (double)st->cross_2m / (double)total);
+	}
+
+	for (i = 0; i < PERF_BR_MAX; i++) {
+		if (st->counts[i] > 0)
+			fprintf(fp, "\n%8s: %5.1f%%",
+				branch_type_name(i),
+				100.0 *
+				(double)st->counts[i] / (double)total);
+	}
+
+	for (i = 0; i < PERF_BR_NEW_MAX; i++) {
+		if (st->new_counts[i] > 0)
+			fprintf(fp, "\n%8s: %5.1f%%",
+				branch_new_type_name(i),
+				100.0 *
+				(double)st->new_counts[i] / (double)total);
+	}
+
+}
+
+static int count_str_scnprintf(int idx, const char *str, char *bf, int size)
+{
+	return scnprintf(bf, size, "%s%s", (idx) ? " " : " (", str);
+}
+
+int branch_type_str(const struct branch_type_stat *st, char *bf, int size)
+{
+	int i, j = 0, printed = 0;
+	u64 total = 0;
+
+	for (i = 0; i < PERF_BR_MAX; i++)
+		total += st->counts[i];
+
+	for (i = 0; i < PERF_BR_NEW_MAX; i++)
+		total += st->new_counts[i];
+
+	if (total == 0)
+		return 0;
+
+	if (st->cond_fwd > 0)
+		printed += count_str_scnprintf(j++, "COND_FWD", bf + printed, size - printed);
+
+	if (st->cond_bwd > 0)
+		printed += count_str_scnprintf(j++, "COND_BWD", bf + printed, size - printed);
+
+	for (i = 0; i < PERF_BR_MAX; i++) {
+		if (i == PERF_BR_COND)
+			continue;
+
+		if (st->counts[i] > 0)
+			printed += count_str_scnprintf(j++, branch_type_name(i), bf + printed, size - printed);
+	}
+
+	for (i = 0; i < PERF_BR_NEW_MAX; i++) {
+		if (st->new_counts[i] > 0)
+			printed += count_str_scnprintf(j++, branch_new_type_name(i), bf + printed, size - printed);
+	}
+
+	if (st->cross_4k > 0)
+		printed += count_str_scnprintf(j++, "CROSS_4K", bf + printed, size - printed);
+
+	if (st->cross_2m > 0)
+		printed += count_str_scnprintf(j++, "CROSS_2M", bf + printed, size - printed);
+
+	return printed;
+}
+
+const char *branch_spec_desc(int spec)
+{
+	const char *branch_spec_outcomes[PERF_BR_SPEC_MAX] = {
+		"N/A",
+		"SPEC_WRONG_PATH",
+		"NON_SPEC_CORRECT_PATH",
+		"SPEC_CORRECT_PATH",
+	};
+
+	if (spec >= 0 && spec < PERF_BR_SPEC_MAX)
+		return branch_spec_outcomes[spec];
+
+	return NULL;
+}