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, 289 insertions, 0 deletions

diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
new file mode 100644
index 000000000..670a10c3e
--- /dev/null
+++ b/drivers/cpufreq/cpufreq_stats.c
@@ -0,0 +1,289 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  drivers/cpufreq/cpufreq_stats.c
+ *
+ *  Copyright (C) 2003-2004 Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>.
+ *  (C) 2004 Zou Nan hai <nanhai.zou@intel.com>.
+ */
+
+#include <linux/cpu.h>
+#include <linux/cpufreq.h>
+#include <linux/module.h>
+#include <linux/sched/clock.h>
+#include <linux/slab.h>
+
+struct cpufreq_stats {
+	unsigned int total_trans;
+	unsigned long long last_time;
+	unsigned int max_state;
+	unsigned int state_num;
+	unsigned int last_index;
+	u64 *time_in_state;
+	unsigned int *freq_table;
+	unsigned int *trans_table;
+
+	/* Deferred reset */
+	unsigned int reset_pending;
+	unsigned long long reset_time;
+};
+
+static void cpufreq_stats_update(struct cpufreq_stats *stats,
+				 unsigned long long time)
+{
+	unsigned long long cur_time = local_clock();
+
+	stats->time_in_state[stats->last_index] += cur_time - time;
+	stats->last_time = cur_time;
+}
+
+static void cpufreq_stats_reset_table(struct cpufreq_stats *stats)
+{
+	unsigned int count = stats->max_state;
+
+	memset(stats->time_in_state, 0, count * sizeof(u64));
+	memset(stats->trans_table, 0, count * count * sizeof(int));
+	stats->last_time = local_clock();
+	stats->total_trans = 0;
+
+	/* Adjust for the time elapsed since reset was requested */
+	WRITE_ONCE(stats->reset_pending, 0);
+	/*
+	 * Prevent the reset_time read from being reordered before the
+	 * reset_pending accesses in cpufreq_stats_record_transition().
+	 */
+	smp_rmb();
+	cpufreq_stats_update(stats, READ_ONCE(stats->reset_time));
+}
+
+static ssize_t show_total_trans(struct cpufreq_policy *policy, char *buf)
+{
+	struct cpufreq_stats *stats = policy->stats;
+
+	if (READ_ONCE(stats->reset_pending))
+		return sprintf(buf, "%d\n", 0);
+	else
+		return sprintf(buf, "%u\n", stats->total_trans);
+}
+cpufreq_freq_attr_ro(total_trans);
+
+static ssize_t show_time_in_state(struct cpufreq_policy *policy, char *buf)
+{
+	struct cpufreq_stats *stats = policy->stats;
+	bool pending = READ_ONCE(stats->reset_pending);
+	unsigned long long time;
+	ssize_t len = 0;
+	int i;
+
+	for (i = 0; i < stats->state_num; i++) {
+		if (pending) {
+			if (i == stats->last_index) {
+				/*
+				 * Prevent the reset_time read from occurring
+				 * before the reset_pending read above.
+				 */
+				smp_rmb();
+				time = local_clock() - READ_ONCE(stats->reset_time);
+			} else {
+				time = 0;
+			}
+		} else {
+			time = stats->time_in_state[i];
+			if (i == stats->last_index)
+				time += local_clock() - stats->last_time;
+		}
+
+		len += sprintf(buf + len, "%u %llu\n", stats->freq_table[i],
+			       nsec_to_clock_t(time));
+	}
+	return len;
+}
+cpufreq_freq_attr_ro(time_in_state);
+
+/* We don't care what is written to the attribute */
+static ssize_t store_reset(struct cpufreq_policy *policy, const char *buf,
+			   size_t count)
+{
+	struct cpufreq_stats *stats = policy->stats;
+
+	/*
+	 * Defer resetting of stats to cpufreq_stats_record_transition() to
+	 * avoid races.
+	 */
+	WRITE_ONCE(stats->reset_time, local_clock());
+	/*
+	 * The memory barrier below is to prevent the readers of reset_time from
+	 * seeing a stale or partially updated value.
+	 */
+	smp_wmb();
+	WRITE_ONCE(stats->reset_pending, 1);
+
+	return count;
+}
+cpufreq_freq_attr_wo(reset);
+
+static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
+{
+	struct cpufreq_stats *stats = policy->stats;
+	bool pending = READ_ONCE(stats->reset_pending);
+	ssize_t len = 0;
+	int i, j, count;
+
+	len += sysfs_emit_at(buf, len, "   From  :    To\n");
+	len += sysfs_emit_at(buf, len, "         : ");
+	for (i = 0; i < stats->state_num; i++) {
+		if (len >= PAGE_SIZE - 1)
+			break;
+		len += sysfs_emit_at(buf, len, "%9u ", stats->freq_table[i]);
+	}
+	if (len >= PAGE_SIZE - 1)
+		return PAGE_SIZE - 1;
+
+	len += sysfs_emit_at(buf, len, "\n");
+
+	for (i = 0; i < stats->state_num; i++) {
+		if (len >= PAGE_SIZE - 1)
+			break;
+
+		len += sysfs_emit_at(buf, len, "%9u: ", stats->freq_table[i]);
+
+		for (j = 0; j < stats->state_num; j++) {
+			if (len >= PAGE_SIZE - 1)
+				break;
+
+			if (pending)
+				count = 0;
+			else
+				count = stats->trans_table[i * stats->max_state + j];
+
+			len += sysfs_emit_at(buf, len, "%9u ", count);
+		}
+		if (len >= PAGE_SIZE - 1)
+			break;
+		len += sysfs_emit_at(buf, len, "\n");
+	}
+
+	if (len >= PAGE_SIZE - 1) {
+		pr_warn_once("cpufreq transition table exceeds PAGE_SIZE. Disabling\n");
+		return -EFBIG;
+	}
+	return len;
+}
+cpufreq_freq_attr_ro(trans_table);
+
+static struct attribute *default_attrs[] = {
+	&total_trans.attr,
+	&time_in_state.attr,
+	&reset.attr,
+	&trans_table.attr,
+	NULL
+};
+static const struct attribute_group stats_attr_group = {
+	.attrs = default_attrs,
+	.name = "stats"
+};
+
+static int freq_table_get_index(struct cpufreq_stats *stats, unsigned int freq)
+{
+	int index;
+	for (index = 0; index < stats->max_state; index++)
+		if (stats->freq_table[index] == freq)
+			return index;
+	return -1;
+}
+
+void cpufreq_stats_free_table(struct cpufreq_policy *policy)
+{
+	struct cpufreq_stats *stats = policy->stats;
+
+	/* Already freed */
+	if (!stats)
+		return;
+
+	pr_debug("%s: Free stats table\n", __func__);
+
+	sysfs_remove_group(&policy->kobj, &stats_attr_group);
+	kfree(stats->time_in_state);
+	kfree(stats);
+	policy->stats = NULL;
+}
+
+void cpufreq_stats_create_table(struct cpufreq_policy *policy)
+{
+	unsigned int i = 0, count;
+	struct cpufreq_stats *stats;
+	unsigned int alloc_size;
+	struct cpufreq_frequency_table *pos;
+
+	count = cpufreq_table_count_valid_entries(policy);
+	if (!count)
+		return;
+
+	/* stats already initialized */
+	if (policy->stats)
+		return;
+
+	stats = kzalloc_obj(*stats);
+	if (!stats)
+		return;
+
+	alloc_size = count * sizeof(int) + count * sizeof(u64);
+
+	alloc_size += count * count * sizeof(int);
+
+	/* Allocate memory for time_in_state/freq_table/trans_table in one go */
+	stats->time_in_state = kzalloc(alloc_size, GFP_KERNEL);
+	if (!stats->time_in_state)
+		goto free_stat;
+
+	stats->freq_table = (unsigned int *)(stats->time_in_state + count);
+
+	stats->trans_table = stats->freq_table + count;
+
+	stats->max_state = count;
+
+	/* Find valid-unique entries */
+	cpufreq_for_each_valid_entry(pos, policy->freq_table)
+		if (policy->freq_table_sorted != CPUFREQ_TABLE_UNSORTED ||
+		    freq_table_get_index(stats, pos->frequency) == -1)
+			stats->freq_table[i++] = pos->frequency;
+
+	stats->state_num = i;
+	stats->last_time = local_clock();
+	stats->last_index = freq_table_get_index(stats, policy->cur);
+
+	policy->stats = stats;
+	if (!sysfs_create_group(&policy->kobj, &stats_attr_group))
+		return;
+
+	/* We failed, release resources */
+	policy->stats = NULL;
+	kfree(stats->time_in_state);
+free_stat:
+	kfree(stats);
+}
+
+void cpufreq_stats_record_transition(struct cpufreq_policy *policy,
+				     unsigned int new_freq)
+{
+	struct cpufreq_stats *stats = policy->stats;
+	int old_index, new_index;
+
+	if (unlikely(!stats))
+		return;
+
+	if (unlikely(READ_ONCE(stats->reset_pending)))
+		cpufreq_stats_reset_table(stats);
+
+	old_index = stats->last_index;
+	new_index = freq_table_get_index(stats, new_freq);
+
+	/* We can't do stats->time_in_state[-1]= .. */
+	if (unlikely(old_index == -1 || new_index == -1 || old_index == new_index))
+		return;
+
+	cpufreq_stats_update(stats, stats->last_time);
+
+	stats->last_index = new_index;
+	stats->trans_table[old_index * stats->max_state + new_index]++;
+	stats->total_trans++;
+}