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, 155 insertions, 0 deletions

diff --git a/drivers/cpufreq/cpufreq_userspace.c b/drivers/cpufreq/cpufreq_userspace.c
new file mode 100644
index 000000000..d738d3199
--- /dev/null
+++ b/drivers/cpufreq/cpufreq_userspace.c
@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ *  linux/drivers/cpufreq/cpufreq_userspace.c
+ *
+ *  Copyright (C)  2001 Russell King
+ *            (C)  2002 - 2004 Dominik Brodowski <linux@brodo.de>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/cpufreq.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/slab.h>
+
+struct userspace_policy {
+	unsigned int is_managed;
+	unsigned int setspeed;
+	struct mutex mutex;
+};
+
+/**
+ * cpufreq_set - set the CPU frequency
+ * @policy: pointer to policy struct where freq is being set
+ * @freq: target frequency in kHz
+ *
+ * Sets the CPU frequency to freq.
+ */
+static int cpufreq_set(struct cpufreq_policy *policy, unsigned int freq)
+{
+	int ret = -EINVAL;
+	struct userspace_policy *userspace = policy->governor_data;
+
+	pr_debug("cpufreq_set for cpu %u, freq %u kHz\n", policy->cpu, freq);
+
+	mutex_lock(&userspace->mutex);
+	if (!userspace->is_managed)
+		goto err;
+
+	userspace->setspeed = freq;
+
+	ret = __cpufreq_driver_target(policy, freq, CPUFREQ_RELATION_L);
+ err:
+	mutex_unlock(&userspace->mutex);
+	return ret;
+}
+
+static ssize_t show_speed(struct cpufreq_policy *policy, char *buf)
+{
+	struct userspace_policy *userspace = policy->governor_data;
+
+	return sprintf(buf, "%u\n", userspace->setspeed);
+}
+
+static int cpufreq_userspace_policy_init(struct cpufreq_policy *policy)
+{
+	struct userspace_policy *userspace;
+
+	userspace = kzalloc_obj(*userspace);
+	if (!userspace)
+		return -ENOMEM;
+
+	mutex_init(&userspace->mutex);
+
+	policy->governor_data = userspace;
+	return 0;
+}
+
+/*
+ * Any routine that writes to the policy struct will hold the "rwsem" of
+ * policy struct that means it is free to free "governor_data" here.
+ */
+static void cpufreq_userspace_policy_exit(struct cpufreq_policy *policy)
+{
+	kfree(policy->governor_data);
+	policy->governor_data = NULL;
+}
+
+static int cpufreq_userspace_policy_start(struct cpufreq_policy *policy)
+{
+	struct userspace_policy *userspace = policy->governor_data;
+
+	BUG_ON(!policy->cur);
+	pr_debug("started managing cpu %u\n", policy->cpu);
+
+	mutex_lock(&userspace->mutex);
+	userspace->is_managed = 1;
+	userspace->setspeed = policy->cur;
+	mutex_unlock(&userspace->mutex);
+	return 0;
+}
+
+static void cpufreq_userspace_policy_stop(struct cpufreq_policy *policy)
+{
+	struct userspace_policy *userspace = policy->governor_data;
+
+	pr_debug("managing cpu %u stopped\n", policy->cpu);
+
+	mutex_lock(&userspace->mutex);
+	userspace->is_managed = 0;
+	userspace->setspeed = 0;
+	mutex_unlock(&userspace->mutex);
+}
+
+static void cpufreq_userspace_policy_limits(struct cpufreq_policy *policy)
+{
+	struct userspace_policy *userspace = policy->governor_data;
+
+	mutex_lock(&userspace->mutex);
+
+	pr_debug("limit event for cpu %u: %u - %u kHz, currently %u kHz, last set to %u kHz\n",
+		 policy->cpu, policy->min, policy->max, policy->cur, userspace->setspeed);
+
+	if (policy->max < userspace->setspeed)
+		__cpufreq_driver_target(policy, policy->max,
+					CPUFREQ_RELATION_H);
+	else if (policy->min > userspace->setspeed)
+		__cpufreq_driver_target(policy, policy->min,
+					CPUFREQ_RELATION_L);
+	else
+		__cpufreq_driver_target(policy, userspace->setspeed,
+					CPUFREQ_RELATION_L);
+
+	mutex_unlock(&userspace->mutex);
+}
+
+static struct cpufreq_governor cpufreq_gov_userspace = {
+	.name		= "userspace",
+	.init		= cpufreq_userspace_policy_init,
+	.exit		= cpufreq_userspace_policy_exit,
+	.start		= cpufreq_userspace_policy_start,
+	.stop		= cpufreq_userspace_policy_stop,
+	.limits		= cpufreq_userspace_policy_limits,
+	.store_setspeed	= cpufreq_set,
+	.show_setspeed	= show_speed,
+	.owner		= THIS_MODULE,
+	.flags		= CPUFREQ_GOV_STRICT_TARGET,
+};
+
+MODULE_AUTHOR("Dominik Brodowski <linux@brodo.de>, "
+		"Russell King <rmk@arm.linux.org.uk>");
+MODULE_DESCRIPTION("CPUfreq policy governor 'userspace'");
+MODULE_LICENSE("GPL");
+
+#ifdef CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE
+struct cpufreq_governor *cpufreq_default_governor(void)
+{
+	return &cpufreq_gov_userspace;
+}
+#endif
+
+cpufreq_governor_init(cpufreq_gov_userspace);
+cpufreq_governor_exit(cpufreq_gov_userspace);