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, 144 insertions, 0 deletions

diff --git a/lib/tests/test_ratelimit.c b/lib/tests/test_ratelimit.c
new file mode 100644
index 000000000..33cea5f3d
--- /dev/null
+++ b/lib/tests/test_ratelimit.c
@@ -0,0 +1,144 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <kunit/test.h>
+
+#include <linux/ratelimit.h>
+#include <linux/module.h>
+#include <linux/kthread.h>
+#include <linux/cpumask.h>
+
+/* a simple boot-time regression test */
+
+#define TESTRL_INTERVAL (5 * HZ)
+static DEFINE_RATELIMIT_STATE(testrl, TESTRL_INTERVAL, 3);
+
+#define test_ratelimited(test, expected) \
+	KUNIT_ASSERT_EQ(test, ___ratelimit(&testrl, "test_ratelimit_smoke"), (expected))
+
+static void test_ratelimit_smoke(struct kunit *test)
+{
+	// Check settings.
+	KUNIT_ASSERT_GE(test, TESTRL_INTERVAL, 100);
+
+	// Test normal operation.
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, false);
+
+	schedule_timeout_idle(TESTRL_INTERVAL / 2);
+	test_ratelimited(test, false);
+
+	schedule_timeout_idle(TESTRL_INTERVAL * 3 / 4);
+	test_ratelimited(test, true);
+
+	schedule_timeout_idle(2 * TESTRL_INTERVAL);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+
+	schedule_timeout_idle(TESTRL_INTERVAL / 2 );
+	test_ratelimited(test, true);
+	schedule_timeout_idle(TESTRL_INTERVAL * 3 / 4);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, false);
+
+	// Test disabling.
+	testrl.burst = 0;
+	test_ratelimited(test, false);
+	testrl.burst = 2;
+	testrl.interval = 0;
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+
+	// Testing re-enabling.
+	testrl.interval = TESTRL_INTERVAL;
+	test_ratelimited(test, true);
+	test_ratelimited(test, true);
+	test_ratelimited(test, false);
+	test_ratelimited(test, false);
+}
+
+static struct ratelimit_state stressrl = RATELIMIT_STATE_INIT_FLAGS("stressrl", HZ / 10, 3,
+								    RATELIMIT_MSG_ON_RELEASE);
+
+static int doneflag;
+static const int stress_duration = 2 * HZ;
+
+struct stress_kthread {
+	unsigned long nattempts;
+	unsigned long nunlimited;
+	unsigned long nlimited;
+	unsigned long nmissed;
+	struct task_struct *tp;
+};
+
+static int test_ratelimit_stress_child(void *arg)
+{
+	struct stress_kthread *sktp = arg;
+
+	set_user_nice(current, MAX_NICE);
+	WARN_ON_ONCE(!sktp->tp);
+
+	while (!READ_ONCE(doneflag)) {
+		sktp->nattempts++;
+		if (___ratelimit(&stressrl, __func__))
+			sktp->nunlimited++;
+		else
+			sktp->nlimited++;
+		cond_resched();
+	}
+
+	sktp->nmissed = ratelimit_state_reset_miss(&stressrl);
+	return 0;
+}
+
+static void test_ratelimit_stress(struct kunit *test)
+{
+	int i;
+	const int n_stress_kthread = cpumask_weight(cpu_online_mask);
+	struct stress_kthread skt = { 0 };
+	struct stress_kthread *sktp = kzalloc_objs(*sktp, n_stress_kthread);
+
+	KUNIT_EXPECT_NOT_NULL_MSG(test, sktp, "Memory allocation failure");
+	for (i = 0; i < n_stress_kthread; i++) {
+		sktp[i].tp = kthread_run(test_ratelimit_stress_child, &sktp[i], "%s/%i",
+					 "test_ratelimit_stress_child", i);
+		KUNIT_EXPECT_NOT_NULL_MSG(test, sktp, "kthread creation failure");
+		pr_alert("Spawned test_ratelimit_stress_child %d\n", i);
+	}
+	schedule_timeout_idle(stress_duration);
+	WRITE_ONCE(doneflag, 1);
+	for (i = 0; i < n_stress_kthread; i++) {
+		kthread_stop(sktp[i].tp);
+		skt.nattempts += sktp[i].nattempts;
+		skt.nunlimited += sktp[i].nunlimited;
+		skt.nlimited += sktp[i].nlimited;
+		skt.nmissed += sktp[i].nmissed;
+	}
+	KUNIT_ASSERT_EQ_MSG(test, skt.nunlimited + skt.nlimited, skt.nattempts,
+			    "Outcomes not equal to attempts");
+	KUNIT_ASSERT_EQ_MSG(test, skt.nlimited, skt.nmissed, "Misses not equal to limits");
+}
+
+static struct kunit_case ratelimit_test_cases[] = {
+	KUNIT_CASE_SLOW(test_ratelimit_smoke),
+	KUNIT_CASE_SLOW(test_ratelimit_stress),
+	{}
+};
+
+static struct kunit_suite ratelimit_test_suite = {
+	.name = "lib_ratelimit",
+	.test_cases = ratelimit_test_cases,
+};
+
+kunit_test_suites(&ratelimit_test_suite);
+
+MODULE_DESCRIPTION("___ratelimit() KUnit test suite");
+MODULE_LICENSE("GPL");