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, 198 insertions, 0 deletions

diff --git a/lib/kunit/backtrace-suppression-test.c b/lib/kunit/backtrace-suppression-test.c
new file mode 100644
index 000000000..7a2a59c6a
--- /dev/null
+++ b/lib/kunit/backtrace-suppression-test.c
@@ -0,0 +1,198 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit test for suppressing warning tracebacks.
+ *
+ * Copyright (C) 2024, Guenter Roeck
+ * Author: Guenter Roeck <linux@roeck-us.net>
+ */
+
+#include <kunit/test.h>
+#include <linux/bug.h>
+#include <linux/completion.h>
+#include <linux/kthread.h>
+
+static void backtrace_suppression_test_warn_direct(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		WARN(1, "This backtrace should be suppressed");
+		/*
+		 * Count must be checked inside the scope; the handle
+		 * is not accessible after the block exits.
+		 */
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+}
+
+static noinline void trigger_backtrace_warn(void)
+{
+	WARN(1, "This backtrace should be suppressed");
+}
+
+static void backtrace_suppression_test_warn_indirect(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		trigger_backtrace_warn();
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+}
+
+static void backtrace_suppression_test_warn_multi(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		WARN(1, "This backtrace should be suppressed");
+		trigger_backtrace_warn();
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 2);
+	}
+}
+
+static void backtrace_suppression_test_warn_on_direct(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+	if (!IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE) && !IS_ENABLED(CONFIG_KALLSYMS))
+		kunit_skip(test, "requires CONFIG_DEBUG_BUGVERBOSE or CONFIG_KALLSYMS");
+
+	kunit_warning_suppress(test) {
+		WARN_ON(1);
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+}
+
+static noinline void trigger_backtrace_warn_on(void)
+{
+	WARN_ON(1);
+}
+
+static void backtrace_suppression_test_warn_on_indirect(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+	if (!IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE))
+		kunit_skip(test, "requires CONFIG_DEBUG_BUGVERBOSE");
+
+	kunit_warning_suppress(test) {
+		trigger_backtrace_warn_on();
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+}
+
+static void backtrace_suppression_test_count(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 0);
+
+		WARN(1, "suppressed");
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+
+		WARN(1, "suppressed again");
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 2);
+	}
+}
+
+static void backtrace_suppression_test_active_state(struct kunit *test)
+{
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+
+	kunit_warning_suppress(test) {
+		KUNIT_EXPECT_TRUE(test, kunit_has_active_suppress_warning());
+	}
+
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+
+	kunit_warning_suppress(test) {
+		KUNIT_EXPECT_TRUE(test, kunit_has_active_suppress_warning());
+	}
+
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+}
+
+static void backtrace_suppression_test_multi_scope(struct kunit *test)
+{
+	struct kunit_suppressed_warning *sw1, *sw2;
+
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+	if (!IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE))
+		kunit_skip(test, "requires CONFIG_DEBUG_BUGVERBOSE");
+
+	sw1 = kunit_start_suppress_warning(test);
+	trigger_backtrace_warn_on();
+	WARN(1, "suppressed by sw1");
+	kunit_end_suppress_warning(test, sw1);
+
+	sw2 = kunit_start_suppress_warning(test);
+	WARN(1, "suppressed by sw2");
+	kunit_end_suppress_warning(test, sw2);
+
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw1), 2);
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw2), 1);
+}
+
+struct cross_kthread_data {
+	bool was_active;
+	struct completion done;
+};
+
+static int cross_kthread_fn(void *data)
+{
+	struct cross_kthread_data *d = data;
+
+	d->was_active = kunit_has_active_suppress_warning();
+	complete(&d->done);
+	while (!kthread_should_stop())
+		schedule();
+	return 0;
+}
+
+static void backtrace_suppression_test_cross_kthread(struct kunit *test)
+{
+	struct cross_kthread_data data;
+	struct task_struct *task;
+
+	data.was_active = false;
+	init_completion(&data.done);
+
+	kunit_warning_suppress(test) {
+		task = kthread_run(cross_kthread_fn, &data, "kunit-cross-test");
+		KUNIT_ASSERT_FALSE(test, IS_ERR(task));
+		wait_for_completion(&data.done);
+		kthread_stop(task);
+	}
+
+	KUNIT_EXPECT_FALSE(test, data.was_active);
+}
+
+static struct kunit_case backtrace_suppression_test_cases[] = {
+	KUNIT_CASE(backtrace_suppression_test_warn_direct),
+	KUNIT_CASE(backtrace_suppression_test_warn_indirect),
+	KUNIT_CASE(backtrace_suppression_test_warn_multi),
+	KUNIT_CASE(backtrace_suppression_test_warn_on_direct),
+	KUNIT_CASE(backtrace_suppression_test_warn_on_indirect),
+	KUNIT_CASE(backtrace_suppression_test_count),
+	KUNIT_CASE(backtrace_suppression_test_active_state),
+	KUNIT_CASE(backtrace_suppression_test_multi_scope),
+	KUNIT_CASE(backtrace_suppression_test_cross_kthread),
+	{}
+};
+
+static struct kunit_suite backtrace_suppression_test_suite = {
+	.name = "backtrace-suppression-test",
+	.test_cases = backtrace_suppression_test_cases,
+};
+kunit_test_suites(&backtrace_suppression_test_suite);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("KUnit test to verify warning backtrace suppression");