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, 411 insertions, 0 deletions

diff --git a/lib/tests/test_kprobes.c b/lib/tests/test_kprobes.c
new file mode 100644
index 000000000..06e729e4d
--- /dev/null
+++ b/lib/tests/test_kprobes.c
@@ -0,0 +1,411 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * test_kprobes.c - simple sanity test for k*probes
+ *
+ * Copyright IBM Corp. 2008
+ */
+
+#include <linux/kernel.h>
+#include <linux/kprobes.h>
+#include <linux/random.h>
+#include <kunit/test.h>
+
+#define div_factor 3
+
+#define KP_CLEAR(_kp) \
+do { \
+	(_kp).addr = NULL; \
+	(_kp).flags = 0; \
+} while (0)
+
+static u32 rand1, preh_val, posth_val;
+static u32 (*target)(u32 value);
+static u32 (*recursed_target)(u32 value);
+static u32 (*target2)(u32 value);
+static struct kunit *current_test;
+
+static unsigned long (*internal_target)(void);
+static unsigned long (*stacktrace_target)(void);
+static unsigned long (*stacktrace_driver)(void);
+static unsigned long target_return_address[2];
+
+static noinline u32 kprobe_target(u32 value)
+{
+	return (value / div_factor);
+}
+
+static noinline u32 kprobe_recursed_target(u32 value)
+{
+	return (value / div_factor);
+}
+
+static int kp_pre_handler(struct kprobe *p, struct pt_regs *regs)
+{
+	KUNIT_EXPECT_FALSE(current_test, preemptible());
+
+	preh_val = recursed_target(rand1);
+	return 0;
+}
+
+static void kp_post_handler(struct kprobe *p, struct pt_regs *regs,
+		unsigned long flags)
+{
+	u32 expval = recursed_target(rand1);
+
+	KUNIT_EXPECT_FALSE(current_test, preemptible());
+	KUNIT_EXPECT_EQ(current_test, preh_val, expval);
+
+	posth_val = preh_val + div_factor;
+}
+
+static struct kprobe kp = {
+	.symbol_name = "kprobe_target",
+	.pre_handler = kp_pre_handler,
+	.post_handler = kp_post_handler
+};
+
+static void test_kprobe(struct kunit *test)
+{
+	current_test = test;
+	KUNIT_EXPECT_EQ(test, 0, register_kprobe(&kp));
+	target(rand1);
+	unregister_kprobe(&kp);
+	KUNIT_EXPECT_NE(test, 0, preh_val);
+	KUNIT_EXPECT_NE(test, 0, posth_val);
+}
+
+static noinline u32 kprobe_target2(u32 value)
+{
+	return (value / div_factor) + 1;
+}
+
+static noinline unsigned long kprobe_stacktrace_internal_target(void)
+{
+	if (!target_return_address[0])
+		target_return_address[0] = (unsigned long)__builtin_return_address(0);
+	return target_return_address[0];
+}
+
+static noinline unsigned long kprobe_stacktrace_target(void)
+{
+	if (!target_return_address[1])
+		target_return_address[1] = (unsigned long)__builtin_return_address(0);
+
+	if (internal_target)
+		internal_target();
+
+	return target_return_address[1];
+}
+
+static noinline unsigned long kprobe_stacktrace_driver(void)
+{
+	if (stacktrace_target)
+		stacktrace_target();
+
+	/* This is for preventing inlining the function */
+	return (unsigned long)__builtin_return_address(0);
+}
+
+static int kp_pre_handler2(struct kprobe *p, struct pt_regs *regs)
+{
+	preh_val = (rand1 / div_factor) + 1;
+	return 0;
+}
+
+static void kp_post_handler2(struct kprobe *p, struct pt_regs *regs,
+		unsigned long flags)
+{
+	KUNIT_EXPECT_EQ(current_test, preh_val, (rand1 / div_factor) + 1);
+	posth_val = preh_val + div_factor;
+}
+
+static struct kprobe kp2 = {
+	.symbol_name = "kprobe_target2",
+	.pre_handler = kp_pre_handler2,
+	.post_handler = kp_post_handler2
+};
+
+static void test_kprobes(struct kunit *test)
+{
+	struct kprobe *kps[2] = {&kp, &kp2};
+
+	current_test = test;
+
+	KUNIT_EXPECT_EQ(test, 0, register_kprobes(kps, 2));
+	preh_val = 0;
+	posth_val = 0;
+	target(rand1);
+
+	KUNIT_EXPECT_NE(test, 0, preh_val);
+	KUNIT_EXPECT_NE(test, 0, posth_val);
+
+	preh_val = 0;
+	posth_val = 0;
+	target2(rand1);
+
+	KUNIT_EXPECT_NE(test, 0, preh_val);
+	KUNIT_EXPECT_NE(test, 0, posth_val);
+	unregister_kprobes(kps, 2);
+}
+
+static struct kprobe kp_missed = {
+	.symbol_name = "kprobe_recursed_target",
+	.pre_handler = kp_pre_handler,
+	.post_handler = kp_post_handler,
+};
+
+static void test_kprobe_missed(struct kunit *test)
+{
+	current_test = test;
+	preh_val = 0;
+	posth_val = 0;
+
+	KUNIT_EXPECT_EQ(test, 0, register_kprobe(&kp_missed));
+
+	recursed_target(rand1);
+
+	KUNIT_EXPECT_EQ(test, 2, kp_missed.nmissed);
+	KUNIT_EXPECT_NE(test, 0, preh_val);
+	KUNIT_EXPECT_NE(test, 0, posth_val);
+
+	unregister_kprobe(&kp_missed);
+}
+
+#ifdef CONFIG_KRETPROBES
+static u32 krph_val;
+
+static int entry_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
+{
+	KUNIT_EXPECT_FALSE(current_test, preemptible());
+	krph_val = (rand1 / div_factor);
+	return 0;
+}
+
+static int return_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
+{
+	unsigned long ret = regs_return_value(regs);
+
+	KUNIT_EXPECT_FALSE(current_test, preemptible());
+	KUNIT_EXPECT_EQ(current_test, ret, rand1 / div_factor);
+	KUNIT_EXPECT_NE(current_test, krph_val, 0);
+	krph_val = rand1;
+	return 0;
+}
+
+static struct kretprobe rp = {
+	.handler	= return_handler,
+	.entry_handler  = entry_handler,
+	.kp.symbol_name = "kprobe_target"
+};
+
+static void test_kretprobe(struct kunit *test)
+{
+	current_test = test;
+	KUNIT_EXPECT_EQ(test, 0, register_kretprobe(&rp));
+	target(rand1);
+	unregister_kretprobe(&rp);
+	KUNIT_EXPECT_EQ(test, krph_val, rand1);
+}
+
+static int return_handler2(struct kretprobe_instance *ri, struct pt_regs *regs)
+{
+	unsigned long ret = regs_return_value(regs);
+
+	KUNIT_EXPECT_EQ(current_test, ret, (rand1 / div_factor) + 1);
+	KUNIT_EXPECT_NE(current_test, krph_val, 0);
+	krph_val = rand1;
+	return 0;
+}
+
+static struct kretprobe rp2 = {
+	.handler	= return_handler2,
+	.entry_handler  = entry_handler,
+	.kp.symbol_name = "kprobe_target2"
+};
+
+static void test_kretprobes(struct kunit *test)
+{
+	struct kretprobe *rps[2] = {&rp, &rp2};
+
+	current_test = test;
+	KUNIT_EXPECT_EQ(test, 0, register_kretprobes(rps, 2));
+
+	krph_val = 0;
+	target(rand1);
+	KUNIT_EXPECT_EQ(test, krph_val, rand1);
+
+	krph_val = 0;
+	target2(rand1);
+	KUNIT_EXPECT_EQ(test, krph_val, rand1);
+	unregister_kretprobes(rps, 2);
+}
+
+#ifdef CONFIG_ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
+#define STACK_BUF_SIZE 16
+static unsigned long stack_buf[STACK_BUF_SIZE];
+
+static int stacktrace_return_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
+{
+	unsigned long retval = regs_return_value(regs);
+	int i, ret;
+
+	KUNIT_EXPECT_FALSE(current_test, preemptible());
+	KUNIT_EXPECT_EQ(current_test, retval, target_return_address[1]);
+
+	/*
+	 * Test stacktrace inside the kretprobe handler, this will involves
+	 * kretprobe trampoline, but must include correct return address
+	 * of the target function.
+	 */
+	ret = stack_trace_save(stack_buf, STACK_BUF_SIZE, 0);
+	KUNIT_EXPECT_NE(current_test, ret, 0);
+
+	for (i = 0; i < ret; i++) {
+		if (stack_buf[i] == target_return_address[1])
+			break;
+	}
+	KUNIT_EXPECT_NE(current_test, i, ret);
+
+#if !IS_MODULE(CONFIG_KPROBES_SANITY_TEST)
+	/*
+	 * Test stacktrace from pt_regs at the return address. Thus the stack
+	 * trace must start from the target return address.
+	 */
+	ret = stack_trace_save_regs(regs, stack_buf, STACK_BUF_SIZE, 0);
+	KUNIT_EXPECT_NE(current_test, ret, 0);
+	KUNIT_EXPECT_EQ(current_test, stack_buf[0], target_return_address[1]);
+#endif
+
+	return 0;
+}
+
+static struct kretprobe rp3 = {
+	.handler	= stacktrace_return_handler,
+	.kp.symbol_name = "kprobe_stacktrace_target"
+};
+
+static void test_stacktrace_on_kretprobe(struct kunit *test)
+{
+	unsigned long myretaddr = (unsigned long)__builtin_return_address(0);
+
+	current_test = test;
+
+	/*
+	 * Run the stacktrace_driver() to record correct return address in
+	 * stacktrace_target() and ensure stacktrace_driver() call is not
+	 * inlined by checking the return address of stacktrace_driver()
+	 * and the return address of this function is different.
+	 */
+	KUNIT_ASSERT_NE(test, myretaddr, stacktrace_driver());
+
+	KUNIT_ASSERT_EQ(test, 0, register_kretprobe(&rp3));
+	KUNIT_ASSERT_NE(test, myretaddr, stacktrace_driver());
+	unregister_kretprobe(&rp3);
+}
+
+static int stacktrace_internal_return_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
+{
+	unsigned long retval = regs_return_value(regs);
+	int i, ret;
+
+	KUNIT_EXPECT_FALSE(current_test, preemptible());
+	KUNIT_EXPECT_EQ(current_test, retval, target_return_address[0]);
+
+	/*
+	 * Test stacktrace inside the kretprobe handler for nested case.
+	 * The unwinder will find the kretprobe_trampoline address on the
+	 * return address, and kretprobe must solve that.
+	 */
+	ret = stack_trace_save(stack_buf, STACK_BUF_SIZE, 0);
+	KUNIT_EXPECT_NE(current_test, ret, 0);
+
+	for (i = 0; i < ret - 1; i++) {
+		if (stack_buf[i] == target_return_address[0]) {
+			KUNIT_EXPECT_EQ(current_test, stack_buf[i + 1], target_return_address[1]);
+			break;
+		}
+	}
+	KUNIT_EXPECT_NE(current_test, i, ret);
+
+#if !IS_MODULE(CONFIG_KPROBES_SANITY_TEST)
+	/* Ditto for the regs version. */
+	ret = stack_trace_save_regs(regs, stack_buf, STACK_BUF_SIZE, 0);
+	KUNIT_EXPECT_NE(current_test, ret, 0);
+	KUNIT_EXPECT_EQ(current_test, stack_buf[0], target_return_address[0]);
+	KUNIT_EXPECT_EQ(current_test, stack_buf[1], target_return_address[1]);
+#endif
+
+	return 0;
+}
+
+static struct kretprobe rp4 = {
+	.handler	= stacktrace_internal_return_handler,
+	.kp.symbol_name = "kprobe_stacktrace_internal_target"
+};
+
+static void test_stacktrace_on_nested_kretprobe(struct kunit *test)
+{
+	unsigned long myretaddr = (unsigned long)__builtin_return_address(0);
+	struct kretprobe *rps[2] = {&rp3, &rp4};
+
+	current_test = test;
+
+	//KUNIT_ASSERT_NE(test, myretaddr, stacktrace_driver());
+
+	KUNIT_ASSERT_EQ(test, 0, register_kretprobes(rps, 2));
+	KUNIT_ASSERT_NE(test, myretaddr, stacktrace_driver());
+	unregister_kretprobes(rps, 2);
+}
+#endif /* CONFIG_ARCH_CORRECT_STACKTRACE_ON_KRETPROBE */
+
+#endif /* CONFIG_KRETPROBES */
+
+static int kprobes_test_init(struct kunit *test)
+{
+	KP_CLEAR(kp);
+	KP_CLEAR(kp2);
+	KP_CLEAR(kp_missed);
+#ifdef CONFIG_KRETPROBES
+	KP_CLEAR(rp.kp);
+	KP_CLEAR(rp2.kp);
+#ifdef CONFIG_ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
+	KP_CLEAR(rp3.kp);
+	KP_CLEAR(rp4.kp);
+#endif
+#endif
+
+	target = kprobe_target;
+	target2 = kprobe_target2;
+	recursed_target = kprobe_recursed_target;
+	stacktrace_target = kprobe_stacktrace_target;
+	internal_target = kprobe_stacktrace_internal_target;
+	stacktrace_driver = kprobe_stacktrace_driver;
+	rand1 = get_random_u32_above(div_factor);
+	return 0;
+}
+
+static struct kunit_case kprobes_testcases[] = {
+	KUNIT_CASE(test_kprobe),
+	KUNIT_CASE(test_kprobes),
+	KUNIT_CASE(test_kprobe_missed),
+#ifdef CONFIG_KRETPROBES
+	KUNIT_CASE(test_kretprobe),
+	KUNIT_CASE(test_kretprobes),
+#ifdef CONFIG_ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
+	KUNIT_CASE(test_stacktrace_on_kretprobe),
+	KUNIT_CASE(test_stacktrace_on_nested_kretprobe),
+#endif
+#endif
+	{}
+};
+
+static struct kunit_suite kprobes_test_suite = {
+	.name = "kprobes_test",
+	.init = kprobes_test_init,
+	.test_cases = kprobes_testcases,
+};
+
+kunit_test_suites(&kprobes_test_suite);
+
+MODULE_DESCRIPTION("simple sanity test for k*probes");
+MODULE_LICENSE("GPL");