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, 439 insertions, 0 deletions

diff --git a/lib/test_meminit.c b/lib/test_meminit.c
new file mode 100644
index 000000000..68c3b9da0
--- /dev/null
+++ b/lib/test_meminit.c
@@ -0,0 +1,439 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test cases for SL[AOU]B/page initialization at alloc/free time.
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/vmalloc.h>
+
+#define GARBAGE_INT (0x09A7BA9E)
+#define GARBAGE_BYTE (0x9E)
+
+#define REPORT_FAILURES_IN_FN() \
+	do {	\
+		if (failures)	\
+			pr_info("%s failed %d out of %d times\n",	\
+				__func__, failures, num_tests);		\
+		else		\
+			pr_info("all %d tests in %s passed\n",		\
+				num_tests, __func__);			\
+	} while (0)
+
+/* Calculate the number of uninitialized bytes in the buffer. */
+static int __init count_nonzero_bytes(void *ptr, size_t size)
+{
+	int i, ret = 0;
+	unsigned char *p = (unsigned char *)ptr;
+
+	for (i = 0; i < size; i++)
+		if (p[i])
+			ret++;
+	return ret;
+}
+
+/* Fill a buffer with garbage, skipping |skip| first bytes. */
+static void __init fill_with_garbage_skip(void *ptr, int size, size_t skip)
+{
+	unsigned int *p = (unsigned int *)((char *)ptr + skip);
+	int i = 0;
+
+	WARN_ON(skip > size);
+	size -= skip;
+
+	while (size >= sizeof(*p)) {
+		p[i] = GARBAGE_INT;
+		i++;
+		size -= sizeof(*p);
+	}
+	if (size)
+		memset(&p[i], GARBAGE_BYTE, size);
+}
+
+static void __init fill_with_garbage(void *ptr, size_t size)
+{
+	fill_with_garbage_skip(ptr, size, 0);
+}
+
+static int __init do_alloc_pages_order(int order, int *total_failures)
+{
+	struct page *page;
+	void *buf;
+	size_t size = PAGE_SIZE << order;
+
+	page = alloc_pages(GFP_KERNEL, order);
+	if (!page)
+		goto err;
+	buf = page_address(page);
+	fill_with_garbage(buf, size);
+	__free_pages(page, order);
+
+	page = alloc_pages(GFP_KERNEL, order);
+	if (!page)
+		goto err;
+	buf = page_address(page);
+	if (count_nonzero_bytes(buf, size))
+		(*total_failures)++;
+	fill_with_garbage(buf, size);
+	__free_pages(page, order);
+	return 1;
+err:
+	(*total_failures)++;
+	return 1;
+}
+
+/* Test the page allocator by calling alloc_pages with different orders. */
+static int __init test_pages(int *total_failures)
+{
+	int failures = 0, num_tests = 0;
+	int i;
+
+	for (i = 0; i < NR_PAGE_ORDERS; i++)
+		num_tests += do_alloc_pages_order(i, &failures);
+
+	REPORT_FAILURES_IN_FN();
+	*total_failures += failures;
+	return num_tests;
+}
+
+/* Test kmalloc() with given parameters. */
+static int __init do_kmalloc_size(size_t size, int *total_failures)
+{
+	void *buf;
+
+	buf = kmalloc(size, GFP_KERNEL);
+	if (!buf)
+		goto err;
+	fill_with_garbage(buf, size);
+	kfree(buf);
+
+	buf = kmalloc(size, GFP_KERNEL);
+	if (!buf)
+		goto err;
+	if (count_nonzero_bytes(buf, size))
+		(*total_failures)++;
+	fill_with_garbage(buf, size);
+	kfree(buf);
+	return 1;
+err:
+	(*total_failures)++;
+	return 1;
+}
+
+/* Test vmalloc() with given parameters. */
+static int __init do_vmalloc_size(size_t size, int *total_failures)
+{
+	void *buf;
+
+	buf = vmalloc(size);
+	if (!buf)
+		goto err;
+	fill_with_garbage(buf, size);
+	vfree(buf);
+
+	buf = vmalloc(size);
+	if (!buf)
+		goto err;
+	if (count_nonzero_bytes(buf, size))
+		(*total_failures)++;
+	fill_with_garbage(buf, size);
+	vfree(buf);
+	return 1;
+err:
+	(*total_failures)++;
+	return 1;
+}
+
+/* Test kmalloc()/vmalloc() by allocating objects of different sizes. */
+static int __init test_kvmalloc(int *total_failures)
+{
+	int failures = 0, num_tests = 0;
+	int i, size;
+
+	for (i = 0; i < 20; i++) {
+		size = 1 << i;
+		num_tests += do_kmalloc_size(size, &failures);
+		num_tests += do_vmalloc_size(size, &failures);
+	}
+
+	REPORT_FAILURES_IN_FN();
+	*total_failures += failures;
+	return num_tests;
+}
+
+#define CTOR_BYTES (sizeof(unsigned int))
+#define CTOR_PATTERN (0x41414141)
+/* Initialize the first 4 bytes of the object. */
+static void test_ctor(void *obj)
+{
+	*(unsigned int *)obj = CTOR_PATTERN;
+}
+
+/*
+ * Check the invariants for the buffer allocated from a slab cache.
+ * If the cache has a test constructor, the first 4 bytes of the object must
+ * always remain equal to CTOR_PATTERN.
+ * If the cache isn't an RCU-typesafe one, or if the allocation is done with
+ * __GFP_ZERO, then the object contents must be zeroed after allocation.
+ * If the cache is an RCU-typesafe one, the object contents must never be
+ * zeroed after the first use. This is checked by memcmp() in
+ * do_kmem_cache_size().
+ */
+static bool __init check_buf(void *buf, int size, bool want_ctor,
+			     bool want_rcu, bool want_zero)
+{
+	int bytes;
+	bool fail = false;
+
+	bytes = count_nonzero_bytes(buf, size);
+	WARN_ON(want_ctor && want_zero);
+	if (want_zero)
+		return bytes;
+	if (want_ctor) {
+		if (*(unsigned int *)buf != CTOR_PATTERN)
+			fail = 1;
+	} else {
+		if (bytes)
+			fail = !want_rcu;
+	}
+	return fail;
+}
+
+#define BULK_SIZE 100
+static void *bulk_array[BULK_SIZE];
+
+/*
+ * Test kmem_cache with given parameters:
+ *  want_ctor - use a constructor;
+ *  want_rcu - use SLAB_TYPESAFE_BY_RCU;
+ *  want_zero - use __GFP_ZERO.
+ */
+static int __init do_kmem_cache_size(size_t size, bool want_ctor,
+				     bool want_rcu, bool want_zero,
+				     int *total_failures)
+{
+	struct kmem_cache *c;
+	int iter;
+	bool fail = false;
+	gfp_t alloc_mask = GFP_KERNEL | (want_zero ? __GFP_ZERO : 0);
+	void *buf, *buf_copy;
+
+	c = kmem_cache_create("test_cache", size, 1,
+			      want_rcu ? SLAB_TYPESAFE_BY_RCU : 0,
+			      want_ctor ? test_ctor : NULL);
+	for (iter = 0; iter < 10; iter++) {
+		/* Do a test of bulk allocations */
+		if (!want_rcu && !want_ctor) {
+			if (!kmem_cache_alloc_bulk(c, alloc_mask, BULK_SIZE,
+					bulk_array)) {
+				fail = true;
+			} else {
+				int i;
+				for (i = 0; i < BULK_SIZE; i++)
+					fail |= check_buf(bulk_array[i], size, want_ctor, want_rcu, want_zero);
+				kmem_cache_free_bulk(c, BULK_SIZE, bulk_array);
+			}
+		}
+
+		buf = kmem_cache_alloc(c, alloc_mask);
+		/* Check that buf is zeroed, if it must be. */
+		fail |= check_buf(buf, size, want_ctor, want_rcu, want_zero);
+		fill_with_garbage_skip(buf, size, want_ctor ? CTOR_BYTES : 0);
+
+		if (!want_rcu) {
+			kmem_cache_free(c, buf);
+			continue;
+		}
+
+		/*
+		 * If this is an RCU cache, use a critical section to ensure we
+		 * can touch objects after they're freed.
+		 */
+		rcu_read_lock();
+		/*
+		 * Copy the buffer to check that it's not wiped on
+		 * free().
+		 */
+		buf_copy = kmalloc(size, GFP_ATOMIC);
+		if (buf_copy)
+			memcpy(buf_copy, buf, size);
+
+		kmem_cache_free(c, buf);
+		/*
+		 * Check that |buf| is intact after kmem_cache_free().
+		 * |want_zero| is false, because we wrote garbage to
+		 * the buffer already.
+		 */
+		fail |= check_buf(buf, size, want_ctor, want_rcu,
+				  false);
+		if (buf_copy) {
+			fail |= (bool)memcmp(buf, buf_copy, size);
+			kfree(buf_copy);
+		}
+		rcu_read_unlock();
+	}
+	kmem_cache_destroy(c);
+
+	*total_failures += fail;
+	return 1;
+}
+
+/*
+ * Check that the data written to an RCU-allocated object survives
+ * reallocation.
+ */
+static int __init do_kmem_cache_rcu_persistent(int size, int *total_failures)
+{
+	struct kmem_cache *c;
+	void *buf, *buf_contents, *saved_ptr;
+	void **used_objects;
+	int i, iter, maxiter = 1024;
+	bool fail = false;
+
+	c = kmem_cache_create("test_cache", size, size, SLAB_TYPESAFE_BY_RCU,
+			      NULL);
+	buf = kmem_cache_alloc(c, GFP_KERNEL);
+	if (!buf)
+		goto out;
+	saved_ptr = buf;
+	fill_with_garbage(buf, size);
+	buf_contents = kmalloc(size, GFP_KERNEL);
+	if (!buf_contents) {
+		kmem_cache_free(c, buf);
+		goto out;
+	}
+	used_objects = kmalloc_array(maxiter, sizeof(void *), GFP_KERNEL);
+	if (!used_objects) {
+		kmem_cache_free(c, buf);
+		kfree(buf_contents);
+		goto out;
+	}
+	memcpy(buf_contents, buf, size);
+	kmem_cache_free(c, buf);
+	/*
+	 * Run for a fixed number of iterations. If we never hit saved_ptr,
+	 * assume the test passes.
+	 */
+	for (iter = 0; iter < maxiter; iter++) {
+		buf = kmem_cache_alloc(c, GFP_KERNEL);
+		used_objects[iter] = buf;
+		if (buf == saved_ptr) {
+			fail = memcmp(buf_contents, buf, size);
+			for (i = 0; i <= iter; i++)
+				kmem_cache_free(c, used_objects[i]);
+			goto free_out;
+		}
+	}
+
+	for (iter = 0; iter < maxiter; iter++)
+		kmem_cache_free(c, used_objects[iter]);
+
+free_out:
+	kfree(buf_contents);
+	kfree(used_objects);
+out:
+	kmem_cache_destroy(c);
+	*total_failures += fail;
+	return 1;
+}
+
+static int __init do_kmem_cache_size_bulk(int size, int *total_failures)
+{
+	struct kmem_cache *c;
+	int i, iter, maxiter = 1024;
+	int bytes;
+	bool fail = false;
+	void *objects[10];
+
+	c = kmem_cache_create("test_cache", size, size, 0, NULL);
+	for (iter = 0; (iter < maxiter) && !fail; iter++) {
+		if (!kmem_cache_alloc_bulk(c, GFP_KERNEL, ARRAY_SIZE(objects),
+				objects))
+			continue;
+
+		for (i = 0; i < ARRAY_SIZE(objects); i++) {
+			bytes = count_nonzero_bytes(objects[i], size);
+			if (bytes)
+				fail = true;
+			fill_with_garbage(objects[i], size);
+		}
+
+		kmem_cache_free_bulk(c, ARRAY_SIZE(objects), objects);
+	}
+	kmem_cache_destroy(c);
+	*total_failures += fail;
+	return 1;
+}
+
+/*
+ * Test kmem_cache allocation by creating caches of different sizes, with and
+ * without constructors, with and without SLAB_TYPESAFE_BY_RCU.
+ */
+static int __init test_kmemcache(int *total_failures)
+{
+	int failures = 0, num_tests = 0;
+	int i, flags, size;
+	bool ctor, rcu, zero;
+
+	for (i = 0; i < 10; i++) {
+		size = 8 << i;
+		for (flags = 0; flags < 8; flags++) {
+			ctor = flags & 1;
+			rcu = flags & 2;
+			zero = flags & 4;
+			if (ctor && zero)
+				continue;
+			num_tests += do_kmem_cache_size(size, ctor, rcu, zero,
+							&failures);
+		}
+		num_tests += do_kmem_cache_size_bulk(size, &failures);
+	}
+	REPORT_FAILURES_IN_FN();
+	*total_failures += failures;
+	return num_tests;
+}
+
+/* Test the behavior of SLAB_TYPESAFE_BY_RCU caches of different sizes. */
+static int __init test_rcu_persistent(int *total_failures)
+{
+	int failures = 0, num_tests = 0;
+	int i, size;
+
+	for (i = 0; i < 10; i++) {
+		size = 8 << i;
+		num_tests += do_kmem_cache_rcu_persistent(size, &failures);
+	}
+	REPORT_FAILURES_IN_FN();
+	*total_failures += failures;
+	return num_tests;
+}
+
+/*
+ * Run the tests. Each test function returns the number of executed tests and
+ * updates |failures| with the number of failed tests.
+ */
+static int __init test_meminit_init(void)
+{
+	int failures = 0, num_tests = 0;
+
+	num_tests += test_pages(&failures);
+	num_tests += test_kvmalloc(&failures);
+	num_tests += test_kmemcache(&failures);
+	num_tests += test_rcu_persistent(&failures);
+
+	if (failures == 0)
+		pr_info("all %d tests passed!\n", num_tests);
+	else
+		pr_info("failures: %d out of %d\n", failures, num_tests);
+
+	return failures ? -EINVAL : 0;
+}
+module_init(test_meminit_init);
+
+MODULE_DESCRIPTION("Test cases for SL[AOU]B/page initialization at alloc/free time");
+MODULE_LICENSE("GPL");