[PATCH v4 4/7] mm/slab: Add tests for the existing kmem_buckets behaviour

[Kees Cook <kees@kernel.org>]

kmem_buckets has had no test coverage since it was added. Add tests,
including stuff unique to the bucket design:

 - A bucket allocation comes from a cache of the set's own, and that
   cache carries SLAB_NO_MERGE.

 - Each size class is served by the set, including 96 and 192, which are
   not powers of two and are filled in from an aligned index by
   kmem_buckets_create(). Sizes above KMALLOC_MAX_CACHE_SIZE go to the
   page allocator instead, bucket set or not.

 - With CONFIG_SLAB_BUCKETS=n, kmem_buckets_create() still returns a
   non-NULL (zero size alloc pointer), so that callers only have to check
   for failure, and allocations through it come from the general caches.

 - Destroying a set takes its caches down rather than only freeing the
   set, which is what a module creating one on each load depends on.
   A set freed without its caches would leave the names taken, and the
   next load would warn about every one of them.

The tests skip rather than compile out, which is useful for testing
the CONFIG_SLAB_BUCKETS=n behaviors.

Built and tests passing (with expected skips) on ARCH=x86_64 defconfig
with GCC 16.2.0, with CONFIG_SLAB_BUCKETS as y and n.

Assisted-by: LLM
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Harry Yoo <harry@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Hao Li <hao.li@linux.dev>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <linux-mm@kvack.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: <linux-hardening@vger.kernel.org>
---
 lib/tests/slub_kunit.c | 214 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 214 insertions(+)

diff --git a/lib/tests/slub_kunit.c b/lib/tests/slub_kunit.c
index e3b63f0338d5..d6467dd5cf9b 100644
--- a/lib/tests/slub_kunit.c
+++ b/lib/tests/slub_kunit.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <kunit/test.h>
 #include <kunit/test-bug.h>
+#include <kunit/resource.h>
 #include <linux/mm.h>
 #include <linux/slab.h>
 #include <linux/module.h>
@@ -474,6 +475,214 @@ static int test_init(struct kunit *test)
 	return 0;
 }
 
+/* Destroy buckets on test exit so a failed KUNIT_ASSERT_*() doesn't leak. */
+KUNIT_DEFINE_ACTION_WRAPPER(destroy_buckets, kmem_buckets_destroy, kmem_buckets *);
+
+#define KUNIT_ASSERT_BUCKETS_CREATED(test, b)					\
+	do {									\
+		KUNIT_ASSERT_NOT_NULL(test, b);					\
+		KUNIT_ASSERT_EQ(test, 0,					\
+				kunit_add_action_or_reset(test,			\
+							  destroy_buckets, b)); \
+	} while (0)
+
+/*
+ * The cache an allocation came from, or NULL if it came from no cache at
+ * all, e.g. a size too big for any of them is served by the page allocator.
+ */
+static struct kmem_cache *cache_of(void *p)
+{
+	struct slab *slab = virt_to_slab(p);
+
+	return slab ? slab->slab_cache : NULL;
+}
+
+/*
+ * A bucket set exists to keep its allocations out of the caches everything
+ * else uses, so check the two things that make that true: they come from a
+ * cache of the set's own, and that cache is never merged into another.
+ */
+static void test_kmem_buckets_isolation(struct kunit *test)
+{
+	struct kmem_cache *bucket_cache, *general_cache;
+	kmem_buckets *b;
+	void *p, *q;
+
+	if (!IS_ENABLED(CONFIG_SLAB_BUCKETS))
+		kunit_skip(test, "needs CONFIG_SLAB_BUCKETS");
+
+	b = kmem_buckets_create("isolated_buckets", 0, 0, INT_MAX, NULL);
+	KUNIT_ASSERT_BUCKETS_CREATED(test, b);
+
+	/*
+	 * Free each allocation before asserting on the next one: the cache
+	 * outlives its objects, so nothing below needs them, and an assertion
+	 * that leaves one behind would make the deferred teardown report a
+	 * cache that is still in use.
+	 */
+	p = kmem_buckets_alloc(b, 128, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, p);
+	bucket_cache = cache_of(p);
+	kfree(p);
+	KUNIT_ASSERT_NOT_NULL(test, bucket_cache);
+
+	KUNIT_EXPECT_TRUE_MSG(test, strstarts(bucket_cache->name, "isolated_buckets-"),
+			      "expected a bucket cache, got %s", bucket_cache->name);
+
+	/*
+	 * Cache merging is on by default, and a bucket cache merged into a
+	 * same-sized general one would quietly undo the whole separation.
+	 */
+	KUNIT_EXPECT_TRUE(test, bucket_cache->flags & SLAB_NO_MERGE);
+
+	q = kmalloc(128, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, q);
+	general_cache = cache_of(q);
+	kfree(q);
+	KUNIT_ASSERT_NOT_NULL(test, general_cache);
+
+	KUNIT_EXPECT_PTR_NE(test, bucket_cache, general_cache);
+}
+
+/*
+ * Every size class gets its own cache in the set, including the ones that
+ * are not powers of two and are filled in from an aligned index. Sizes past
+ * the largest cache are served by the page allocator, bucket set or not.
+ */
+static void test_kmem_buckets_sizes(struct kunit *test)
+{
+	static const size_t sizes[] = { 8, 96, 192, 1024, 4096 };
+	struct kmem_cache *c;
+	kmem_buckets *b;
+	void *p;
+	int i;
+
+	if (!IS_ENABLED(CONFIG_SLAB_BUCKETS))
+		kunit_skip(test, "needs CONFIG_SLAB_BUCKETS");
+
+	b = kmem_buckets_create("sized_buckets", 0, 0, INT_MAX, NULL);
+	KUNIT_ASSERT_BUCKETS_CREATED(test, b);
+
+	for (i = 0; i < ARRAY_SIZE(sizes); i++) {
+		p = kmem_buckets_alloc(b, sizes[i], GFP_KERNEL);
+		KUNIT_ASSERT_NOT_NULL(test, p);
+		c = cache_of(p);
+		kfree(p);
+		KUNIT_ASSERT_NOT_NULL(test, c);
+
+		KUNIT_EXPECT_TRUE_MSG(test, strstarts(c->name, "sized_buckets-"),
+				      "size %zu: expected a bucket cache, got %s",
+				      sizes[i], c->name);
+		KUNIT_EXPECT_GE(test, c->object_size, sizes[i]);
+	}
+
+	/* Too big for any cache: a folio from the page allocator, not a slab. */
+	p = kmem_buckets_alloc(b, KMALLOC_MAX_CACHE_SIZE + 1, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, p);
+	c = cache_of(p);
+	kfree(p);
+
+	KUNIT_EXPECT_NULL(test, c);
+}
+
+/*
+ * A bucket cache stands in for a kmalloc cache, so it has to be aligned like
+ * one. The DMA layer decides whether a buffer needs bouncing from its size,
+ * on the grounds that a kmalloc cache of that size is already aligned for
+ * the device, so a weaker alignment here is not something a caller can see
+ * coming. Without slab debugging the size implies the alignment and this
+ * holds either way; with it, only the cache's own alignment does.
+ */
+static void test_kmem_buckets_alignment(struct kunit *test)
+{
+	static const size_t sizes[] = { 128, 512, 2048 };
+	struct kmem_cache *bucket_cache, *general_cache;
+	kmem_buckets *b;
+	void *p;
+	int i;
+
+	if (!IS_ENABLED(CONFIG_SLAB_BUCKETS))
+		kunit_skip(test, "needs CONFIG_SLAB_BUCKETS");
+
+	b = kmem_buckets_create("aligned_buckets", 0, 0, INT_MAX, NULL);
+	KUNIT_ASSERT_BUCKETS_CREATED(test, b);
+
+	for (i = 0; i < ARRAY_SIZE(sizes); i++) {
+		p = kmem_buckets_alloc(b, sizes[i], GFP_KERNEL);
+		KUNIT_ASSERT_NOT_NULL(test, p);
+		bucket_cache = cache_of(p);
+		KUNIT_EXPECT_TRUE_MSG(test,
+				      IS_ALIGNED((unsigned long)p, ARCH_DMA_MINALIGN),
+				      "size %zu: object %p is not %d byte aligned",
+				      sizes[i], p, (int)ARCH_DMA_MINALIGN);
+		kfree(p);
+
+		p = kmalloc(sizes[i], GFP_KERNEL);
+		KUNIT_ASSERT_NOT_NULL(test, p);
+		general_cache = cache_of(p);
+		kfree(p);
+
+		KUNIT_ASSERT_NOT_NULL(test, bucket_cache);
+		KUNIT_ASSERT_NOT_NULL(test, general_cache);
+		KUNIT_EXPECT_EQ_MSG(test, bucket_cache->align, general_cache->align,
+				    "size %zu: bucket cache aligned to %u, %s to %u",
+				    sizes[i], bucket_cache->align,
+				    general_cache->name, general_cache->align);
+	}
+}
+
+/*
+ * With the feature compiled out, kmem_buckets_create() still returns
+ * something non-NULL so that callers only have to check for failure, and
+ * allocations through it work (i.e. come from the general caches).
+ */
+static void test_kmem_buckets_disabled(struct kunit *test)
+{
+	kmem_buckets *b;
+	struct kmem_cache *c;
+	void *p;
+
+	if (IS_ENABLED(CONFIG_SLAB_BUCKETS))
+		kunit_skip(test, "only meaningful without CONFIG_SLAB_BUCKETS");
+
+	b = kmem_buckets_create("disabled_buckets", 0, 0, INT_MAX, NULL);
+	KUNIT_ASSERT_BUCKETS_CREATED(test, b);
+
+	p = kmem_buckets_alloc(b, 128, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, p);
+	c = cache_of(p);
+	kfree(p);
+	KUNIT_ASSERT_NOT_NULL(test, c);
+
+	KUNIT_EXPECT_TRUE_MSG(test, !strstarts(c->name, "disabled_buckets-"),
+			      "expected a general cache, got %s", c->name);
+}
+
+/* Destroying a set has to take its caches down, not just free the set. */
+static void test_kmem_buckets_destroy(struct kunit *test)
+{
+	kmem_buckets *b;
+	void *p;
+
+	if (!IS_ENABLED(CONFIG_SLAB_BUCKETS))
+		kunit_skip(test, "needs CONFIG_SLAB_BUCKETS");
+
+	b = kmem_buckets_create("destroyed_buckets", 0, 0, INT_MAX, NULL);
+	KUNIT_ASSERT_NOT_NULL(test, b);
+
+	/*
+	 * Deliberately leaked, as test_leak_destroy() leaks its own: the
+	 * teardown below has to find it. kmem_cache_destroy() unlists the
+	 * cache either way, so the name is still released.
+	 */
+	p = kmem_buckets_alloc(b, 128, GFP_KERNEL);
+	KUNIT_EXPECT_NOT_NULL(test, p);
+
+	kmem_buckets_destroy(b);
+
+	KUNIT_EXPECT_EQ(test, 2, slab_errors);
+}
+
 static struct kunit_case test_cases[] = {
 	KUNIT_CASE(test_clobber_zone),
 
@@ -495,6 +704,11 @@ static struct kunit_case test_cases[] = {
 #if defined(CONFIG_KPROBES) && defined(CONFIG_SMP)
 	KUNIT_CASE_SLOW(test_kmalloc_nolock_and_friends_kprobe),
 #endif
+	KUNIT_CASE(test_kmem_buckets_isolation),
+	KUNIT_CASE(test_kmem_buckets_sizes),
+	KUNIT_CASE(test_kmem_buckets_alignment),
+	KUNIT_CASE(test_kmem_buckets_disabled),
+	KUNIT_CASE(test_kmem_buckets_destroy),
 	{}
 };
 
-- 
2.34.1