Re: [PATCH 5/5] mm/slub, kunit: Add testcase for krealloc redzone and zeroing

[Vlastimil Babka]

On 9/9/24 03:29, Feng Tang wrote:
> Danilo Krummrich raised issue about krealloc+GFP_ZERO [1], and Vlastimil
> suggested to add some test case which can sanity test the kmalloc-redzone
> and zeroing by utilizing the kmalloc's 'orig_size' debug feature.
> 
> It covers the grow and shrink case of krealloc() re-using current kmalloc
> object, and the case of re-allocating a new bigger object.
> 
> User can add "slub_debug" kernel cmdline parameter to test it.
> 
> [1]. https://lore.kernel.org/lkml/20240812223707.32049-1-dakr@xxxxxxxxxx/
> 
> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> ---
>  lib/slub_kunit.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
> 
> diff --git a/lib/slub_kunit.c b/lib/slub_kunit.c
> index 6e3a1e5a7142..03e0089149ad 100644
> --- a/lib/slub_kunit.c
> +++ b/lib/slub_kunit.c
> @@ -186,6 +186,51 @@ static void test_leak_destroy(struct kunit *test)
>  	KUNIT_EXPECT_EQ(test, 1, slab_errors);
>  }
>  
> +static void test_krealloc_redzone_zeroing(struct kunit *test)
> +{
> +	char *p;
> +	int i;
> +
> +	KUNIT_TEST_REQUIRES(test, __slub_debug_enabled());

AFAICS this is insufficient, because the static key may be enabled due to
debugging enabled for different caches than kmalloc, or it might not include
both red zone and object tracking.

But it should be possible to instead create a fake kmalloc cache of size 64
and use __kmalloc_cache_noprof() like test_kmalloc_redzone_access()?

> +
> +	/* Allocate a 64B kmalloc object */
> +	p = kzalloc(48, GFP_KERNEL);
> +	if (unlikely(is_kfence_address(p))) {
> +		kfree(p);
> +		return;
> +	}
> +	memset(p, 0xff, 48);
> +
> +	kasan_disable_current();
> +	OPTIMIZER_HIDE_VAR(p);
> +
> +	/* Test shrink */
> +	p = krealloc(p, 40, GFP_KERNEL | __GFP_ZERO);
> +	for (i = 40; i < 64; i++)
> +		KUNIT_EXPECT_EQ(test, p[i], SLUB_RED_ACTIVE);
> +
> +	/* Test grow within the same 64B kmalloc object */
> +	p = krealloc(p, 56, GFP_KERNEL | __GFP_ZERO);
> +	for (i = 40; i < 56; i++)
> +		KUNIT_EXPECT_EQ(test, p[i], 0);
> +	for (i = 56; i < 64; i++)
> +		KUNIT_EXPECT_EQ(test, p[i], SLUB_RED_ACTIVE);
> +
> +	/* Test grow with allocating a bigger 128B object */
> +	p = krealloc(p, 112, GFP_KERNEL | __GFP_ZERO);

The only downside is that krealloc() here might use kmalloc-128 cache that's
not doing red zoning and object tracking....

> +	if (unlikely(is_kfence_address(p)))
> +		goto exit;
> +
> +	for (i = 56; i < 112; i++)
> +		KUNIT_EXPECT_EQ(test, p[i], 0);

... but this test is still valid and necessary

> +	for (i = 112; i < 128; i++)
> +		KUNIT_EXPECT_EQ(test, p[i], SLUB_RED_ACTIVE);

... we might skip this test as the red zoning is not done by __do_krealloc()
anyway in the alloc_new case.

> +
> +exit:
> +	kfree(p);

Ideally we'd also validate the fake kmalloc cache we created and expect zero
slab_errors.

Hopefully this approach works and I'm not missing something...

> +	kasan_enable_current();
> +}
> +
>  static int test_init(struct kunit *test)
>  {
>  	slab_errors = 0;
> @@ -196,6 +241,7 @@ static int test_init(struct kunit *test)
>  }
>  
>  static struct kunit_case test_cases[] = {
> +	KUNIT_CASE(test_krealloc_redzone_zeroing),
>  	KUNIT_CASE(test_clobber_zone),
>  
>  #ifndef CONFIG_KASAN