Re: [PATCH v4 2/3] mm/slub, kunit: add a KUnit test for SLUB debugging functionality

From: Daniel Latypov
Date: Tue Apr 13 2021 - 17:34:04 EST

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On Tue, Apr 13, 2021 at 3:07 AM <glittao@xxxxxxxxx> wrote:
>
> From: Oliver Glitta <glittao@xxxxxxxxx>
>
> SLUB has resiliency_test() function which is hidden behind #ifdef
> SLUB_RESILIENCY_TEST that is not part of Kconfig, so nobody
> runs it. KUnit should be a proper replacement for it.
>
> Try changing byte in redzone after allocation and changing
> pointer to next free node, first byte, 50th byte and redzone
> byte. Check if validation finds errors.
>
> There are several differences from the original resiliency test:
> Tests create own caches with known state instead of corrupting
> shared kmalloc caches.
>
> The corruption of freepointer uses correct offset, the original
> resiliency test got broken with freepointer changes.
>
> Scratch changing random byte test, because it does not have
> meaning in this form where we need deterministic results.
>
> Add new option CONFIG_SLUB_KUNIT_TEST in Kconfig.
> Because the test deliberatly modifies non-allocated objects, it depends on

nit: *deliberately

> !KASAN which would have otherwise prevented that.
>
> Use kunit_resource to count errors in cache and silence bug reports.
> Count error whenever slab_bug() or slab_fix() is called or when
> the count of pages is wrong.
>
> Signed-off-by: Oliver Glitta <glittao@xxxxxxxxx>

Acked-by: Daniel Latypov <dlatypov@xxxxxxxxxx>

Looks good to me!
My one minor suggestion: perhaps let's log a summary of the error or
the func name in slab_add_kunit_errors().

> ---
> Changes since v3
>
> Use kunit_resource to silence bug reports and count errors suggested by
> Marco Elver.
> Make the test depends on !KASAN thanks to report from the kernel test robot.
>
> Changes since v2
>
> Use bit operation & instead of logical && as reported by kernel test
> robot and Dan Carpenter
>
> Changes since v1
>
> Conversion from kselftest to KUnit test suggested by Marco Elver.
> Error silencing.
> Error counting improvements.
> lib/Kconfig.debug | 12 ++++
> lib/Makefile | 1 +
> lib/slub_kunit.c | 150 ++++++++++++++++++++++++++++++++++++++++++++++
> mm/slab.h | 1 +
> mm/slub.c | 50 ++++++++++++++--
> 5 files changed, 209 insertions(+), 5 deletions(-)
> create mode 100644 lib/slub_kunit.c
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 2779c29d9981..9b8a0d754278 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2371,6 +2371,18 @@ config BITS_TEST
>
> If unsure, say N.
>
> +config SLUB_KUNIT_TEST
> + tristate "KUnit test for SLUB cache error detection" if !KUNIT_ALL_TESTS
> + depends on SLUB_DEBUG && KUNIT && !KASAN
> + default KUNIT_ALL_TESTS
> + help
> + This builds SLUB allocator unit test.
> + Tests SLUB cache debugging functionality.
> + For more information on KUnit and unit tests in general please refer
> + to the KUnit documentation in Documentation/dev-tools/kunit/.
> +
> + If unsure, say N.
> +
> config TEST_UDELAY
> tristate "udelay test driver"
> help
> diff --git a/lib/Makefile b/lib/Makefile
> index b5307d3eec1a..1e59c6714ed8 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -352,5 +352,6 @@ obj-$(CONFIG_LIST_KUNIT_TEST) += list-test.o
> obj-$(CONFIG_LINEAR_RANGES_TEST) += test_linear_ranges.o
> obj-$(CONFIG_BITS_TEST) += test_bits.o
> obj-$(CONFIG_CMDLINE_KUNIT_TEST) += cmdline_kunit.o
> +obj-$(CONFIG_SLUB_KUNIT_TEST) += slub_kunit.o
>
> obj-$(CONFIG_GENERIC_LIB_DEVMEM_IS_ALLOWED) += devmem_is_allowed.o
> diff --git a/lib/slub_kunit.c b/lib/slub_kunit.c
> new file mode 100644
> index 000000000000..cb9ae9f7e8a6
> --- /dev/null
> +++ b/lib/slub_kunit.c
> @@ -0,0 +1,150 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <kunit/test.h>
> +#include <linux/mm.h>
> +#include <linux/slab.h>
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include "../mm/slab.h"
> +
> +static struct kunit_resource resource;
> +static int slab_errors;
> +
> +static void test_clobber_zone(struct kunit *test)
> +{
> + struct kmem_cache *s = kmem_cache_create("TestSlub_RZ_alloc", 64, 0,
> + SLAB_RED_ZONE, NULL);
> + u8 *p = kmem_cache_alloc(s, GFP_KERNEL);
> +
> + p[64] = 0x12;
> +
> + validate_slab_cache(s);
> + KUNIT_EXPECT_EQ(test, 2, slab_errors);
> +
> + kmem_cache_free(s, p);
> + kmem_cache_destroy(s);

Might not be worth doing for now:
I see kmem_cache_destroy() has a `if (err) { pr_err(...); dump_stack(); }` call.
Does it make sense to cause that to fail the test?

I see it's defined in mm/slab_common.c, so we might not want to touch
that in this patch.

> +}
> +
> +static void test_next_pointer(struct kunit *test)
> +{
> + struct kmem_cache *s = kmem_cache_create("TestSlub_next_ptr_free", 64, 0,
> + SLAB_POISON, NULL);
> + u8 *p = kmem_cache_alloc(s, GFP_KERNEL);
> + unsigned long tmp;
> + unsigned long *ptr_addr;
> +
> + kmem_cache_free(s, p);
> +
> + ptr_addr = (unsigned long *)(p + s->offset);
> + tmp = *ptr_addr;
> + p[s->offset] = 0x12;
> +

I really like this test!
I think it'll be a good example to point to wrt handle mutating state
in tests (clear comments, using whitespace to break up steps, setting
up clear EXPECT calls, etc.)

> + /*
> + * Expecting three errors.
> + * One for the corrupted freechain and the other one for the wrong
> + * count of objects in use. The third error is fixing broken cache.
> + */
> + validate_slab_cache(s);
> + KUNIT_EXPECT_EQ(test, 3, slab_errors);
> +
> + /*
> + * Try to repair corrupted freepointer.
> + * Still expecting two errors. The first for the wrong count
> + * of objects in use.
> + * The second error is for fixing broken cache.
> + */
> + *ptr_addr = tmp;
> + slab_errors = 0;
> +
> + validate_slab_cache(s);
> + KUNIT_EXPECT_EQ(test, 2, slab_errors);
> +
> + /*
> + * Previous validation repaired the count of objects in use.
> + * Now expecting no error.
> + */
> + slab_errors = 0;
> + validate_slab_cache(s);
> + KUNIT_EXPECT_EQ(test, 0, slab_errors);
> +
> + kmem_cache_destroy(s);
> +}
> +
> +static void test_first_word(struct kunit *test)
> +{
> + struct kmem_cache *s = kmem_cache_create("TestSlub_1th_word_free", 64, 0,
> + SLAB_POISON, NULL);
> + u8 *p = kmem_cache_alloc(s, GFP_KERNEL);
> +
> + kmem_cache_free(s, p);
> + *p = 0x78;
> +
> + validate_slab_cache(s);
> + KUNIT_EXPECT_EQ(test, 2, slab_errors);
> +
> + kmem_cache_destroy(s);
> +}
> +
> +static void test_clobber_50th_byte(struct kunit *test)
> +{
> + struct kmem_cache *s = kmem_cache_create("TestSlub_50th_word_free", 64, 0,
> + SLAB_POISON, NULL);
> + u8 *p = kmem_cache_alloc(s, GFP_KERNEL);
> +
> + kmem_cache_free(s, p);
> + p[50] = 0x9a;
> +
> + validate_slab_cache(s);
> + KUNIT_EXPECT_EQ(test, 2, slab_errors);
> + kmem_cache_destroy(s);
> +}
> +
> +static void test_clobber_redzone_free(struct kunit *test)
> +{
> + struct kmem_cache *s = kmem_cache_create("TestSlub_RZ_free", 64, 0,
> + SLAB_RED_ZONE, NULL);
> + u8 *p = kmem_cache_alloc(s, GFP_KERNEL);
> +
> + kmem_cache_free(s, p);
> + p[64] = 0xab;
> +
> + validate_slab_cache(s);
> + KUNIT_EXPECT_EQ(test, 2, slab_errors);
> + kmem_cache_destroy(s);
> +}
> +
> +static int test_init(struct kunit *test)
> +{
> + slab_errors = 0;
> +
> + /* FIXME: remove when CONFIG_KASAN requirement is dropped. */
> + current->kunit_test = test;
> +
> + kunit_add_named_resource(test, NULL, NULL, &resource,
> + "slab_errors", &slab_errors);
> + return 0;
> +}
> +
> +static void test_exit(struct kunit *test)
> +{
> + /* FIXME: remove when CONFIG_KASAN requirement is dropped. */
> + current->kunit_test = NULL;
> +}
> +
> +static struct kunit_case test_cases[] = {
> + KUNIT_CASE(test_clobber_zone),
> + KUNIT_CASE(test_next_pointer),
> + KUNIT_CASE(test_first_word),
> + KUNIT_CASE(test_clobber_50th_byte),
> + KUNIT_CASE(test_clobber_redzone_free),
> + {}
> +};
> +
> +static struct kunit_suite test_suite = {
> + .name = "slub_test",
> + .init = test_init,
> + .exit = test_exit,
> + .test_cases = test_cases,
> +};
> +kunit_test_suite(test_suite);
> +
> +MODULE_LICENSE("GPL");
> diff --git a/mm/slab.h b/mm/slab.h
> index 076582f58f68..95cf42eb8396 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -215,6 +215,7 @@ DECLARE_STATIC_KEY_TRUE(slub_debug_enabled);
> DECLARE_STATIC_KEY_FALSE(slub_debug_enabled);
> #endif
> extern void print_tracking(struct kmem_cache *s, void *object);
> +long validate_slab_cache(struct kmem_cache *s);
> #else
> static inline void print_tracking(struct kmem_cache *s, void *object)
> {
> diff --git a/mm/slub.c b/mm/slub.c
> index 3021ce9bf1b3..d7df8841d90a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -35,6 +35,7 @@
> #include <linux/prefetch.h>
> #include <linux/memcontrol.h>
> #include <linux/random.h>
> +#include <kunit/test.h>
>
> #include <trace/events/kmem.h>
>
> @@ -447,6 +448,26 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
> static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
> static DEFINE_SPINLOCK(object_map_lock);
>
> +#if IS_ENABLED(CONFIG_KUNIT)
> +static bool slab_add_kunit_errors(void)
> +{
> + struct kunit_resource *resource;
> +
> + if (likely(!current->kunit_test))
> + return false;
> +
> + resource = kunit_find_named_resource(current->kunit_test, "slab_errors");
> + if (!resource)
> + return false;
> +
> + (*(int *)resource->data)++;
> + kunit_put_resource(resource);
> + return true;
> +}
> +#else
> +static inline bool slab_add_kunit_errors(void) { return false; }
> +#endif
> +
> /*
> * Determine a map of object in use on a page.
> *
> @@ -676,6 +697,9 @@ static void slab_fix(struct kmem_cache *s, char *fmt, ...)
> struct va_format vaf;
> va_list args;
>
> + if (slab_add_kunit_errors())
> + return;
> +
> va_start(args, fmt);
> vaf.fmt = fmt;
> vaf.va = &args;
> @@ -739,6 +763,9 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
> void object_err(struct kmem_cache *s, struct page *page,
> u8 *object, char *reason)
> {
> + if (slab_add_kunit_errors())
> + return;
> +
> slab_bug(s, "%s", reason);

Would it be a good idea for us to log the error text in slab_add_kunit_errors()?
Otherwise we could end up with getting the same error count but from
an unexpected set of code paths.

Boils down to using this macro
kunit_info(current->kunit_test, "%s", reason);

Perhaps we could get away with just duplicating (a subset of) what
we'd pass into slab_bug(), i.e.
if (slab_add_kunit_errors("%s", reason)) return;
if (slab_add_kunit_errors("%s overwritten", what) return;
If the string we're printing is too annoying to duplicate, can get
away with just using __func__ as well.

Or a more messy alternative would be to add some #if'd code in
slab_bug(), but I don't know if that's as good of an idea.

> print_trailer(s, page, object);
> }
> @@ -749,6 +776,9 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct page *page,
> va_list args;
> char buf[100];
>
> + if (slab_add_kunit_errors())
> + return;
> +
> va_start(args, fmt);
> vsnprintf(buf, sizeof(buf), fmt, args);
> va_end(args);
> @@ -798,12 +828,16 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
> while (end > fault && end[-1] == value)
> end--;
>
> + if (slab_add_kunit_errors())
> + goto skip_bug_print;
> +
> slab_bug(s, "%s overwritten", what);
> pr_err("INFO: 0x%p-0x%p @offset=%tu. First byte 0x%x instead of 0x%x\n",
> - fault, end - 1, fault - addr,
> - fault[0], value);
> + fault, end - 1, fault - addr,
> + fault[0], value);
> print_trailer(s, page, object);
>
> +skip_bug_print:
> restore_bytes(s, what, value, fault, end);
> return 0;
> }
> @@ -4650,9 +4684,11 @@ static int validate_slab_node(struct kmem_cache *s,
> validate_slab(s, page);
> count++;
> }
> - if (count != n->nr_partial)
> + if (count != n->nr_partial) {
> pr_err("SLUB %s: %ld partial slabs counted but counter=%ld\n",
> s->name, count, n->nr_partial);
> + slab_add_kunit_errors();
> + }
>
> if (!(s->flags & SLAB_STORE_USER))
> goto out;
> @@ -4661,16 +4697,18 @@ static int validate_slab_node(struct kmem_cache *s,
> validate_slab(s, page);
> count++;
> }
> - if (count != atomic_long_read(&n->nr_slabs))
> + if (count != atomic_long_read(&n->nr_slabs)) {
> pr_err("SLUB: %s %ld slabs counted but counter=%ld\n",
> s->name, count, atomic_long_read(&n->nr_slabs));
> + slab_add_kunit_errors();
> + }
>
> out:
> spin_unlock_irqrestore(&n->list_lock, flags);
> return count;
> }
>
> -static long validate_slab_cache(struct kmem_cache *s)
> +long validate_slab_cache(struct kmem_cache *s)
> {
> int node;
> unsigned long count = 0;
> @@ -4682,6 +4720,8 @@ static long validate_slab_cache(struct kmem_cache *s)
>
> return count;
> }
> +EXPORT_SYMBOL(validate_slab_cache);
> +
> /*
> * Generate lists of code addresses where slabcache objects are allocated
> * and freed.
> --
> 2.31.1.272.g89b43f80a5
>

Next message: Mike Kravetz: "Re: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality"
Previous message: Kubalewski, Arkadiusz: "RE: [Intel-wired-lan] [PATCH] i40e: The state of phy may not be correct during power-on"
In reply to: Marco Elver: "Re: [PATCH v4 2/3] mm/slub, kunit: add a KUnit test for SLUB debugging functionality"
Next in thread: Oliver Glitta: "Re: [PATCH v4 2/3] mm/slub, kunit: add a KUnit test for SLUB debugging functionality"
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