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Sender: Guenter Roeck <groeck7@gmail.com>
From: Guenter Roeck <linux@roeck-us.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org,
	Guenter Roeck <linux@roeck-us.net>,
	Oliver Glitta <glittao@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Daniel Latypov <dlatypov@google.com>,
	Marco Elver <elver@google.com>
Subject: [PATCH] mm/slub, kunit: Use inverted data to corrupt kmem cache
Date: Tue,  2 Apr 2024 06:38:39 -0700
Message-Id: <20240402133839.283793-1-linux@roeck-us.net>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

Two failure patterns are seen randomly when running slub_kunit tests with
CONFIG_SLAB_FREELIST_RANDOM and CONFIG_SLAB_FREELIST_HARDENED enabled.

Pattern 1:
     # test_clobber_zone: pass:1 fail:0 skip:0 total:1
     ok 1 test_clobber_zone
     # test_next_pointer: EXPECTATION FAILED at lib/slub_kunit.c:72
     Expected 3 == slab_errors, but
         slab_errors == 0 (0x0)
     # test_next_pointer: EXPECTATION FAILED at lib/slub_kunit.c:84
     Expected 2 == slab_errors, but
         slab_errors == 0 (0x0)
     # test_next_pointer: pass:0 fail:1 skip:0 total:1
     not ok 2 test_next_pointer

In this case, test_next_pointer() overwrites p[s->offset], but the data
at p[s->offset] is already 0x12.

Pattern 2:
     ok 1 test_clobber_zone
     # test_next_pointer: EXPECTATION FAILED at lib/slub_kunit.c:72
     Expected 3 == slab_errors, but
         slab_errors == 2 (0x2)
     # test_next_pointer: pass:0 fail:1 skip:0 total:1
     not ok 2 test_next_pointer

In this case, p[s->offset] has a value other than 0x12, but one of the
expected failures is nevertheless missing.

Invert data instead of writing a fixed value to corrupt the cache data
structures to fix the problem.

Fixes: 1f9f78b1b376 ("mm/slub, kunit: add a KUnit test for SLUB debugging functionality")
Cc: Oliver Glitta <glittao@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
CC: Daniel Latypov <dlatypov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 lib/slub_kunit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/slub_kunit.c b/lib/slub_kunit.c
index d4a3730b08fa..4ce960438806 100644
--- a/lib/slub_kunit.c
+++ b/lib/slub_kunit.c
@@ -55,7 +55,7 @@ static void test_next_pointer(struct kunit *test)
 
 	ptr_addr = (unsigned long *)(p + s->offset);
 	tmp = *ptr_addr;
-	p[s->offset] = 0x12;
+	p[s->offset] = ~p[s->offset];
 
 	/*
 	 * Expecting three errors.
-- 
2.39.2