[PATCH 6.18.y] mm/damon/tests/core-kunit: catch test failure in test_merge_regions_of()

[SJ Park <sj@kernel.org>]

KUNIT_EXPECT_EQ() does not abort the execution of test code when the
expectation is not met.  But damon_test_merge_regions_of() code after its
initial KUNIT_EXPECT_EQ() call assumes the expectation is met.  It does a
per-region test with a hard-coded number of regions that is correct only
if the expectation was met.  As a result, __nth_region_of() could return
NULL, and the test code can dereference NULL pointers.  Fix the issue by
catching the expectation failure and skip the per-region tests.

The user impact on realistic setups should be negligible, as it is a unit
test.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260715031002.108504-3-sj@kernel.org
Link: https://lore.kernel.org/20260710144937.26981-1-sj@kernel.org [1]
Fixes: 17ccae8bb5c9 ("mm/damon: add kunit tests")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit b640708929aa956235866bf7717d89018c661083)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/tests/core-kunit.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 6687778752728..c1f0be6746abd 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -264,11 +264,14 @@ static void damon_test_merge_regions_of(struct kunit *test)
 	damon_merge_regions_of(t, 9, 9999);
 	/* 0-112, 114-130, 130-156, 156-170 */
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 5u);
+	if (damon_nr_regions(t) != 6)
+		goto out;
 	for (i = 0; i < 5; i++) {
 		r = __nth_region_of(t, i);
 		KUNIT_EXPECT_EQ(test, r->ar.start, saddrs[i]);
 		KUNIT_EXPECT_EQ(test, r->ar.end, eaddrs[i]);
 	}
+out:
 	damon_free_target(t);
 }
 
-- 
2.47.3