[PATCH 3/7] mm/damon/vaddr-kunit: check region count in three_regions test

[SJ Park <sj@kernel.org>]

damon_do_test_apply_three_regions() iterates regions after
damon_set_regions() call assuming the function would succeed at setting
the number of regions the same to the expected one.  It might have
failed.  In this case, __nth_region_of() in the iteration could return
NULL and NULL dereference can happen in the test.

The consequent user impact (NULL dereference) is quite bad.  The
realistic user impact would be limited, though.  It would affect only
test run setups.

Fix it by testing if the number of regions was also changed as expected
and exit early for the failure.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260713144757.39740-1-sj@kernel.org

Fixes: 17ccae8bb5c9 ("mm/damon: add kunit tests")
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/tests/vaddr-kunit.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h
index 61f844336ffb5..6a95441d193ae 100644
--- a/mm/damon/tests/vaddr-kunit.h
+++ b/mm/damon/tests/vaddr-kunit.h
@@ -158,12 +158,17 @@ static void damon_do_test_apply_three_regions(struct kunit *test,
 		kunit_skip(test, "second damon_set_regions() fail");
 	}
 
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), nr_expected / 2);
+	if (damon_nr_regions(t) != nr_expected / 2)
+		goto out;
+
 	for (i = 0; i < nr_expected / 2; i++) {
 		r = __nth_region_of(t, i);
 		KUNIT_EXPECT_EQ(test, r->ar.start, expected[i * 2]);
 		KUNIT_EXPECT_EQ(test, r->ar.end, expected[i * 2 + 1]);
 	}
 
+out:
 	damon_destroy_target(t, NULL);
 }
 
-- 
2.47.3