[PATCH 6.6.y] 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.

Link: https://lore.kernel.org/20260718001442.87129-4-sj@kernel.org
Link: https://lore.kernel.org/20260713144757.39740-1-sj@kernel.org [1]
Fixes: 17ccae8bb5c9 ("mm/damon: add kunit tests")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 5fea07e460874c8c7cf00f728efbe22abc62c8d8)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/vaddr-test.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/damon/vaddr-test.h b/mm/damon/vaddr-test.h
index 292736d3f3954..c0546447b21e6 100644
--- a/mm/damon/vaddr-test.h
+++ b/mm/damon/vaddr-test.h
@@ -149,12 +149,17 @@ static void damon_do_test_apply_three_regions(struct kunit *test,
 
 	damon_set_regions(t, three_regions, 3);
 
+	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);
 }
 
-- 
2.47.3