[RFC PATCH v1.1 4/7] mm/damon/core-kunit: handle region split failure in filter_out()

[SJ Park <sj@kernel.org>]

dasmon_test_filter_out() test checks if damos_filter_match() of an
address filter splits the region as expected under a given condition.
But, the test continued regardless of the split successes.  As a result,
the later part of the test could dereference invalid pointers that
returned from damon_next_region().  Further, it could corrupt memory
from damon_destroy_region().

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

Fix it by exiting early for the number of regions test failure.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260714142352.100478-1-sj@kernel.org

Fixes: 26713c890875 ("mm/damon/core-test: add a unit test for __damos_filter_out()")
Cc: <stable@vger.kernel.org> # 6.6.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/tests/core-kunit.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 322a295ac2cec..eddf3a0484583 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -1365,6 +1365,8 @@ static void damos_test_filter_out(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, r->ar.start, 1);
 	KUNIT_EXPECT_EQ(test, r->ar.end, 2);
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 2);
+	if (damon_nr_regions(t) != 2)
+		goto out;
 	r2 = damon_next_region(r);
 	KUNIT_EXPECT_EQ(test, r2->ar.start, 2);
 	KUNIT_EXPECT_EQ(test, r2->ar.end, 4);
@@ -1379,11 +1381,14 @@ static void damos_test_filter_out(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, r->ar.start, 2);
 	KUNIT_EXPECT_EQ(test, r->ar.end, 6);
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 2);
+	if (damon_nr_regions(t) != 2)
+		goto out;
 	r2 = damon_next_region(r);
 	KUNIT_EXPECT_EQ(test, r2->ar.start, 6);
 	KUNIT_EXPECT_EQ(test, r2->ar.end, 8);
 	damon_destroy_region(r2, t);
 
+out:
 	damon_free_target(t);
 	damos_free_filter(f);
 }
-- 
2.47.3