[PATCH v2 2/2] time/kunit: Add time64_to_tm() case beyond 32-bit day count

[Karl Mehltretter]

The existing date range test covers +/-80,000 years around the epoch,
far below the day count at which a 32-bit long wraps (2^31 days,
roughly 5.88 million years).

Add a case at exactly 2^31 days after the epoch. Without the preceding
fix a 32-bit kernel returns tm_year=-5879541 and wrong month, day,
day-of-year and weekday; with it the result matches 64-bit kernels.

Assisted-by: LLM
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
 kernel/time/time_test.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/time/time_test.c b/kernel/time/time_test.c
index 1b99180da288..8b718767b3ba 100644
--- a/kernel/time/time_test.c
+++ b/kernel/time/time_test.c
@@ -87,8 +87,24 @@ static void time64_to_tm_test_date_range(struct kunit *test)
 	}
 }
 
+static void time64_to_tm_test_wide_day_count(struct kunit *test)
+{
+	/* 2^31 days: the first count that does not fit in a 32-bit long. */
+	time64_t timestamp = (1LL << 31) * 86400;
+	struct tm result;
+
+	time64_to_tm(timestamp, 0, &result);
+
+	KUNIT_EXPECT_EQ(test, result.tm_year, 5879680);
+	KUNIT_EXPECT_EQ(test, result.tm_mon, 6);
+	KUNIT_EXPECT_EQ(test, result.tm_mday, 12);
+	KUNIT_EXPECT_EQ(test, result.tm_yday, 193);
+	KUNIT_EXPECT_EQ(test, result.tm_wday, 6);
+}
+
 static struct kunit_case time_test_cases[] = {
 	KUNIT_CASE_SLOW(time64_to_tm_test_date_range),
+	KUNIT_CASE(time64_to_tm_test_wide_day_count),
 	{}
 };
 
-- 
2.39.5 (Apple Git-154)