[PATCH] time: Prevent time64_to_tm() day truncation on 32-bit

[Karl Mehltretter <kmehltretter@gmail.com>]

time64_to_tm() accepts a 64-bit seconds value, but stores the quotient in
long. On a 32-bit kernel the day count therefore wraps when it reaches
2^31, even though the corresponding year remains representable in
struct tm.

This is reachable when formatting externally supplied timestamps. For
example, NILFS recovery copies the little-endian 64-bit ss_create field
from an on-disk segment summary into time64_t. Its sysfs attributes then
print that value with %ptTs, whose formatter calls time64_to_tm(). A
corrupted image can consequently produce an architecture-dependent printed
date.

Keep the day quotient in s64 while leaving the seconds-within-day remainder
as long. Use div_s64_rem() for the weekday calculation so 32-bit builds do
not require compiler runtime division helpers. Add a KUnit case at the
first positive day count outside signed 32-bit range. The baseline i386
kernel returns a negative year and wrong calendar fields; the fixed kernel
returns the same result as x86_64.

Fixes: e6c2682a1da3 ("time: Add time64_to_tm()")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Review notes:

  - On a test-only baseline, QEMU/i386 passed the existing case but failed
    the wide-day case with tm_year=-5879541 and four other wrong fields.
    Baseline x86_64 and fixed i386, x86_64 and lockdep x86_64 passed 2/2.
  - A direct s64 modulo emitted __moddi3 on i386 and __aeabi_ldivmod on ARM
    with Clang 21. The submitted form uses div_s64_rem(); W=1 builds of
    both objects contain neither compiler-runtime reference.
  - The narrow type came from time_to_tm() in the Fixes commit. The 2021
    arithmetic rewrite kept it and tested only +/-80,000 years, below the
    approximately 5.88-million-year 32-bit day boundary.

 kernel/time/time_test.c | 16 ++++++++++++++++
 kernel/time/timeconv.c  |  6 ++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/kernel/time/time_test.c b/kernel/time/time_test.c
index 1b99180da2881..8b718767b3baf 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),
 	{}
 };
 
diff --git a/kernel/time/timeconv.c b/kernel/time/timeconv.c
index 59b922c826e77..292a855827b7a 100644
--- a/kernel/time/timeconv.c
+++ b/kernel/time/timeconv.c
@@ -49,7 +49,8 @@ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
 	u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
 	u64 u64tmp, udays, century, year;
 	bool is_Jan_or_Feb, is_leap_year;
-	long days, rem;
+	s64 days;
+	long rem;
 	int remainder;
 
 	days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
@@ -70,7 +71,8 @@ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
 	result->tm_sec = rem % 60;
 
 	/* January 1, 1970 was a Thursday. */
-	result->tm_wday = (4 + days) % 7;
+	div_s64_rem(days + 4, 7, &remainder);
+	result->tm_wday = remainder;
 	if (result->tm_wday < 0)
 		result->tm_wday += 7;
 

base-commit: cf72cbb39da84b6f02f90c07f33b102fc10b16f0
-- 
2.53.0