[PATCH v2 1/2] time: Prevent time64_to_tm() day truncation on 32-bit

[Karl Mehltretter]

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.

Fixes: e6c2682a1da3 ("time: Add time64_to_tm()")
Assisted-by: LLM
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
 kernel/time/timeconv.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timeconv.c b/kernel/time/timeconv.c
index 59b922c826e7..f1cb61e0b114 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;
+	long rem;
+	s64 days;
 	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;
 
-- 
2.39.5 (Apple Git-154)