[PATCH 1/2] ksmbd: fix negative timestamps being saved to disk

[koraynilay <koray.fra@gmail.com>]

When setting a timestamp before epoch 0, ksmbd converts it from NT
format to Unix format without normalizing it, leading to the nanoseconds
field to be saved (on-disk too) as a negative number, which leads to a
garbled stat command output and a reported +4 seconds in the timestamp.

This bug also violates a struct timespec64 invariant, as documented in
the set_normalized_timespec64() kernel-doc:
[...]
 * Note: The tv_nsec part is always in the range of 0 <= tv_nsec < NSEC_PER_SEC.
 * For negative values only the tv_sec field is negative !
 */

Reproducer, with /tmp as the local filesystem folder exported as [test]
in ksmbd.conf and /mnt as the smb folder mounted with mount -t cifs:

  mount -t cifs //127.0.0.1/test /mnt -o username=user,password=pass
  touch /mnt/test.txt
  touch -d "1969-07-20 20:17:40.123456789" /mnt/test.txt
  stat -c "Server local (/tmp):   %y" /tmp/test.txt
  stat -c "CIFS client  (/mnt):   %y" /mnt/test.txt --cached=never
  smbclient //127.0.0.1/test -U user%pass -c "allinfo test.txt"

which prints (stat output is garbled because of negative value):

  Server local (/tmp):   1969-07-20 20:17:41.()*+,--00 +0000
  CIFS client  (/mnt):   1969-07-20 20:17:45.+(/+)*/00 +0000
  altname: TESTT~K6.TXT
  [...]
  access_time:    Sun Jul 20 20:17:44 1969 UTC
  write_time:     Sun Jul 20 20:17:44 1969 UTC
  [...]

Fix this by using set_normalized_timespec64() in ksmbd_NTtimeToUnix().

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Assisted-by: Gemini:3.8-flash-high antigravity-cli-1.5.3
Signed-off-by: koraynilay <koray.fra@gmail.com>
---
Yet another small fix I found while implementing ptime[1].
(Still not the last one :c)

Thanks.

[1]: https://lore.kernel.org/all/20261101015551.786044-1-koray.fra@gmail.com/T/#t

 fs/smb/server/misc.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/smb/server/misc.c b/fs/smb/server/misc.c
index 5037f37c9226..94781246b4d6 100644
--- a/fs/smb/server/misc.c
+++ b/fs/smb/server/misc.c
@@ -336,8 +336,7 @@ struct timespec64 ksmbd_NTtimeToUnix(__le64 ntutc)
 	if (t < 0) {
 		abs_t = -t;
 		ts.tv_nsec = do_div(abs_t, 10000000) * 100;
-		ts.tv_nsec = -ts.tv_nsec;
-		ts.tv_sec = -abs_t;
+		set_normalized_timespec64(&ts, -abs_t, -ts.tv_nsec);
 	} else {
 		abs_t = t;
 		ts.tv_nsec = do_div(abs_t, 10000000) * 100;

base-commit: f4692d0e3b5e16d32d4ec7b1e0e58d7dbe74aaf4
-- 
2.56.0