linux-kernel - [PATCH v2] timekeeping: Make sure wall_to

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Date:   Mon, 13 Dec 2021 21:57:27 +0800
From:   Yu Liao <liaoyu15@...wei.com>
To:     <john.stultz@...aro.org>
CC:     <tglx@...utronix.de>, <linux-kernel@...r.kernel.org>,
        <liwei391@...wei.com>, <liaoyu15@...wei.com>
Subject: [PATCH v2] timekeeping: Make sure wall_to_monotonic isn't positive

After commit e1d7ba873555 ("time: Always make sure wall_to_monotonic
isn't positive"), we can still change wall_to_monotonic to positive
by running the following code:
    int main(void)
    {
        struct timespec time;

        clock_gettime(CLOCK_MONOTONIC, &time);
        time.tv_nsec = 0;
        clock_settime(CLOCK_REALTIME, &time);
        return 0;
    }

The reason is that the parameters of timespec64_compare() may be
unnormalized, and timespec64_compare() simply compare tv_sec,
for example:
  wall_to_monotonic = {tv_sec = -10, tv_nsec = 900000000}
  ts_delta = {tv_sec = -9, tv_nsec = -900000000}

timespec64_compare() result is wall_to_monotonic < ts_delta, but
actually wall_to_monotonic > ts_delta. After normalized, the result
of timespec64_compare() is correct:
  wall_to_monotonic = {tv_sec = -10, tv_nsec = 900000000}
  ts_delta = {tv_sec = -10, tv_nsec = 100000000}

Use timespec64_sub() to ensure ts_delta normalized, and the code
looks more concise.

Signed-off-by: Yu Liao <liaoyu15@...wei.com>
---
 kernel/time/timekeeping.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index b348749a9fc6..dcdcb85121e4 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1306,8 +1306,7 @@ int do_settimeofday64(const struct timespec64 *ts)
 	timekeeping_forward_now(tk);
 
 	xt = tk_xtime(tk);
-	ts_delta.tv_sec = ts->tv_sec - xt.tv_sec;
-	ts_delta.tv_nsec = ts->tv_nsec - xt.tv_nsec;
+	ts_delta = timespec64_sub(*ts, xt);
 
 	if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
 		ret = -EINVAL;
-- 
2.25.1