linux-kernel - [PATCH] time: Normalize timespec64 before timespec64

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Date:   Thu, 2 Dec 2021 21:07:07 +0800
From:   Yu Liao <liaoyu15@...wei.com>
To:     <john.stultz@...aro.org>, <tglx@...utronix.de>, <sboyd@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <wangxiongfeng2@...wei.com>,
        <liaoyu15@...wei.com>
Subject: [PATCH] time: Normalize timespec64 before timespec64_compare()

Passing unnormalized timespec64 to timespec64_compare() may cause
incorrect results.

For example:
  wall_to_monotonic = {tv_sec = -10, tv_nsec = 900000000}
  ts_delta = {tv_sec = -9, tv_nsec = -900000000}

timespec64_compare() returns -1, but actually wall_to_monotonic > ts_delta.
This will cause wall_to_monotonic to become a positive number.

Use timespec64_sub() instead of direct subtraction to avoid this.

Signed-off-by: Yu Liao <liaoyu15@...wei.com>
---
 include/linux/time64.h    | 2 ++
 kernel/time/timekeeping.c | 3 +--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/time64.h b/include/linux/time64.h
index 81b9686a2079..f74a36d253cd 100644
--- a/include/linux/time64.h
+++ b/include/linux/time64.h
@@ -50,6 +50,8 @@ static inline int timespec64_equal(const struct timespec64 *a,
  * lhs < rhs:  return <0
  * lhs == rhs: return 0
  * lhs > rhs:  return >0
+ *
+ * Note: Both lhs and rhs must be normalized.
  */
 static inline int timespec64_compare(const struct timespec64 *lhs, const struct timespec64 *rhs)
 {
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