[PATCH v2 2/2] tsacct: keep task begin times stable across suspend

[Hungyu Lin <dennylin0707@gmail.com>]

bacct_add_tsk() derives ac_btime and ac_btime64 by subtracting a
CLOCK_MONOTONIC elapsed duration from CLOCK_REALTIME. MONOTONIC stops
during suspend while REALTIME continues, so querying an existing task
after resume moves its reported begin time forward by the time spent
suspended.

Calculate the begin time from the wall-clock boot timestamp and the
task's CLOCK_BOOTTIME start timestamp instead. This accounts for suspend
and adds the nanoseconds before truncating to seconds. Keep ac_etime and
ac_tgetime on CLOCK_MONOTONIC and retain the existing rebasing when the
wall clock is adjusted.

The new begin time can be negative: setting CLOCK_REALTIME is bounded
by CLOCK_MONOTONIC, which excludes suspend, rather than CLOCK_BOOTTIME.
A permitted backward clock adjustment after suspend can therefore put
the task's begin time before the epoch. Clamp ac_btime64 at zero too,
instead of wrapping in the unsigned field, and retain its positive range.

Fixes: f3cef7a99469 ("[PATCH] csa: basic accounting over taskstats")
Cc: stable@vger.kernel.org
Reported-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20130611200600.GA15168@redhat.com
Suggested-by: Thomas Gleixner <tglx@kernel.org>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.1911071335320.4256@nanos.tec.linutronix.de
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
Changes in v2:
- Clamp negative ac_btime64 values to zero without a 32-bit upper limit.
- Precede this change with the getboottime64() sampling fix.
- Add Cc: stable and rebase onto mainline d89e1d186ec9.

 kernel/tsacct.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 5c153106e642..48320684c7c0 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -24,7 +24,7 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	const struct cred *tcred;
 	u64 utime, stime, utimescaled, stimescaled;
 	u64 now_ns, delta;
-	time64_t btime;
+	struct timespec64 btime;
 
 	BUILD_BUG_ON(TS_COMM_LEN < TASK_COMM_LEN);
 
@@ -39,9 +39,11 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	do_div(delta, NSEC_PER_USEC);
 	stats->ac_etime = delta;
 	/* Convert to seconds for btime (note y2106 limit) */
-	btime = ktime_get_real_seconds() - div_u64(delta, USEC_PER_SEC);
-	stats->ac_btime = clamp_t(time64_t, btime, 0, U32_MAX);
-	stats->ac_btime64 = btime;
+	getboottime64(&btime);
+	btime = timespec64_add(btime, ns_to_timespec64(tsk->start_boottime));
+	stats->ac_btime = clamp_t(time64_t, btime.tv_sec, 0, U32_MAX);
+	/* Setting the clock back after suspend can make btime negative. */
+	stats->ac_btime64 = max_t(time64_t, btime.tv_sec, 0);
 
 	if (tsk->flags & PF_EXITING)
 		stats->ac_exitcode = tsk->exit_code;
-- 
2.43.0