[PATCH] kernel: use ktime_get_real_ts64() to calculate acct.ac_btime

Jan Stancek
2019-11-01T23:39:39Z

fill_ac() calculates process creation time from current time as:
ac->ac_btime = get_seconds() - elapsed

get_seconds() doesn't accumulate nanoseconds as regular time getters.
This creates race for user-space (e.g. LTP acct02), which typically
uses gettimeofday(), because process creation time sometimes appear
to be dated 'in past':

acct("myfile");
time_t start_time = time(NULL);
if (fork()==0) {
sleep(1);
exit(0);
}
waitpid(NULL);
acct(NULL);

// acct.ac_btime == 1572616777
// start_time == 1572616778

Testing: 10 hours of LTP acct02 on s390x with CONFIG_HZ=100,
test failed on unpatched kernel in 15 minutes

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Fontana <rfontana@redhat.com>
---
kernel/acct.c | 4 +++-
kernel/tsacct.c | 4 +++-
2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/acct.c b/kernel/acct.c
index 81f9831a7859..991c898160cd 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -417,6 +417,7 @@ static void fill_ac(acct_t *ac)
struct pacct_struct *pacct = &current->signal->pacct;
u64 elapsed, run_time;
struct tty_struct *tty;
+ struct timespec64 ts;

/*
* Fill the accounting struct with the needed info as recorded
@@ -448,7 +449,8 @@ static void fill_ac(acct_t *ac)
}
#endif
do_div(elapsed, AHZ);
- ac->ac_btime = get_seconds() - elapsed;
+ ktime_get_real_ts64(&ts);
+ ac->ac_btime = ts.tv_sec - elapsed;
#if ACCT_VERSION==2
ac->ac_ahz = AHZ;
#endif
diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 7be3e7530841..4d10854255ab 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -24,6 +24,7 @@ void bacct_add_tsk(struct user_namespace *user_ns,
const struct cred *tcred;
u64 utime, stime, utimescaled, stimescaled;
u64 delta;
+ struct timespec64 ts;

BUILD_BUG_ON(TS_COMM_LEN < TASK_COMM_LEN);

@@ -34,7 +35,8 @@ void bacct_add_tsk(struct user_namespace *user_ns,
stats->ac_etime = delta;
/* Convert to seconds for btime */
do_div(delta, USEC_PER_SEC);
- stats->ac_btime = get_seconds() - delta;
+ ktime_get_real_ts64(&ts);
+ stats->ac_btime = ts.tv_sec - delta;
if (thread_group_leader(tsk)) {
stats->ac_exitcode = tsk->exit_code;
if (tsk->flags & PF_FORKNOEXEC)
--
1.8.3.1