[ 07/49] sched/cputime: Do not scale when utime == 0

Greg Kroah-Hartman
2013-09-29T19:52:37Z

From: Stanislaw Gruszka <sgruszka@redhat.com>

commit 5a8e01f8fa51f5cbce8f37acc050eb2319d12956 upstream.

scale_stime() silently assumes that stime < rtime, otherwise
when stime == rtime and both values are big enough (operations
on them do not fit in 32 bits), the resulting scaling stime can
be bigger than rtime. In consequence utime = rtime - stime
results in negative value.

User space visible symptoms of the bug are overflowed TIME
values on ps/top, for example:

$ ps aux | grep rcu
root 8 0.0 0.0 0 0 ? S 12:42 0:00 [rcuc/0]
root 9 0.0 0.0 0 0 ? S 12:42 0:00 [rcub/0]
root 10 62422329 0.0 0 0 ? R 12:42 21114581:37 [rcu_preempt]
root 11 0.1 0.0 0 0 ? S 12:42 0:02 [rcuop/0]
root 12 62422329 0.0 0 0 ? S 12:42 21114581:35 [rcuop/1]
root 10 62422329 0.0 0 0 ? R 12:42 21114581:37 [rcu_preempt]

or overflowed utime values read directly from /proc/$PID/stat

Reference:

https://lkml.org/lkml/2013/8/20/259

Reported-and-tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: stable@vger.kernel.org
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/20130904131602.GC2564@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched/cputime.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -558,7 +558,7 @@ static void cputime_adjust(struct task_c
struct cputime *prev,
cputime_t *ut, cputime_t *st)
{
- cputime_t rtime, stime, utime, total;
+ cputime_t rtime, stime, utime;

if (vtime_accounting_enabled()) {
*ut = curr->utime;
@@ -566,9 +566,6 @@ static void cputime_adjust(struct task_c
return;
}

- stime = curr->stime;
- total = stime + curr->utime;
-
/*
* Tick based cputime accounting depend on random scheduling
* timeslices of a task to be interrupted or not by the timer.
@@ -589,13 +586,19 @@ static void cputime_adjust(struct task_c
if (prev->stime + prev->utime >= rtime)
goto out;

- if (total) {
+ stime = curr->stime;
+ utime = curr->utime;
+
+ if (utime == 0) {
+ stime = rtime;
+ } else if (stime == 0) {
+ utime = rtime;
+ } else {
+ cputime_t total = stime + utime;
+
stime = scale_stime((__force u64)stime,
(__force u64)rtime, (__force u64)total);
utime = rtime - stime;
- } else {
- stime = rtime;
- utime = 0;
}

/*