sched/cputime: Do not scale when utime == 0
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>
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@ -551,10 +551,7 @@ static void cputime_adjust(struct task_cputime *curr,
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struct cputime *prev,
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cputime_t *ut, cputime_t *st)
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{
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cputime_t rtime, stime, utime, total;
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stime = curr->stime;
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total = stime + curr->utime;
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cputime_t rtime, stime, utime;
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/*
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* Tick based cputime accounting depend on random scheduling
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@ -576,13 +573,19 @@ static void cputime_adjust(struct task_cputime *curr,
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if (prev->stime + prev->utime >= rtime)
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goto out;
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if (total) {
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stime = curr->stime;
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utime = curr->utime;
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if (utime == 0) {
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stime = rtime;
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} else if (stime == 0) {
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utime = rtime;
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} else {
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cputime_t total = stime + utime;
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stime = scale_stime((__force u64)stime,
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(__force u64)rtime, (__force u64)total);
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utime = rtime - stime;
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} else {
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stime = rtime;
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utime = 0;
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}
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/*
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