Re: [PATCH] perf trace: Fix potential u64 underflow in duration calculation

[Michael Petlan <mpetlan@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 2171 bytes --]

On Tue, 31 Mar 2026, Ian Rogers wrote:
> On Tue, Mar 31, 2026 at 3:10 AM Michael Petlan <mpetlan@redhat.com> wrote:
> >
> > Although it happens very rarely, in case of out-of-order events (i.e.
> > due to CPU migration when a syscall is executed), the calculation of
> > event duration might underflow and thus a bogus value is printed:
> >
> >     2.804 ( 0.001 ms): :49553/49553 rt_sigaction(sig: QUIT, act: 0x7fff403ed6e0, oact: 0x7fff403ed780, sigsetsize: 8) = 0
> >     2.807 ( 0.001 ms): :49553/49553 rt_sigaction(sig: CHLD, act: 0x7fff403ed6e0, oact: 0x7fff403ed780, sigsetsize: 8) = 0
> >     2.815 (18446744073709.438 ms): :49553/49553 execve(filename: 0xbb173a30, argv: 0x55aabb171930, envp: 0x55aabb171120) = 0
> >     2.815 ( 0.534 ms): pwd/49553  ... [continued]: execve())                                           = 0
> >
> > Check for possible underflow first and in case of a bogus value, do
> > not print it.
> >
> > Signed-off-by: Michael Petlan <mpetlan@redhat.com>
> 
> Reviewed-by: Ian Rogers <irogers@google.com>
> 
> Sashiko notes [1] that maybe this should be >= so that 0ns system
> calls still have a duration. I'm not sure how sensible that is.
>
OK, although there is nothing of zero duration, coarse clock resolution
is a reasonable case. Sending v2.
Thanks.
Michael

> Thanks,
> Ian
> 
> [1] https://sashiko.dev/#/patchset/20260331101020.1270363-1-mpetlan%40redhat.com
> 
> 
> > ---
> >  tools/perf/builtin-trace.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> > index f487fbaa0ad6..c9de6bb9b0ea 100644
> > --- a/tools/perf/builtin-trace.c
> > +++ b/tools/perf/builtin-trace.c
> > @@ -2956,7 +2956,7 @@ static int trace__sys_exit(struct trace *trace, struct evsel *evsel,
> >                 ++trace->stats.vfs_getname;
> >         }
> >
> > -       if (ttrace->entry_time) {
> > +       if (ttrace->entry_time && sample->time > ttrace->entry_time) {
> >                 duration = sample->time - ttrace->entry_time;
> >                 if (trace__filter_duration(trace, duration))
> >                         goto out;
> > --
> > 2.47.3
> >
> 
>