netdev - Re: [Potential Spoof] [PATCH bpf-next] selftests/bpf: fix nanosleep for real this time

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Date:   Fri, 13 Mar 2020 17:10:29 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [Potential Spoof] [PATCH bpf-next] selftests/bpf: fix nanosleep
 for real this time

On Fri, Mar 13, 2020 at 5:05 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Fri, Mar 13, 2020 at 04:35:35PM -0700, Andrii Nakryiko wrote:
> > Amazingly, some libc implementations don't call __NR_nanosleep syscall from
> > their nanosleep() APIs. Hammer it down with explicit syscall() call and never
> > get back to it again. Also simplify code for timespec initialization.
> >
> > I verified that nanosleep is called w/ printk and in exactly same Linux image
> > that is used in Travis CI. So it should both sleep and call correct syscall.
> >
> > Fixes: 4e1fd25d19e8 ("selftests/bpf: Fix usleep() implementation")
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > ---
> >  tools/testing/selftests/bpf/test_progs.c | 16 ++++++----------
> >  1 file changed, 6 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/testing/selftests/bpf/test_progs.c b/tools/testing/selftests/bpf/test_progs.c
> > index f85a06512541..6956d722a463 100644
> > --- a/tools/testing/selftests/bpf/test_progs.c
> > +++ b/tools/testing/selftests/bpf/test_progs.c
> > @@ -35,16 +35,12 @@ struct prog_test_def {
> >   */
> >  int usleep(useconds_t usec)
> >  {
> > -     struct timespec ts;
> > -
> > -     if (usec > 999999) {
> > -             ts.tv_sec = usec / 1000000;
> > -             ts.tv_nsec = usec % 1000000;
> > -     } else {
> > -             ts.tv_sec = 0;
> > -             ts.tv_nsec = usec;
> > -     }
> > -     return nanosleep(&ts, NULL);
> > +     struct timespec ts = {
> > +             .tv_sec = usec / 1000000,
> > +             .tv_nsec = usec % 1000000,
> usec is in micro and tv_nsec is in nano?
>

Yes, this is implementation of usleep() (microsecond sleep), so usec
is microseconds. We call nanosleep internally, though, which accepts
seconds and nanoseconds units. Did I mess up math here?

But either way, sending v2, there is another place we explicitly are
calling nanosleep as well, fixing that one as well.

> > +     };
> > +
> > +     return syscall(__NR_nanosleep, &ts, NULL);
> >  }
> >