linux-kernel - Re: [PATCH] selftests/seccomp: Actually sleep for 1/10th second

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Date:   Mon, 28 Jan 2019 08:13:01 +1300
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Shuah Khan <shuah@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH] selftests/seccomp: Actually sleep for 1/10th second

On Sun, Jan 27, 2019 at 11:25 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Sun, Jan 27, 2019 at 1:44 AM Kees Cook <keescook@...omium.org> wrote:
> >
> > Clang noticed that some none-zero sleep()s were actually using zero
> > anyway. This switches to nanosleep() to gain sub-second granularity.
> >
> > seccomp_bpf.c:2625:9: warning: implicit conversion from 'double' to
> >       'unsigned int' changes value from 0.1 to 0 [-Wliteral-conversion]
> >                 sleep(0.1);
> >                 ~~~~~ ^~~
> >
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  tools/testing/selftests/seccomp/seccomp_bpf.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > index 067cb4607d6c..a9f278c13f13 100644
> > --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> > +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > @@ -2569,6 +2569,7 @@ TEST_F(TSYNC, two_siblings_not_under_filter)
> >  {
> >         long ret, sib;
> >         void *status;
> > +       struct timespec delay = { .tv_nsec = 100000000 };
>
> "Omitted fields are implicitly initialized the same as for objects
> that have static storage duration."
> https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html
> https://godbolt.org/z/cuGqxM
> (So this wont sleep an arbitrary amount of seconds, phew)

Yup. :) Even an empty initializer works ... = { };
(Except that padding bytes aren't always included in the zeroing...)

>
> >
> >         ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
> >                 TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
> > @@ -2622,7 +2623,7 @@ TEST_F(TSYNC, two_siblings_not_under_filter)
> >         EXPECT_EQ(SIBLING_EXIT_UNKILLED, (long)status);
> >         /* Poll for actual task death. pthread_join doesn't guarantee it. */
> >         while (!kill(self->sibling[sib].system_tid, 0))
> > -               sleep(0.1);
> > +               nanosleep(&delay, NULL);
> >         /* Switch to the remaining sibling */
> >         sib = !sib;
> >
> > @@ -2647,7 +2648,7 @@ TEST_F(TSYNC, two_siblings_not_under_filter)
> >         EXPECT_EQ(0, (long)status);
> >         /* Poll for actual task death. pthread_join doesn't guarantee it. */
> >         while (!kill(self->sibling[sib].system_tid, 0))
> > -               sleep(0.1);
> > +               nanosleep(&delay, NULL);
>
> Interesting bug.  If the sleeps weren't doing anything, are they even
> needed? Does adding the tests cause them to continue to pass, or start
> to fail?  If they weren't doing anything, and the tests were passing,
> maybe it's just better to remove them?

It was just spinning. This restores the intention of not being so
aggressive in the wait loop. While the sleep could be removed, that
wasn't the intention.

-- 
Kees Cook