Re: [musl] Question about musl's time() implementation in time.c

[Arnd Bergmann <arnd@kernel.org>]

On Fri, Jun 10, 2022 at 10:53 AM Zev Levy Stevenson <zevlevys@gmail.com> wrote:
>
> Thank you for the responses, those reasons make sense to me. We are using a very customized toolchain but the kernel itself is standard.
> We looked into it a bit further and we were able to reproduce the issue with a clean musl-gcc toolchain for x86_64 (version 1.2.2) on a Linux kernel that we took from a standard Ubuntu distribution.
> Specifically, tests in the libc-test suite (https://wiki.musl-libc.org/libc-test.html) using the time() function fail sometimes, e.g. src/functional/utime.c, which fails on about ~3-4 runs in every 1,000 runs. This can be reduced to this type of code failing:
>
> t = time(0);
> if(futimens(fd, ((struct timespec[2]){{.tv_nsec=UTIME_NOW},{.tv_nsec=UTIME_OMIT}})) != 0) return 1;
> if (fstat(fd, &st) != 0) return 1;
> if (st.st_atim.tv_sec < t) printf("time inconsistency\n");
>
> When replacing the call to time(0) with a raw call to the Linux time() syscall the issue seems to disappear. On the other hand, using the clock_gettime syscall results in the same issue.
> Perhaps this is an issue with the Linux implementation of these syscalls / vdso functions, in which case further research may be required, or maybe such consistency when using different methods for measuring the system time doesn't have to be guaranteed, in which case the tests should probably be modified to allow for small inaccuracies such as the one described above.

I think the problem here is likely the inconsistency between the
COCK_REALTIME and
CLOCK_REALTIME_COARSE domains.

The time() syscall in the kernel is based on CLOCK_REALTIME_COARSE, while musl
uses CLOCK_REALTIME. The futimens() syscall in turn uses
CLOCK_REALTIME_COARSE.

The coarse time can be up to one timer tick behind, so reading
CLOCK_REALTIME first
can give you the exact second with a small nanosecond value, while the
utime will still
set the previous value.

Can you change the test case to check if the later time is less than
clock_getres(CLOCK_REALTIME_COARSE, ...) behind?

        Arnd