wait4 testcase for y2038 systems

[Alistair Francis]

On Fri, Mar 6, 2020 at 6:13 PM Dmitry V. Levin <ldv at altlinux.org> wrote:
>
> Hi,
>
> On Fri, Mar 06, 2020 at 05:38:33PM -0800, Alistair Francis wrote:
> > Hey,
> >
> > I have a question.
> >
> > Since the 5.1 kernel new 32-bit architectures (like RV32) don't have
> > the wait4 kernel syscall as it isn't y2038 safe. Other 32-bit
> > architectures will eventually also stop using the wait4 kernel syscall
> > (by 2038 at least).
> >
> > glibc still supports the wait4() function call, internally glibc will
> > convert the programs wait4() call into waitid kernel calls.
>
> glibc will support wait4() function call using __NR_wait4 at least until
> the minimal Linux kernel version supported by glibc raises to 5.4, which
> is not going to happen any time soon.

That's not what gibc does. On older architectures it will continue to
support the __NR_wait4 syscall. On new 32-bit architectures (like
RV32) that require the 5.1+ kernel there is no __NR_wait4 syscall and
glibc will use __NR_waitid.

You can see the code here:
https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/wait4.c;h=3a8bed1169c799a8fb20360c06f2808cf648c874;hb=HEAD

Note that due to a bug in __NR_waitid it will actually only do it for
the 5.4+ kernel, which is the minimum version for RV32.

>
> To be honest, I don't see any difference between __NR_wait4 and
> __NR_waitid from y2038 perspective as both of them use the same
> struct rusage.

The time in rusage is still 32-bit for both calls. This is because the
time is a diff (so it won't really overflow).

As __NR_waitid is a superset of __NR_wait4 I think it was just easier
to have only one. I also think originally there was a plan for
__NR_waitid_time64 but that seems to have been dropped.

>
> > This means the strace wait4 testcase fails as it is run with these arguments:
> >   strace -o log -e trace=wait4 -esignal=none ./wait4
> >
> > The wait4 test case prints out wait4 calls, but the actual wait4
> > function that is called doesn't do any wait4 kernel calls instead it
> > does waitid kernel calls.
> >
> > What would be the best way to handle this test for y2038 32-bit systems?
>
> The main purpose of the strace test suite is to test strace.  Yes,
> sometimes we discover bugs in other projects [1], but that's not the
> purpose of the strace test suite.  A test of wait4 syscall decoder shall
> invoke wait4 syscall.  If libc wait4() cannot be relied upon, the test
> shall invoke __NR_wait4 syscall directly, like many other tests do when

That is the problem, there is no __NR_wait4 syscall for new 32-bit archs.

> they cannot use libc functions.  For example, tests/waitid.c invokes
> __NR_waitid syscall directly because libc waitid() doesn't expose the 5th
> argument.

In this case we are fine as __NR_waitid still exists.

Alistair

>
> [1] https://strace.io/wiki/TestSuiteBattlestars
>
> --
> ldv