linux-kernel - Re: [PATCH 7/8] socket: Add SO_TIMESTAMP[NS]

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Message-ID: <CAF=yD-+m=mn9UyB39u_kuRVSsqWFg8ZBW812p_c=h9h=9_6XLA@mail.gmail.com>
Date:   Sun, 25 Nov 2018 19:25:45 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Deepa Dinamani <deepa.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        jejb@...isc-linux.org, ralf@...ux-mips.org, rth@...ddle.net,
        linux-alpha@...r.kernel.org, linux-mips@...ux-mips.org,
        linux-parisc@...r.kernel.org, linux-rdma@...r.kernel.org,
        sparclinux@...r.kernel.org
Subject: Re: [PATCH 7/8] socket: Add SO_TIMESTAMP[NS]_NEW

> > > The existing timestamp options: SO_TIMESTAMP* fail to provide proper
> > > timestamps beyond year 2038 on 32 bit ABIs.
> > > But, these work fine on 64 bit native ABIs.
> > > So now we need a way of updating these timestamps so that we do not
> > > break existing userspace: 64 bit ABIs should not have to change
> > > userspace, 32 bit ABIs should work as is until 2038 after which they
> > > have bad timestamps.
> > > So we introduce new y2038 safe timestamp options for 32 bit ABIs. We
> > > assume that 32 bit applications will switch to new ABIs at some point,
> > > but leave the older timestamps as is.
> > > I can update the commit text as per above.
> >
> > So on 32-bit platforms SO_TIMESTAMP_NEW introduces a new struct
> > sock_timeval with both 64-bit fields.
> >
> > Does this not break existing applications that compile against SO_TIMESTAMP
> > and expect struct timeval? For one example, the selftests under tools/testing.
> >
> > The kernel will now convert SO_TIMESTAMP (previously constant 29) to
> > different SO_TIMESTAMP_NEW (62) and returns a different struct. Perhaps
> > with a library like libc in the middle this can be fixed up
> > transparently, but for
> > applications that don't have a more recent libc or use a library at
> > all, it breaks
> > the ABI.
> >
> > I suspect that these finer ABI points may have been discussed outside the
> > narrow confines of socket timestamping. But on its own, this does worry me.
>
> The entire purpose of the complexities in the patch set is to not break
> the user space ABI after an application gets recompiled with a 64-bit
> time_t defined by a new libc version:
>
> #define SO_TIMESTAMP (sizeof(time_t) == sizeof(__kernel_long_t) ? \
>             SO_TIMESTAMP_OLD : SO_TIMESTAMP_NEW)
>
> This delays the evaluation of SO_TIMESTAMP to the point where
> it is first used, the assumption being that at this point we have included
> the libc header file that defines both 'time_t' and 'struct timeval'.
> [If we have not included that header, we get a compile-time error,
> which is also necessary because the compiler has no way of deciding
> whether to use SO_TIMESTAMP_OLD or SO_TIMESTAMP_NEW
> in that case].
>
> If the application is built with a 32-bit time_t, or with on a 64-bit
> architecture (all of which have 64-bit time_t and __kernel_long_t),
> or on x32 (which also has 64-bit time_t and __kernel_long_t),
> the result is SO_TIMESTAMP_OLD, so we tell the kernel
> to send back a timestamp in the old format, and everything works
> as it did before.
>
> The only thing that changes is 32-bit user space (other than x32) with
> a 64-bit time_t. In this case, the application expects a structure
> that corresponds to the new sock_timeval (which is compatible
> with the user space timeval based on 64-bit time_t), so we must
> use the new constant in order to tell the kernel which one we want.

Thanks. That was exactly the context that I was missing. I hadn't
figured out that the test was based on a libc definition. This all
makes perfect sense.