linux-kernel - netlink backwards compatibility in userspace tools

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Message-ID: <CAHmME9oixZtPVdH24KJQ9NaTuf_ECAOoHwQhuA+Fy-BX+F_3dw@mail.gmail.com>
Date:   Fri, 29 Sep 2017 12:22:42 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Daniel Kahn Gillmor <dkg@...thhorseman.net>
Subject: netlink backwards compatibility in userspace tools

Hi guys,

One handy aspect of Netlink is that it's backwards compatible. This
means that you can run old userspace utilities on new kernels, even if
the new kernel supports new features and netlink attributes. The wire
format is stable enough that the data marshaled can be extended
without breaking compat. Neat.

I was wondering, though, what you think the best stance is toward
these old userspace utilities. What should they do if the kernel sends
it netlink attributes that it does not recognize? At the moment, I'm
doing something like this:

static void warn_unrecognized(void)
{
    static bool once = false;
    if (once)
        return;
    once = true;
    fprintf(stderr,
        "Warning: this program received from your kernel one or more\n"
        "attributes that it did not recognize. It is possible that\n"
        "this version of wg(8) is older than your kernel. You may\n"
        "want to update this program.\n");
}

This seems like a somewhat sensible warning, but then I wonder about
distributions like Debian, which has a long stable life cycle, so it
frequently has very old tools (ancient iproute2 for example). Then,
VPS providers have these Debian images run on top of newer kernels.
People in this situation would undoubtedly see the above warning a lot
and not be able to do anything about it. Not horrible, but a bit
annoying. Is this an okay annoyance? Or is it advised to just have no
warning at all? One idea would be to put it behind an environment
variable flag, but I don't like too many nobs.

I'm generally wondering about attitudes toward this kind of userspace
program behavior in response to newer kernels.

Thanks,
Jason