netdev - Re: [PATCH net-next 1/3] net: rework SIOCGSTAMP ioctl handling

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Date:   Fri, 31 Aug 2018 16:00:21 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Networking <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        linux-arch <linux-arch@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-hams@...r.kernel.org,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        linux-can@...r.kernel.org, dccp@...r.kernel.org,
        linux-wpan@...r.kernel.org, linux-sctp@...r.kernel.org,
        linux-x25@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net: rework SIOCGSTAMP ioctl handling

On Fri, Aug 31, 2018 at 3:38 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Fri, Aug 31, 2018 at 6:31 AM Arnd Bergmann <arnd@...db.de> wrote:
> > On Thu, Aug 30, 2018 at 10:10 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > > On Wed, Aug 29, 2018 at 9:05 AM Arnd Bergmann <arnd@...db.de> wrote:
> > > If this is the only valid implementation of .gettstamp, the indirect
> > > call could be avoided in favor of a simple branch.
> >
> > I thought about that as well, but I could not come up with a
> > good way to encode the difference between socket protocols
> > that allow timestamping and those that don't.
> >
> > I think ideally we would just call sock_gettstamp() unconditonally
> > on every socket, and have that function decide whether timestamps
> > make sense or not. The part I did not understand is which ones
> > actually want the timestamps or not. Most protocols that
> > implement the ioctls also assign skb->tstamp, but there are some
> > protocols in which I could not see skb->tstamp ever being set,
> > and some that set it but don't seem to have the ioctls.
>
> These probably only use cmsgs SCM_TIMESTAMP(NS|IMG)
> to read timestamps.

Good point. FWIW, I have discussed with Deepa how those should
be modified for y2038, but we don't have any current patches for
those.

> > Looking at it again, it seems that sock_gettstamp() should
> > actually deal with this gracefully: it will return a -EINVAL
> > error condition if the timestamp remains at the
> > SK_DEFAULT_STAMP initial value, which is probably
> > just as appropriate (or better) as the current -ENOTTY
> > default, and if we are actually recording timestamps, we
> > might just as well report them.
>
> Yes, that's a nice solution. There is always some risk in changing
> error codes. But ioctl callers should be able to support newly
> implemented functionality. Even if partially implemented and
> returning ENOENT instead of ENOIOCTLCMD.

Ok, so do you think we should stay with the current version
for now, and change the two points later, or should I rework
it to integrate the locking and removing the callback?

I suppose the series actually gets nicer without the
callback, since I can simply add the generic timestamping
implementation first, and then remove the dead ioctl
handlers.

        Arnd