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

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Date:   Fri, 31 Aug 2018 12:31:23 +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 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:
> >
> > The SIOCGSTAMP/SIOCGSTAMPNS ioctl commands are implemented by many
> > socket protocol handlers, and all of those end up calling the same
> > sock_get_timestamp()/sock_get_timestampns() helper functions, which
> > results in a lot of duplicate code.
> >
> > With the introduction of 64-bit time_t on 32-bit architectures, this
> > gets worse, as we then need four different ioctl commands in each
> > socket protocol implementation.
> >
> > To simplify that, let's add a new .gettstamp() operation in
> > struct proto_ops, and move ioctl implementation into the common
> > sock_ioctl()/compat_sock_ioctl_trans() functions that these all go
> > through.
> >
> > We can reuse the sock_get_timestamp() implementation, but generalize
> > it so it can deal with both native and compat mode, as well as
> > timeval and timespec structures.
> >
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
> This also will simplify fixing a recently reported race condition with
> sock_get_timestamp [1]. That calls sock_enable_timestamp, which
> modifies sk->sk_flags, without taking the socket lock. Currently some
> callers of sock_get_timestamp hold the lock (ax25, netrom, qrtr), many
> don't. See also how this patch removes the lock_sock in the netrom
> case. Moving the call to sock_gettstamp outside the protocol handlers
> will allow taking the lock inside the function.

I suppose it would be best to always take that lock then, rather than
removing the lock as my patch does at the moment.

> 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.

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.

> Acked-by: Willem de Bruijn <willemb@...gle.com>

Thanks,

      Arnd