linux-kernel - Re: [PATCH v2 6/8] socket: Add SO_TIMESTAMP[NS]

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Date:   Tue, 18 Dec 2018 17:33:11 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Deepa Dinamani <deepa.kernel@...il.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Richard Henderson <rth@...ddle.net>,
        linux-alpha@...r.kernel.org,
        "open list:RALINK MIPS ARCHITECTURE" <linux-mips@...ux-mips.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v2 6/8] socket: Add SO_TIMESTAMP[NS]_NEW

On Sat, Dec 15, 2018 at 7:52 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> > 3 reasons for not doing this:
> >
> > 1. We do not want to break userspace. If we move this to
> > linux/socket.h all the userspace programs now have to include
> > linux/socket.h or get this definition through a new libc.
> > 2. All the socket options are together in the file asm/socket.h. It
> > doesn't seem good for maintainability to move just a few bits
> > elsewhere.
> > 3. There are only 4 arches (after the series is applied) that have
> > their own asm/socket.h. And, this is because there seems to be
> > significant differences to asm-generic/socket.h that don't seem
> > logically obvious to group and eliminate some of the defines.
>
> Agreed. All good reasons to leave as is.
>
> > Also for the other comment. The reason the conditionals were not
> > consistent is because they were not consistent to begin with.
>
> The only difference I see is an inversion of the test. Nesting order
> is the same:
>
>         int need_software_tstamp = sock_flag(sk, SOCK_RCVTSTAMP);
>         ...
>         if (need_software_tstamp) {
>                 if (!sock_flag(sk, SOCK_RCVTSTAMPNS)) {
>                 } else {
>                 }
>         }
>
> vs
>
>                 if (sock_flag(sk, SOCK_RCVTSTAMP)) {
>                         if (sock_flag(sk, SOCK_RCVTSTAMPNS)) {
>                         } else {
>                         }
>                 }
>
> I suggest just adding something like
>
>         if (need_software_tstamp) {
> +              if (sock_uses_new_tstamp(sk) {
> +                   __sock_recv_timestamp_new(msg, sk,
> ktime_to_timespec64(skb->tstamp));
> +              } else if (!sock_flag(sk, SOCK_RCVTSTAMPNS)) {
> -               if (!sock_flag(sk, SOCK_RCVTSTAMPNS)) {
>                 } else {
>                 }
>
> and
>
>                 if (sock_flag(sk, SOCK_RCVTSTAMP)) {
> +                      if (sock_uses_new_tstamp(sk) {
> +                           __sock_recv_timestamp_new(msg, sk, ts);
> +                      else if (sock_flag(sk, SOCK_RCVTSTAMPNS)) {
> -                       if (sock_flag(sk, SOCK_RCVTSTAMPNS)) {
>                         } else {
>                         }

Generally speaking, I think we want the new time handling
to be written as the default case rather than have it hidden away
in a separate function. If we didn't have the sparc64 quirk with its
unusual timeval definition, we'd only need a special flag for the
old 32-bit format, but that doesn't work as long we have to support
two different 64-bit formats for 64-bit timeval on sparc64
(32 or 64 bit microseconds).

> Note also (2) tentative helper function sock_uses_new_tstamp(const
> struct sock *sk) instead of testing sock_flag(sk, SOCK_TSTAMP_NEW)
> directly. Since the .._NEW variants are equivalent to .._OLD on 64-bit,
> I wonder if we can just compile out the branch. Something like
>
>     static inline bool sock_uses_new_tstamp(const struct sock *sk) {
>             return (sizeof(time_t) != sizeof(__kernel_long_t)) &&
>                        sock_flag(sk, SOCK_TSTAMP_NEW);
>     }

I think that would break compat handling: when we have a 32-bit
user space process, the difference between old and new timestamps
is meaningful even on 64-bit kernels, but the distinction is only made all
the way down in put_cmsg_compat().

      Arnd