netdev - Re: [PATCH net-next] tcp-zerocopy: Update returned getsockopt() optlen.

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Date:   Tue, 25 Feb 2020 08:48:02 -0800
From:   Arjun Roy <arjunroy@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Arjun Roy <arjunroy.kdev@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] tcp-zerocopy: Update returned getsockopt() optlen.

On Mon, Feb 24, 2020 at 10:28 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Mon, Feb 24, 2020 at 10:06 PM Arjun Roy <arjunroy.kdev@...il.com> wrote:
> >
> > From: Arjun Roy <arjunroy@...gle.com>
> >
> > TCP receive zerocopy currently does not update the returned optlen for
> > getsockopt(). Thus, userspace cannot properly determine if all the
> > fields are set in the passed-in struct. This patch sets the optlen
> > before return, in keeping with the expected operation of getsockopt().
> >
> > Signed-off-by: Arjun Roy <arjunroy@...gle.com>
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Signed-off-by: Soheil Hassas Yeganeh <soheil@...gle.com>
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > Fixes: c8856c051454 ("tcp-zerocopy: Return inq along with tcp receive
> > zerocopy")
>
>
> OK, please note for next time :
>
> Fixes: tag should not wrap : It should be a single line.
> Preferably it should be the first tag (before your Sob)
>
> Add v2 as in [PATCH v2 net-next]  :  so that reviewers can easily see
> which version is the more recent one.
>
>
> >
> > +               if (!err) {
> > +                       if (put_user(len, optlen))
> > +                               return -EFAULT;
>
> Sorry for not asking this before during our internal review :
>
> Is the cost of the extra STAC / CLAC (on x86) being high enough that it is worth
> trying to call put_user() only if user provided a different length ?

I'll have to defer to someone with more understanding of the overheads
involved in this case.

-Arjun