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

[Arjun Roy <arjunroy@google.com>]

On Tue, Feb 25, 2020 at 8:48 AM Arjun Roy <arjunroy@google.com> wrote:
>
> On Mon, Feb 24, 2020 at 10:28 PM Eric Dumazet <edumazet@google.com> wrote:
> >
> > On Mon, Feb 24, 2020 at 10:06 PM Arjun Roy <arjunroy.kdev@gmail.com> wrote:
> > >
> > > From: Arjun Roy <arjunroy@google.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@google.com>
> > > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > > Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
> > > Signed-off-by: Willem de Bruijn <willemb@google.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.
>

Actually, now that I think about it, the (hopefully) common case is
indeed that the kernel and userspace agree on the size of the struct,
so I think just having just that one extra branch to check before
issuing a put_user() would be well worth it compared to all the
instructions in put_user(). I'll send a v2 patch with the change.

Thanks,
-Arjun

> -Arjun