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

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Message-ID: <CAOFY-A1UEPx12274Mqw3xefNDAVg2FQGAvV4LHoWMLyihyaD6w@mail.gmail.com>
Date:   Tue, 25 Feb 2020 12:20:22 -0800
From:   Arjun Roy <arjunroy@...gle.com>
To:     Soheil Hassas Yeganeh <soheil@...gle.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Arjun Roy <arjunroy.kdev@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] tcp-zerocopy: Update returned getsockopt() optlen.

On Tue, Feb 25, 2020 at 9:17 AM Soheil Hassas Yeganeh <soheil@...gle.com> wrote:
>
> On Tue, Feb 25, 2020 at 12:04 PM Arjun Roy <arjunroy@...gle.com> wrote:
> >
> > On Tue, Feb 25, 2020 at 8:48 AM Arjun Roy <arjunroy@...gle.com> wrote:
> > >
> > > 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.
> > >
> >
> > 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.
>
> Thank you, Arjun.  Given that most TCP socket options overwrite the
> optlen even when returning error, I think we can avoid having the
> extra branch by simply moving put_user right after the check for "len
> == sizeof(zc)" and before "switch(len)".
>
> Thanks,
> Soheil
>
Unfortunately I don't that works in this case - there's a point before
then that could set len to sizeof(zc) (if len was > sizeof(zc) to
begin with) which would disrupt what we want.

Accounting for that would probably add more complication and still
require a branch, so I'm going with the simpler move in this case.
Will send a v2 patch out momentarily.

Thanks,
-Arjun


> > Thanks,
> > -Arjun
> >
> > > -Arjun