netdev - Re: [PATCH] tun: support not enabling carrier in TUNSETIFF

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Date:   Tue, 20 Sep 2022 09:44:45 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Maciej Żenczykowski <maze@...gle.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Patrick Rohr <prohr@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Network Development Mailing List 
        <netdev@...r.kernel.org>, Lorenzo Colitti <lorenzo@...gle.com>
Subject: Re: [PATCH] tun: support not enabling carrier in TUNSETIFF

On Tue, Sep 20, 2022 at 8:01 AM Maciej Żenczykowski <maze@...gle.com> wrote:
>
> On Mon, Sep 19, 2022 at 10:18 AM Stephen Hemminger
> <stephen@...workplumber.org> wrote:
> > On Fri, 16 Sep 2022 16:45:52 -0700
> > Patrick Rohr <prohr@...gle.com> wrote:
> > >  #define IFF_DETACH_QUEUE 0x0400
> > > +/* Used in TUNSETIFF to bring up tun/tap without carrier */
> > > +#define IFF_NO_CARRIER IFF_DETACH_QUEUE
> >
> > Overloading a flag in existing user API is likely to break
> > some application somewhere...
>
> We could of course burn a bit (0x0040 and 0x0080 are both currently
> utterly unused)... but that just seemed wasteful...
> Do you think that would be better?
>
> I find it exceedingly unlikely that any application is specifying this
> flag to TUNSETIFF currently.
>
> This flag has barely any hits in the code base, indeed ignoring the
> Documentation, tests, and #define's we have:
>
> $ git grep IFF_DETACH_QUEUE
> drivers/net/tap.c:928:  else if (flags & IFF_DETACH_QUEUE)
> drivers/net/tun.c:2954: } else if (ifr->ifr_flags & IFF_DETACH_QUEUE) {
> drivers/net/tun.c:3115:                 ifr.ifr_flags |= IFF_DETACH_QUEUE;
>
> The first two implement ioctl(TUNSETQUEUE) -- that's the only spot
> where IFF_DETACH_QUEUE is currently supposed to be used.
>
> The third one is the most interesting, see drivers/net/tun.c:3111
>
>  case TUNGETIFF:
>          tun_get_iff(tun, &ifr);
>          if (tfile->detached)
>                  ifr.ifr_flags |= IFF_DETACH_QUEUE;
>          if (!tfile->socket.sk->sk_filter)
>                  ifr.ifr_flags |= IFF_NOFILTER;
>
> This means TUNGETIFF can return this flag for a detached queue.  However:
>
> (a) multiqueue tun/tap is pretty niche, and detached queues are even more niche.
>
> (b) the TUNGETIFF returned ifr_flags field already cannot be safely
> used as input to TUNSETIFF,

Yes, but it could be used by userspace to recover the multiqueue state
via TUNSETQUEUE for a feature like checkpoint.

> because IFF_NOFILTER == IFF_NO_PI ==
> 0x1000
>
> (this overlap of IFF_NO_PI and IFF_NOFILTER is why we thought it'd be
> ok to overlap here as well)
>
> (c) if this actually turns out to be a problem it shouldn't be that
> hard to fix the 1 or 2 userspace programs to mask out the flag
> and not pass in garbage... Do we really want / need to maintain
> compatibility with extremely badly written userspace?

Not sure, but instead of trying to answer this hard question, having a
new flag seems to be easier.

> It's really hard to even imagine how such code would come into existence...
>
> Arguably the TUNSETIFF api should have always returned an error for
> invalid flags... should we make that change now?

Probably too late to do that.

Thanks

>