linux-kernel - Re: [PATCH] PPPoL2TP: Add more code snippets

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Date:   Tue, 18 Apr 2023 11:11:48 +0200
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     James Chapman <jchapman@...alix.com>, tparkin@...alix.com,
        edumazet@...gle.com, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, corbet@....net, netdev@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PPPoL2TP: Add more code snippets

Guillaume Nault, le mar. 18 avril 2023 11:06:51 +0200, a ecrit:
> On Tue, Apr 18, 2023 at 10:53:23AM +0200, Samuel Thibault wrote:
> > Guillaume Nault, le mar. 18 avril 2023 10:34:03 +0200, a ecrit:
> > > On Mon, Apr 17, 2023 at 12:07:04AM +0200, Samuel Thibault wrote:
> > > >          sax.sa_family = AF_PPPOX;
> > > >          sax.sa_protocol = PX_PROTO_OL2TP;
> > > >          sax.pppol2tp.fd = tunnel_fd;
> > > > @@ -406,12 +407,64 @@ Sample userspace code:
> > > >          /* session_fd is the fd of the session's PPPoL2TP socket.
> > > >           * tunnel_fd is the fd of the tunnel UDP / L2TPIP socket.
> > > >           */
> > > > -        fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
> > > > -        if (fd < 0 ) {
> > > > +        ret = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
> > > > +        if (ret < 0 ) {
> > > 
> > > Now you also need to close session_fd.
> > 
> > ? No, we need it for PPPIOCGCHAN, and also PPPIOCGL2TPSTATS.
> 
> connect() failed. You can't do anything with this socket.

Ah, you were talking about the failure case, ok.

> > > > +The ppp<ifunit> interface can then be configured as usual with SIOCSIFMTU,
> > > > +SIOCSIFADDR, SIOCSIFDSTADDR, SIOCSIFNETMASK, and activated by setting IFF_UP
> > > > +with SIOCSIFFLAGS
> > > > +
> > > > +  - Tunnel switching is supported by bridging channels::
> > > 
> > > This is a PPP feature not an L2TP one.
> > > 
> > > PPPIOCBRIDGECHAN's description
> > > belongs to Documentation/networking/ppp_generic.rst, where it's already
> > > documented.
> > 
> > Yes but that's hard to find out when you're looking from the L2TP end.
> 
> That's why I proposed linking to ppp_generic.rst.

Yes, but it's still not obvious to L2TP people that it's a ppp channel
that you have to bridge. Really, having that 20-line snippet available
would have saved me some head-scratching time.

Samuel