netdev - Re: [PATCH] PPPoL2TP: Add more code snippets

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Date:   Tue, 18 Apr 2023 11:06:51 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     Samuel Thibault <samuel.thibault@...-lyon.org>,
        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

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.

> I'll put return session_fd instead.

What's the point of returning session_fd if connect() failed?
How will the caller know if session_fd is connected or not?
Why would it even be interested in a half-created session fd?

> > > +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.

> > If necessary, you can link to ppp_generic.rst here.
> > 
> > Also, calling this feature 'tunnel switching' is misleading.
> 
> That's how I have seen it is called in L2TP jargon.

That still doesn't describe the kernel feature. We can add a 'so called
"tunnel switching" in L2TP jargon' into parenthesis to give a hint to
the people using this terminology.

> Samuel
>