netdev - Re: [PATCHv4] PPPoL2TP: Add more code snippets

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Message-ID: <ZctngNzLYe/+Iman@katalix.com>
Date: Tue, 13 Feb 2024 12:58:40 +0000
From: Tom Parkin <tparkin@...alix.com>
To: Samuel Thibault <samuel.thibault@...-lyon.org>,
	James Chapman <jchapman@...alix.com>, edumazet@...gle.com,
	gnault@...hat.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: [PATCHv4] PPPoL2TP: Add more code snippets

On  Tue, Feb 13, 2024 at 12:53:04 +0100, Samuel Thibault wrote:
> Tom Parkin, le mar. 13 févr. 2024 10:51:08 +0000, a ecrit:
> > > +        ret = ioctl(session_fd1, PPPIOCGCHAN, &chindx1);
> > > +        if (ret < 0)
> > > +                return -errno;
> > > +
> > > +        ret = ioctl(session_fd2, PPPIOCGCHAN, &chindx2);
> > > +        if (ret < 0)
> > > +                return -errno;
> > > +
> > > +        ppp_chan_fd = open("/dev/ppp", O_RDWR);
> > > +        if (ppp_chan_fd < 0) {
> > > +                return -errno;
> > > +        }
> > > +
> > > +        ret = ioctl(ppp_chan_fd, PPPIOCATTCHAN, &chindx1);
> > > +        if (ret < 0) {
> > > +                close(ppp_chan_fd);
> > > +                return -errno;
> > > +        }
> > 
> > I think we should drop the PPPIOCATTCHAN ioctl call here.
> > 
> > The input file descriptors are called out as being PPPoX sockets
> > created as described earlier, in which case they should both
> > already be attached to a channel.
> > 
> > It would make more sense IMO to call out the two ppp_chan_fd file
> > descriptors as being input parameters alongside the PPPoX session file
> > descriptors.
> > 
> > > +
> > > +        ret = ioctl(ppp_chan_fd, PPPIOCBRIDGECHAN, &chindx2);
> > > +        close(ppp_chan_fd);
> > > +        if (ret < 0)
> > > +                return -errno;
> > > +
> > > +It can be noted that in this case no PPP interface is needed, and the PPP
> > > +channel does not need to be kept open.  Only the session PPPoX data sockets need
> > > +to be kept open.
> > 
> > Is it true to say that the PPP channel file descriptors can be closed
> > by userspace?
> 
> In our code we do it
> https://code.ffdn.org/sthibaul/l2tpns/-/blob/kernel/l2tpns.c?ref_type=heads#L1295
> and it works all fine indeed (and avoids that fd per session).
> 
> That's actually one of the reason why I made the snipped only take the
> pppox sockets, and make it create the ppp chan fd only temporarily. AIUI
> the pppox socket already has a ppp chan (returned by PPPIOCGCHAN), and
> the ppp chan fd is there only for performing the bridging ioctl.

Thanks for the code reference -- that makes it clearer.  And I'm glad
someone (else) is using PPPIOCBRIDGECHAN :-)

It's a while since I was looking in ppp_generic.c and you're right
about the ppp channel fd.
-- 
Tom Parkin
Katalix Systems Ltd
https://katalix.com
Catalysts for your Embedded Linux software development

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