linux-kernel - Re: [PATCH net-next v3] net: ioctl: Use kernel memory on protocol ioctl callbacks

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Date:   Fri, 26 May 2023 01:49:10 -0700
From:   Breno Leitao <leitao@...ian.org>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     dsahern@...nel.org, Remi Denis-Courmont <courmisch@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexander Aring <alex.aring@...il.com>,
        Stefan Schmidt <stefan@...enfreihafen.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        Mat Martineau <martineau@...nel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Xin Long <lucien.xin@...il.com>, leit@...com, axboe@...nel.dk,
        asml.silence@...il.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, dccp@...r.kernel.org,
        linux-wpan@...r.kernel.org, mptcp@...ts.linux.dev,
        linux-sctp@...r.kernel.org
Subject: Re: [PATCH net-next v3] net: ioctl: Use kernel memory on protocol
 ioctl callbacks

On Thu, May 25, 2023 at 11:05:40AM -0400, Willem de Bruijn wrote:
> On Thu, May 25, 2023 at 8:55 AM Breno Leitao <leitao@...ian.org> wrote:
> > @@ -1547,6 +1547,28 @@ int ip_mroute_setsockopt(struct sock *sk, int optname, sockptr_t optval,
> >         return ret;
> >  }
> >
> > +/* Execute if this ioctl is a special mroute ioctl */
> > +int ipmr_sk_ioctl(struct sock *sk, unsigned int cmd, void __user *arg)
> > +{
> > +       switch (cmd) {
> > +       /* These userspace buffers will be consumed by ipmr_ioctl() */
> > +       case SIOCGETVIFCNT: {
> > +               struct sioc_vif_req buffer;
> > +
> > +               return sock_ioctl_inout(sk, cmd, arg, &buffer,
> > +                                     sizeof(buffer));
> > +               }
> 
> More importantly, if we go down the path of demultiplexing in protocol
> independent code to call protocol specific handlers, then there there
> is no need to have them call protocol independent helpers like
> sock_ioct_inout again. Just call the protocol-specific ioctl handlers
> directly?

That is what I was expecting, but, the code is exactly the same and I
kept it in the generic section. This is what this code needs to do:

 * Copy X byte from userspace
 * sk->sk_prot->ioctl()
 * Copy X bytes back to userspace

I tried to keep the code generic enough that it could be reused.
I can definitely push the same code to two different protocols, if you
prefer, no strong opinion.