netdev - Re: [PATCH net-next 5/5] sctp: add spt_pathcpthld in struct sctp

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Message-ID: <CADvbK_dqNas+vwP2t3LqWyabNnzRDO=PZPe4p+zE-vQJTnfKpA@mail.gmail.com>
Date:   Wed, 11 Sep 2019 17:38:33 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     network dev <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH net-next 5/5] sctp: add spt_pathcpthld in struct sctp_paddrthlds

On Wed, Sep 11, 2019 at 5:21 PM Xin Long <lucien.xin@...il.com> wrote:
>
> On Wed, Sep 11, 2019 at 5:03 PM David Laight <David.Laight@...lab.com> wrote:
> >
> > From: Xin Long [mailto:lucien.xin@...il.com]
> > > Sent: 11 September 2019 09:52
> > > On Tue, Sep 10, 2019 at 9:19 PM David Laight <David.Laight@...lab.com> wrote:
> > > >
> > > > From: Xin Long
> > > > > Sent: 09 September 2019 08:57
> > > > > Section 7.2 of rfc7829: "Peer Address Thresholds (SCTP_PEER_ADDR_THLDS)
> > > > > Socket Option" extends 'struct sctp_paddrthlds' with 'spt_pathcpthld'
> > > > > added to allow a user to change ps_retrans per sock/asoc/transport, as
> > > > > other 2 paddrthlds: pf_retrans, pathmaxrxt.
> > > > >
> > > > > Note that ps_retrans is not allowed to be greater than pf_retrans.
> > > > >
> > > > > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > > > > ---
> > > > >  include/uapi/linux/sctp.h |  1 +
> > > > >  net/sctp/socket.c         | 10 ++++++++++
> > > > >  2 files changed, 11 insertions(+)
> > > > >
> > > > > diff --git a/include/uapi/linux/sctp.h b/include/uapi/linux/sctp.h
> > > > > index a15cc28..dfd81e1 100644
> > > > > --- a/include/uapi/linux/sctp.h
> > > > > +++ b/include/uapi/linux/sctp.h
> > > > > @@ -1069,6 +1069,7 @@ struct sctp_paddrthlds {
> > > > >       struct sockaddr_storage spt_address;
> > > > >       __u16 spt_pathmaxrxt;
> > > > >       __u16 spt_pathpfthld;
> > > > > +     __u16 spt_pathcpthld;
> > > > >  };
> > > > >
> > > > >  /*
> > > > > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > > > > index 5e2098b..5b9774d 100644
> > > > > --- a/net/sctp/socket.c
> > > > > +++ b/net/sctp/socket.c
> > > > > @@ -3954,6 +3954,9 @@ static int sctp_setsockopt_paddr_thresholds(struct sock *sk,
> > > >
> > > > This code does:
> > > >         if (optlen < sizeof(struct sctp_paddrthlds))
> > > >                 return -EINVAL;
> > > here will become:
> > >
> > >         if (optlen >= sizeof(struct sctp_paddrthlds)) {
> > >                 optlen = sizeof(struct sctp_paddrthlds);
> > >         } else if (optlen >= ALIGN(offsetof(struct sctp_paddrthlds,
> > >                                             spt_pathcpthld), 4))
> > >                 optlen = ALIGN(offsetof(struct sctp_paddrthlds,
> > >                                         spt_pathcpthld), 4);
> > >                 val.spt_pathcpthld = 0xffff;
> > >         else {
> > >                 return -EINVAL;
> > >         }
> >
> > Hmmm...
> > If the kernel has to default 'val.spt_pathcpthld = 0xffff'
> > then recompiling an existing application with the new uapi
> > header is going to lead to very unexpected behaviour.
> >
> > The best you can hope for is that the application memset the
> > structure to zero.
> > But more likely it is 'random' on-stack data.
> 0xffff is a value to disable the feature 'Primary Path Switchover'.
> you're right that user might set it to zero unexpectly with their
> old application rebuilt.
>
> A safer way is to introduce "sysctl net.sctp.ps_retrans", it won't
> matter if users set spt_pathcpthld properly when they're not aware
> of this feature, like "sysctl net.sctp.pF_retrans". Looks better?
Sorry for confusing,  "sysctl net.sctp.ps_retrans" is already there
(its value is 0xffff by default),
we just need to do this in sctp_setsockopt_paddr_thresholds():

        if (copy_from_user(&val, (struct sctp_paddrthlds __user *)optval,
                           optlen))
                return -EFAULT;

        if (sock_net(sk)->sctp.ps_retrans == 0xffff)
                val.spt_pathcpthld = 0xffff;

        if (val.spt_pathpfthld > val.spt_pathcpthld)
                return -EINVAL;

>
> >
> >         David
> >
> > -
> > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> > Registration No: 1397386 (Wales)