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

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Date:   Wed, 11 Sep 2019 17:21:59 +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 5:03 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?

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