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

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Wed, 11 Sep 2019 09:56:09 -0300

On Wed, Sep 11, 2019 at 05:38:33PM +0800, Xin Long wrote:
> On Wed, Sep 11, 2019 at 5:21 PM Xin Long <[email-masked]> wrote:
> >
> > On Wed, Sep 11, 2019 at 5:03 PM David Laight <[email-masked]> wrote:
> > >
> > > From: Xin Long [mailto:[email-masked]]
> > > > Sent: 11 September 2019 09:52
> > > > On Tue, Sep 10, 2019 at 9:19 PM David Laight <[email-masked]> 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 <[email-masked]>
> > > > > > ---
> > > > > >  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;

I'm confused with the snippets, but if I got them right, this is after
dealing with proper len and could leave val.spt_pathcpthld
uninitialized if the application used the old format and sysctl is !=
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)
>