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

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Thu, 12 Sep 2019 19:51:54 -0300

On Thu, Sep 12, 2019 at 01:47:08AM +0800, Xin Long wrote:
> On Wed, Sep 11, 2019 at 8:56 PM Marcelo Ricardo Leitner
> <[email-masked]> wrote:
> >
> > 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.
> right, how about this in sctp_setsockopt_paddr_thresholds():
> 
>         offset = ALIGN(offsetof(struct sctp_paddrthlds, spt_pathcpthld), 4);
>         if (optlen < offset)
>                 return -EINVAL;
>         if (optlen < sizeof(val) || sock_net(sk)->sctp.ps_retrans == 0xffff) {
>                 optlen = offset;
>                 val.spt_pathcpthld = 0xffff;
>         } else {
>                 optlen = sizeof(val);
>         }
> 
>         if (copy_from_user(&val, (struct sctp_paddrthlds __user *)optval,
>                            optlen))
>                 return -EFAULT;
> 
>         if (val.spt_pathpfthld > val.spt_pathcpthld)
>                 return -EINVAL;
> 
> Which means we will 'skip' spt_pathcpthld if (it's using old format) or
> (ps_retrans is disabled and it's using new format).

This will be inconsistent if we end up having to add another field
after spt_pathcpthld, because it will get ignored if ps_retrans ==
0xffff.  Lets not optimize out the fields please. This is already very
tricky to get right.

> Note that  ps_retrans < pf_retrans is not allowed in rfc7829.
> 
> and in sctp_getsockopt_paddr_thresholds():
> 
>         offset = ALIGN(offsetof(struct sctp_paddrthlds, spt_pathcpthld), 4);
>         if (len < offset)
>                 return -EINVAL;
>         if (len < sizeof(val) || sock_net(sk)->sctp.ps_retrans == 0xffff)
>                 len = offset;
>         else
>                 len = sizeof(val);

Here it is more visible. If net->...ps_retrans is disabled, remaining
fields (currently just this one, but as we are extending it now, we
have to think about the possibility of more as well) will be ignored,
we and we may not want that.

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