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Date:   Tue, 30 Jun 2020 09:57:00 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] icmp: support rfc 4884

On Mon, Jun 29, 2020 at 10:19 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Mon, Jun 29, 2020 at 8:37 PM Tom Herbert <tom@...bertland.com> wrote:
> >
> > On Mon, Jun 29, 2020 at 4:07 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> > >
> > >
> > >
> > > On 6/29/20 2:30 PM, Willem de Bruijn wrote:
> > > > On Mon, Jun 29, 2020 at 5:15 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > >>
> > > >>
> > > >>
> > > >> On 6/29/20 9:57 AM, Willem de Bruijn wrote:
> > > >>> From: Willem de Bruijn <willemb@...gle.com>
> > > >>>
> > > >>> ICMP messages may include an extension structure after the original
> > > >>> datagram. RFC 4884 standardized this behavior.
> > > >>>
> > > >>> It introduces an explicit original datagram length field in the ICMP
> > > >>> header to delineate the original datagram from the extension struct.
> > > >>>
> > > >>> Return this field when reading an ICMP error from the error queue.
> > > >>
> > > >> RFC mentions a 'length' field of 8 bits, your patch chose to export the whole
> > > >> second word of icmp header.
> > > >>
> > > >> Why is this field mapped to a prior one (icmp_hdr(skb)->un.gateway) ?
> > > >>
> > > >> Should we add an element in the union to make this a little bit more explicit/readable ?
> > > >>
> > > >> diff --git a/include/uapi/linux/icmp.h b/include/uapi/linux/icmp.h
> > > >> index 5589eeb791ca580bb182e1dc38c05eab1c75adb9..427ed5a6765316a4c1e2fa06f3b6618447c01564 100644
> > > >> --- a/include/uapi/linux/icmp.h
> > > >> +++ b/include/uapi/linux/icmp.h
> > > >> @@ -76,6 +76,7 @@ struct icmphdr {
> > > >>                 __be16  sequence;
> > > >>         } echo;
> > > >>         __be32  gateway;
> > > >> +       __be32  second_word; /* RFC 4884 4.[123] : <unused:8>,<length:8>,<mtu:16> */
> > > >>         struct {
> > > >>                 __be16  __unused;
> > > >>                 __be16  mtu;
> > > >
> > > > Okay. How about a variant of the existing struct frag?
> > > >
> > > > @@ -80,6 +80,11 @@ struct icmphdr {
> > > >                 __be16  __unused;
> > > >                 __be16  mtu;
> > > >         } frag;
> > > > +       struct {
> > > > +               __u8    __unused;
> > > > +               __u8    length;
> > > > +               __be16  mtu;
> > > > +       } rfc_4884;
> > > >         __u8    reserved[4];
> > > >    } un;
> > > >
> > >
> > > Sure, but my point was later in the code :
> > >
> > > >>> +     if (inet_sk(sk)->recverr_rfc4884)
> > > >>> +             info = ntohl(icmp_hdr(skb)->un.gateway);
> > > >>
> > > >> ntohl(icmp_hdr(skb)->un.second_word);
> > >
> > > If you leave there "info = ntohl(icmp_hdr(skb)->un.gateway)" it is a bit hard for someone
> > > reading linux kernel code to understand why we do this.
> > >
> > It's also potentially problematic. The other bits are Unused, which
> > isn't the same thing as necessarily being zero. Userspace might assume
> > that info is the length without checking its bounded.
>
> It shouldn't. The icmp type and code are passed in sock_extended_err
> as ee_type and ee_code. So it can demultiplex the meaning of the rest
> of the icmp header.
>
> It just needs access to the other 32-bits, which indeed are context
> sensitive. It makes more sense to me to let userspace demultiplex this
> in one place, rather than demultiplex in the kernel and define a new,
> likely no simpler, data structure to share with userspace.
>
> Specific to RFC 4884, the 8-bit length field coexists with the
> 16-bit mtu field in case of ICMP_FRAG_NEEDED, so we cannot just pass
> the first as ee_info in RFC 4884 mode. sock_extended_err additionally
> has ee_data, but after that we're out of fields, too, so this approach
> is not very future proof to additional ICMP extensions.
>
> On your previous point, it might be useful to define struct rfc_4884
> equivalent outside struct icmphdr, so that an application can easily
> cast to that. RFC 4884 itself does not define any extension objects.
> That is out of scope there, and in my opinion, here. Again, better
> left to userspace. Especially because as it describes, it standardized
> the behavior after observing non-compliant, but existing in the wild,
> proprietary extension variants. Users may have to change how they
> interpret the fields based on what they have deployed.

As this just shares the raw icmp header data, I should probably
change the name to something less specific to RFC 4884.

Since it would also help with decoding other extensions, such as
the one you mention in  draft-ietf-6man-icmp-limits-08.

Unfortunately I cannot simply reserve IP_RECVERR with integer 2.
Perhaps IP_RECVERR_EXINFO.

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