Re: [PATCH net-next] icmp: support rfc 4884

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Mon, Jun 29, 2020 at 5:15 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
>
>
> On 6/29/20 9:57 AM, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@google.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;

 };

>
>
> >
> > ICMPv6 by default already returns the entire 32-bit part of the header
> > that includes this field by default. For consistency, do the exact
> > same for ICMP. So far it only returns mtu on ICMP_FRAG_NEEDED and gw
> > on ICMP_PARAMETERPROB.
> >
> > For backwards compatibility, make this optional, set by setsockopt
> > SOL_IP/IP_RECVERR_RFC4884. For API documentation and feature test, see
> > https://github.com/wdebruij/kerneltools/blob/master/tests/recv_icmp.c
> >
> > Alternative implementation to reading from the skb in ip_icmp_error
> > is to pass the field from icmp_unreach, again analogous to ICMPv6. But
> > this would require changes to every $proto_err() callback, which for
> > ICMP_FRAG_NEEDED pass the u32 info arg to a pmtu update function.
> >
> > Signed-off-by: Willem de Bruijn <willemb@google.com>
> > ---
> >  include/net/inet_sock.h |  1 +
> >  include/uapi/linux/in.h |  1 +
> >  net/ipv4/ip_sockglue.c  | 12 ++++++++++++
> >  3 files changed, 14 insertions(+)
> >
> > diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
> > index a7ce00af6c44..a3702d1d4875 100644
> > --- a/include/net/inet_sock.h
> > +++ b/include/net/inet_sock.h
> > @@ -225,6 +225,7 @@ struct inet_sock {
> >                               mc_all:1,
> >                               nodefrag:1;
> >       __u8                    bind_address_no_port:1,
> > +                             recverr_rfc4884:1,
> >                               defer_connect:1; /* Indicates that fastopen_connect is set
> >                                                 * and cookie exists so we defer connect
> >                                                 * until first data frame is written
> > diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
> > index 8533bf07450f..3d0d8231dc19 100644
> > --- a/include/uapi/linux/in.h
> > +++ b/include/uapi/linux/in.h
> > @@ -123,6 +123,7 @@ struct in_addr {
> >  #define IP_CHECKSUM  23
> >  #define IP_BIND_ADDRESS_NO_PORT      24
> >  #define IP_RECVFRAGSIZE      25
> > +#define IP_RECVERR_RFC4884   26
> >
> >  /* IP_MTU_DISCOVER values */
> >  #define IP_PMTUDISC_DONT             0       /* Never send DF frames */
> > diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> > index 84ec3703c909..525140e3947c 100644
> > --- a/net/ipv4/ip_sockglue.c
> > +++ b/net/ipv4/ip_sockglue.c
> > @@ -398,6 +398,9 @@ void ip_icmp_error(struct sock *sk, struct sk_buff *skb, int err,
> >       if (!skb)
> >               return;
> >
> > +     if (inet_sk(sk)->recverr_rfc4884)
> > +             info = ntohl(icmp_hdr(skb)->un.gateway);
>
> ntohl(icmp_hdr(skb)->un.second_word);
>
> > +
> >       serr = SKB_EXT_ERR(skb);
> >       serr->ee.ee_errno = err;
> >       serr->ee.ee_origin = SO_EE_ORIGIN_ICMP;
> > @@ -755,6 +758,7 @@ static int do_ip_setsockopt(struct sock *sk, int level,
> >       case IP_RECVORIGDSTADDR:
> >       case IP_CHECKSUM:
> >       case IP_RECVFRAGSIZE:
> > +     case IP_RECVERR_RFC4884:
> >               if (optlen >= sizeof(int)) {
> >                       if (get_user(val, (int __user *) optval))
> >                               return -EFAULT;
> > @@ -914,6 +918,11 @@ static int do_ip_setsockopt(struct sock *sk, int level,
> >               if (!val)
> >                       skb_queue_purge(&sk->sk_error_queue);
> >               break;
> > +     case IP_RECVERR_RFC4884:
> > +             if (val != 0 && val != 1)
> > +                     goto e_inval;
> > +             inet->recverr_rfc4884 = val;
> > +             break;
> >       case IP_MULTICAST_TTL:
> >               if (sk->sk_type == SOCK_STREAM)
> >                       goto e_inval;
> > @@ -1588,6 +1597,9 @@ static int do_ip_getsockopt(struct sock *sk, int level, int optname,
> >       case IP_RECVERR:
> >               val = inet->recverr;
> >               break;
> > +     case IP_RECVERR_RFC4884:
> > +             val = inet->recverr_rfc4884;
> > +             break;
> >       case IP_MULTICAST_TTL:
> >               val = inet->mc_ttl;
> >               break;
> >