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

[Eric Dumazet <eric.dumazet@gmail.com>]

On 6/29/20 2:30 PM, Willem de Bruijn wrote:
> 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;
> 

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.