Re: [PATCH net-next v2] icmp: support rfc 4884

[David Miller <davem@davemloft.net>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Fri, 10 Jul 2020 09:29:02 -0400

> From: Willem de Bruijn <willemb@google.com>
> 
> Add setsockopt SOL_IP/IP_RECVERR_4884 to return the offset to an
> extension struct if present.
> 
> ICMP messages may include an extension structure after the original
> datagram. RFC 4884 standardized this behavior. It stores the offset
> in words to the extension header in u8 icmphdr.un.reserved[1].
> 
> The field is valid only for ICMP types destination unreachable, time
> exceeded and parameter problem, if length is at least 128 bytes and
> entire packet does not exceed 576 bytes.
> 
> Return the offset to the start of the extension struct when reading an
> ICMP error from the error queue, if it matches the above constraints.
> 
> Do not return the raw u8 field. Return the offset from the start of
> the user buffer, in bytes. The kernel does not return the network and
> transport headers, so subtract those.
> 
> Also validate the headers. Return the offset regardless of validation,
> as an invalid extension must still not be misinterpreted as part of
> the original datagram. Note that !invalid does not imply valid. If
> the extension version does not match, no validation can take place,
> for instance.
> 
> For backward compatibility, make this optional, set by setsockopt
> SOL_IP/IP_RECVERR_RFC4884. For API example and feature test, see
> github.com/wdebruij/kerneltools/blob/master/tests/recv_icmp_v2.c
> 
> For forward compatibility, reserve only setsockopt value 1, leaving
> other bits for additional icmp extensions.
> 
> Changes
>   v1->v2:
>   - convert word offset to byte offset from start of user buffer
>     - return in ee_data as u8 may be insufficient
>   - define extension struct and object header structs
>   - return len only if constraints met
>   - if returning len, also validate
> 
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Applied, thanks Willem.