selftests: net: Add kernel selftest for RFC 4884

selftests: net: Add kernel selftest for RFC 4884

RFC 4884 extended certain ICMP messages with a length attribute that
encodes the length of the "original datagram" field. This is needed so
that new information could be appended to these messages without
applications thinking that it is part of the "original datagram" field.

In version 5.9, the kernel was extended with two new socket options
(SOL_IP/IP_RECVERR_4884 and SOL_IPV6/IPV6_RECVERR_RFC4884) that allow
user space to retrieve this length which is basically the offset to the
ICMP Extension Structure at the end of the ICMP message. This is
required by user space applications that need to parse the information
contained in the ICMP Extension Structure. For example, the RFC 5837
extension for tracepath.

Add a selftest that verifies correct handling of the RFC 4884 length
field for both IPv4 and IPv6, with and without extension structures,
and validates that malformed extensions are correctly reported as invalid.

For each address family, the test creates:
  - a raw socket used to send locally crafted ICMP error packets to the
    loopback address, and
  - a datagram socket used to receive the encapsulated original datagram
    and associated error metadata from the kernel error queue.

ICMP packets are constructed entirely in user space rather than relying
on kernel-generated errors. This allows the test to exercise invalid
scenarios (such as corrupted checksums and incorrect length fields) and
verify that the SO_EE_RFC4884_FLAG_INVALID flag is set as expected.

Output Example:

$ ./icmp_rfc4884
Starting 18 tests from 18 test cases.
  RUN           rfc4884.ipv4_ext_small_payload.rfc4884 ...
            OK  rfc4884.ipv4_ext_small_payload.rfc4884
ok 1 rfc4884.ipv4_ext_small_payload.rfc4884
  RUN           rfc4884.ipv4_ext.rfc4884 ...
            OK  rfc4884.ipv4_ext.rfc4884
ok 2 rfc4884.ipv4_ext.rfc4884
  RUN           rfc4884.ipv4_ext_large_payload.rfc4884 ...
            OK  rfc4884.ipv4_ext_large_payload.rfc4884
ok 3 rfc4884.ipv4_ext_large_payload.rfc4884
  RUN           rfc4884.ipv4_no_ext_small_payload.rfc4884 ...
            OK  rfc4884.ipv4_no_ext_small_payload.rfc4884
ok 4 rfc4884.ipv4_no_ext_small_payload.rfc4884
  RUN           rfc4884.ipv4_no_ext_min_payload.rfc4884 ...
            OK  rfc4884.ipv4_no_ext_min_payload.rfc4884
ok 5 rfc4884.ipv4_no_ext_min_payload.rfc4884
  RUN           rfc4884.ipv4_no_ext_large_payload.rfc4884 ...
            OK  rfc4884.ipv4_no_ext_large_payload.rfc4884
ok 6 rfc4884.ipv4_no_ext_large_payload.rfc4884
  RUN           rfc4884.ipv4_invalid_ext_checksum.rfc4884 ...
            OK  rfc4884.ipv4_invalid_ext_checksum.rfc4884
ok 7 rfc4884.ipv4_invalid_ext_checksum.rfc4884
  RUN           rfc4884.ipv4_invalid_ext_length_small.rfc4884 ...
            OK  rfc4884.ipv4_invalid_ext_length_small.rfc4884
ok 8 rfc4884.ipv4_invalid_ext_length_small.rfc4884
  RUN           rfc4884.ipv4_invalid_ext_length_large.rfc4884 ...
            OK  rfc4884.ipv4_invalid_ext_length_large.rfc4884
ok 9 rfc4884.ipv4_invalid_ext_length_large.rfc4884
  RUN           rfc4884.ipv6_ext_small_payload.rfc4884 ...
            OK  rfc4884.ipv6_ext_small_payload.rfc4884
ok 10 rfc4884.ipv6_ext_small_payload.rfc4884
  RUN           rfc4884.ipv6_ext.rfc4884 ...
            OK  rfc4884.ipv6_ext.rfc4884
ok 11 rfc4884.ipv6_ext.rfc4884
  RUN           rfc4884.ipv6_ext_large_payload.rfc4884 ...
            OK  rfc4884.ipv6_ext_large_payload.rfc4884
ok 12 rfc4884.ipv6_ext_large_payload.rfc4884
  RUN           rfc4884.ipv6_no_ext_small_payload.rfc4884 ...
            OK  rfc4884.ipv6_no_ext_small_payload.rfc4884
ok 13 rfc4884.ipv6_no_ext_small_payload.rfc4884
  RUN           rfc4884.ipv6_no_ext_min_payload.rfc4884 ...
            OK  rfc4884.ipv6_no_ext_min_payload.rfc4884
ok 14 rfc4884.ipv6_no_ext_min_payload.rfc4884
  RUN           rfc4884.ipv6_no_ext_large_payload.rfc4884 ...
            OK  rfc4884.ipv6_no_ext_large_payload.rfc4884
ok 15 rfc4884.ipv6_no_ext_large_payload.rfc4884
  RUN           rfc4884.ipv6_invalid_ext_checksum.rfc4884 ...
            OK  rfc4884.ipv6_invalid_ext_checksum.rfc4884
ok 16 rfc4884.ipv6_invalid_ext_checksum.rfc4884
  RUN           rfc4884.ipv6_invalid_ext_length_small.rfc4884 ...
            OK  rfc4884.ipv6_invalid_ext_length_small.rfc4884
ok 17 rfc4884.ipv6_invalid_ext_length_small.rfc4884
  RUN           rfc4884.ipv6_invalid_ext_length_large.rfc4884 ...
            OK  rfc4884.ipv6_invalid_ext_length_large.rfc4884
ok 18 rfc4884.ipv6_invalid_ext_length_large.rfc4884
 PASSED: 18 / 18 tests passed.
 Totals: pass:18 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260121114644.2863640-1-danieller@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>