Re: [PATCH net v2] calipso: keep BIG TCP payload_len in calipso_skbuff_setattr()

On Fri, 09 Oct 2026 12:59:19 -0300 Joas Antonio dos Santos wrote:

calipso_skbuff_setattr() adjusts payload_len by the length of the
option it inserts or resizes:

	payload = ntohs(ip6_hdr->payload_len);
	ip6_hdr->payload_len = htons(payload + len_delta);

GRO can build BIG TCP packets larger than IPV6_MAXPLEN, and stores
payload_len == 0 for them; ipv6_payload_len() then falls back to
skb->len.  For such a packet forwarded to a destination mapped to
CALIPSO, the code above writes len_delta (or 65536 + len_delta when
shrinking) into payload_len, and ipv6_payload_len() no longer falls
back to skb->len.  nf_tables, conntrack and sch_cake rely on it.

The same arithmetic also wraps when a packet grows past IPV6_MAXPLEN,
and a BIG TCP packet that shrinks below IPV6_MAXPLEN should get its
real length back.

Set payload_len from the skb length with ipv6_set_payload_len(), which
stores 0 above IPV6_MAXPLEN and the real length otherwise, as
ipv6_gro_complete() does.  The network header was just reset to
skb->data, so skb->len - sizeof(*ip6_hdr) is the payload length.

Tested on one kernel (arm64, QEMU) with SELinux permissive, a minimal
mdp-generated policy (network_peer_controls=1), "netlabelctl calipso
add pass doi:16" and a NetLabel mapping of fd00:2::/64 to CALIPSO
DOI 16.  calipso_skbuff_setattr() was called through
netlbl_skbuff_setattr() from a test-only debugfs hook (not part of
this patch) adding a level-only label; payload_len after the call:

  case                         before fix   after fix   real length
  1000, unlabeled                    1016        1016          1016
  70000 GSO, payload_len 0             16           0         70016
  65530 GSO, grows past MAXPLEN        10           0         65546
  65540 GSO with a 24 byte hop
  header, shrinks below MAXPLEN     65528       65532         65532
  1000 with a 24 byte hop header      992         992           992

(0 means ipv6_payload_len() returns the real length.)  On the forward
path, with a client injecting unlabeled UDP through a router to a
server, the router adds the CALIPSO option and the server delivers all
datagrams (Udp6InDatagrams 5, Ip6InTruncatedPkts 0) with this patch.

Fixes: 81fbc812132c ("ipv6/gro: insert temporary HBH/jumbo header")
Suggested-by: Eric Dumazet <edumazet@kernel.org>
Suggested-by: Wang Zhan <redacted>
Signed-off-by: Joas Antonio dos Santos <redacted>
Assisted-by: Claude:claude-opus-5-5

Reviewed-by: Wang Zhan <redacted>

nit: consider dropping the comment, it just repeats what
ipv6_set_payload_len() does.