[PATCH RFC net-next 0/4] tcp: RFC 7323-compliant window retraction handling

[Simon Baatz via B4 Relay]

Hi,

this series implements the receiver-side requirements for TCP window
retraction as specified in RFC 7323 and adds packetdrill tests to
cover the new behavior.

It addresses a regression with somewhat complex causes; see my message
"Re: [regression] [PATCH net-next 7/8] tcp: stronger sk_rcvbuf checks"
(https://lkml.kernel.org/netdev/aXaHEk_eRJyhYfyM@xxxxxxxxxxxxxxxxxxxx/).

Please see the first patch for background and implementation details.

This is an RFC because a few open questions remain:

- Placement of the new rcv_mwnd_seq field in tcp_sock:

  rcv_mwnd_seq is updated together with rcv_wup and rcv_wnd in
  tcp_select_window. However, rcv_wup is documented as RX read_write
  only (even though it is updated in tcp_select_window), and rcv_wnd
  is TX read_write / RX read_mostly.

  rcv_mwnd_seq is only updated in tcp_select_window and, as far as I
  can tell, is not used on the RX fast path.

  If I understand the placement rules correctly, this means that
  rcv_mwnd_seq, rcv_wup, and rcv_wnd end up in different cacheline
  groups, which feels odd. Guidance on where rcv_mwnd_seq should live
  would be appreciated.

- In tcp_minisocks.c, it is not clear to me whether we should change
  "tcptw->tw_rcv_wnd = tcp_receive_window(tp)" to
  "tcptw->tw_rcv_wnd = tcp_max_receive_window(tp)". I could not find a
  case where this makes a practical difference and have left the
  existing behavior unchanged.

- Packetdrill tests: Some of these seem rather brittle to me; I
  included them mostly to document what I have tested. Suggestions
  for making them more robust are welcome.

- MPTCP seems to modify tp->rcv_wnd of subflows. I haven't looked at
  this, since I wanted to get feedback on the overall approach first.

- Although this series addresses a regression triggered by commit
  d2fbaad7cd8 ("tcp: stronger sk_rcvbuf checks") the underlying
  problem is shrinking the window. Thus I added "Fixes" headers for
  the commits that introduced window shrinking.

I would appreciate feedback on the overall approach and on these
questions.

Signed-off-by: Simon Baatz <gmbnomis@xxxxxxxxx>
---
Simon Baatz (4):
      tcp: implement RFC 7323 window retraction receiver requirements
      selftests/net: packetdrill: add tcp_rcv_wnd_shrink_nomem.pkt
      selftests/net: packetdrill: add tcp_rcv_wnd_shrink_allowed.pkt
      selftests/net: packetdrill: add tcp_rcv_toobig_back_to_back.pkt

 .../networking/net_cachelines/tcp_sock.rst         |   1 +
 include/linux/tcp.h                                |   1 +
 include/net/tcp.h                                  |  14 +++
 net/ipv4/tcp_fastopen.c                            |   1 +
 net/ipv4/tcp_input.c                               |   6 +-
 net/ipv4/tcp_minisocks.c                           |   1 +
 net/ipv4/tcp_output.c                              |  12 +++
 .../net/packetdrill/tcp_rcv_big_endseq.pkt         |   2 +-
 .../packetdrill/tcp_rcv_toobig_back_to_back.pkt    |  27 +++++
 .../net/packetdrill/tcp_rcv_wnd_shrink_allowed.pkt |  35 +++++++
 .../net/packetdrill/tcp_rcv_wnd_shrink_nomem.pkt   | 109 +++++++++++++++++++++
 11 files changed, 206 insertions(+), 3 deletions(-)
---
base-commit: 8bf22c33e7a172fbc72464f4cc484d23a6b412ba
change-id: 20260220-tcp_rfc7323_retract_wnd_rfc-c8a2d2baebde

Best regards,
-- 
Simon Baatz <gmbnomis@xxxxxxxxx>