This series keeps sender-visible TCP receive-window accounting tied to the
scaling basis that was in force when the window was advertised.

Problem
-------

`tp->rcv_wnd` is an advertised promise to the sender, but later
receive-memory admission and clamping could reconstruct that promise
through the mutable live `scaling_ratio`. After ratio drift, the stack
could retain or advertise a receive window that no longer matched the
local hard rmem budget.

Fix
---

- store the advertise-time scaling basis alongside `tp->rcv_wnd`
- refresh that pair at the TCP and MPTCP receive-window write sites
- consume the snapshot in receive-memory admission, clamping, and the
  scaled-window quantization path
- preserve the snapshot across `TCP_REPAIR_WINDOW` restore when userspace
  provides it, and fall back safely when legacy userspace cannot
- expose the accounting in tracepoints and cover the ABI/runtime contract
  in selftests

Series layout
-------------

1. track the receive-window snapshot state and helpers
2. refresh the snapshot when TCP advertises or initializes windows
3. use the snapshot in receive-memory admission and clamping
4. extend `TCP_REPAIR_WINDOW` for exact restore plus legacy compatibility
5. refresh the TCP shadow window snapshot in MPTCP
6. expose rmem/backlog state in `rcvbuf_grow` tracepoints
7. cover legacy and extended repair-window layouts in selftests

Testing
-------

- `git diff --check origin/main..HEAD`
- `scripts/checkpatch.pl --strict --show-types` on patches 1-7
- `make -j8 headers`
- `make -j8 net/ipv4/tcp_input.o net/ipv4/tcp_output.o net/ipv4/tcp_minisocks.o net/ipv4/tcp.o`
- `make -j8 C=1 CF='-D__CHECK_ENDIAN__' W=1 net/ipv4/tcp_input.o net/ipv4/tcp_output.o net/ipv4/tcp_minisocks.o net/ipv4/tcp.o`
- `make SPHINXDIRS='networking/net_cachelines' htmldocs`
- `make -j8 vmlinux bzImage modules`
- `make -C tools/testing/selftests/net/tcp_ao -j8`
- `make -C tools/testing/selftests/net/mptcp -j8`
- `packetdrill --dry_run` for `tcp_rcv_toobig.pkt` and
  `tcp_rcv_toobig_default.pkt`
- `virtme-run` guest pass for both packetdrill tests
- feature-enabled guest pass for `restore_ipv4`, `self-connect_ipv4`, and
  `mptcp_sockopt.sh`

Thanks,
Wesley

---
base-commit: 908c344d5cfa0ee6efb3226d22ea661e078ebfa0
-- 
2.43.0

Hi Wesley,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Success! ✅
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/22943222289

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/1943750d8521
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1064867


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)

On Wed, Mar 11, 2026 at 8:56 AM Wesley Atwell <atwellwea@gmail.com> wrote:
>
> This series keeps sender-visible TCP receive-window accounting tied to the
> scaling basis that was in force when the window was advertised.
>
> Problem
> -------
>
> `tp->rcv_wnd` is an advertised promise to the sender, but later
> receive-memory admission and clamping could reconstruct that promise
> through the mutable live `scaling_ratio`. After ratio drift, the stack
> could retain or advertise a receive window that no longer matched the
> local hard rmem budget.
>
> Fix
> ---
>
> - store the advertise-time scaling basis alongside `tp->rcv_wnd`
> - refresh that pair at the TCP and MPTCP receive-window write sites
> - consume the snapshot in receive-memory admission, clamping, and the
>   scaled-window quantization path
> - preserve the snapshot across `TCP_REPAIR_WINDOW` restore when userspace
>   provides it, and fall back safely when legacy userspace cannot
> - expose the accounting in tracepoints and cover the ABI/runtime contract
>   in selftests
>

Your series will heavily conflict with Simon's one

https://patchwork.kernel.org/project/netdevbpf/list/?series=1063486&state=%2A&archive=both

I suggest you rebase/retest/resend after we merge it.

> Series layout
> -------------
>
> 1. track the receive-window snapshot state and helpers
> 2. refresh the snapshot when TCP advertises or initializes windows
> 3. use the snapshot in receive-memory admission and clamping
> 4. extend `TCP_REPAIR_WINDOW` for exact restore plus legacy compatibility
> 5. refresh the TCP shadow window snapshot in MPTCP
> 6. expose rmem/backlog state in `rcvbuf_grow` tracepoints
> 7. cover legacy and extended repair-window layouts in selftests
>
> Testing
> -------
>
> - `git diff --check origin/main..HEAD`
> - `scripts/checkpatch.pl --strict --show-types` on patches 1-7
> - `make -j8 headers`
> - `make -j8 net/ipv4/tcp_input.o net/ipv4/tcp_output.o net/ipv4/tcp_minisocks.o net/ipv4/tcp.o`
> - `make -j8 C=1 CF='-D__CHECK_ENDIAN__' W=1 net/ipv4/tcp_input.o net/ipv4/tcp_output.o net/ipv4/tcp_minisocks.o net/ipv4/tcp.o`
> - `make SPHINXDIRS='networking/net_cachelines' htmldocs`
> - `make -j8 vmlinux bzImage modules`
> - `make -C tools/testing/selftests/net/tcp_ao -j8`
> - `make -C tools/testing/selftests/net/mptcp -j8`
> - `packetdrill --dry_run` for `tcp_rcv_toobig.pkt` and
>   `tcp_rcv_toobig_default.pkt`
> - `virtme-run` guest pass for both packetdrill tests
> - feature-enabled guest pass for `restore_ipv4`, `self-connect_ipv4`, and
>   `mptcp_sockopt.sh`
>
> Thanks,
> Wesley
>
> ---
> base-commit: 908c344d5cfa0ee6efb3226d22ea661e078ebfa0
> --
> 2.43.0
>

On Wed, 11 Mar 2026 09:34:32 +0100 Eric Dumazet wrote:
> Your series will heavily conflict with Simon's one
> 
> https://patchwork.kernel.org/project/netdevbpf/list/?series=1063486&state=%2A&archive=both
> 
> I suggest you rebase/retest/resend after we merge it.

Would it make sense to extend netdevsim and packetdrill to be able to
exercise scaling ratio a little more? Having it optionally clone the
skb and truesize += X would be trivial. IDK how many bugs this would
let us catch tho :(

On Thu, Mar 12, 2026 at 1:41 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Wed, 11 Mar 2026 09:34:32 +0100 Eric Dumazet wrote:
> > Your series will heavily conflict with Simon's one
> >
> > https://patchwork.kernel.org/project/netdevbpf/list/?series=1063486&state=%2A&archive=both
> >
> > I suggest you rebase/retest/resend after we merge it.
>
> Would it make sense to extend netdevsim and packetdrill to be able to
> exercise scaling ratio a little more? Having it optionally clone the
> skb and truesize += X would be trivial. IDK how many bugs this would
> let us catch tho :(

Yes, I think we mentioned this at some point.
packetdrill uses tun device.
Adding a TUN ioctl() to control how many additional bytes are added to
skb->truesize after tun allocates an skb is doable.

On Wed, 11 Mar 2026 01:55:53 -0600 Wesley Atwell wrote:
> Subject: [PATCH net 0/7] tcp: preserve advertised rwnd accounting across receive-memory decisions

when you repost please make sure you use "PATCH net-next v2" 
as the tag / prefix. "net" is a tree we use to fast track fixes.