[PATCH net 0/7] tcp: preserve advertised rwnd accounting across receive-memory decisions

[Wesley Atwell <atwellwea@gmail.com>]

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

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base-commit: 908c344d5cfa0ee6efb3226d22ea661e078ebfa0
-- 
2.43.0