[PATCH 6.1.y 8/8] [AUTOMATED CONFLICT RESOLUTION] Documentation for 870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1

[Sasha Levin <sashal@kernel.org>]

This commit documents the automated conflict resolution performed by claude
for upstream commit 870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1.

claude's resolution process:
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Build passed clean (rc=0 on both x86 and arm64); all warnings are pre-existing/unrelated (dtsi clock warnings, xen/objtool noinstr warnings) and not caused by this change.

## Summary

**Conflict:** Single hunk in `net/ipv4/tcp_output.c:tcp_connect_init()`. Our tree's `window_clamp` assignment (`tp->window_clamp = ...`) lacks a `WRITE_ONCE()` wrapper that upstream added in a separate, unrelated commit not present in this tree. Git couldn't reconcile the two forms of that line since the target commit's context included the `WRITE_ONCE` wrapper.

**Resolution:** Kept our tree's existing line (`tp->window_clamp = dst_metric(dst, RTAX_WINDOW);`, no `WRITE_ONCE`) unchanged, and applied only the actual payload of this commit — replacing `dst_metric_advmss(dst)` with `tcp_dst_advmss(dst)` on that line, matching the other 4 (conflict-free) call sites. The resulting diff is a pure, minimal application of the upstream fix (new `tcp_dst_advmss()` helper in `include/net/tcp.h`, plus 5 call-site substitutions) — no divergence from the original commit beyond that one unavoidable context difference.

Commit `18e91d6e2785f` is on the branch with the original message and author (Yong Wang) preserved, and the build gate is green.
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Backport verification analysis (via claude):
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All 5 file/site changes match exactly (same stat as upstream), and no stray `dst_metric_advmss` call sites were left unconverted in the TCP paths (the remaining hit in `xfrm_policy.c` is outside this commit's scope, untouched upstream too). The build gate passed clean. This is a high-quality backport.

## Analysis

**1. Intent preservation — correct.** The helper `tcp_dst_advmss()` is added verbatim (`max_t(u32, dst_metric_advmss(dst), TCP_MIN_MSS)`), and all 5 call sites that upstream converts (`tcp_ipv4.c`, `tcp_minisocks.c`, `tcp_output.c` x3, `tcp_ipv6.c`) are converted identically. The fix's actual security-relevant payload — clamping route-derived advmss so it can never reach 0 and cause a divide-by-zero in `tcp_select_initial_window()` — is fully and correctly applied.

**2. Differences from upstream — justified.** The only textual difference (beyond expected line-number drift from tree divergence) is in `tcp_connect_init()`:

```
- WRITE_ONCE(tp->window_clamp, dst_metric(dst, RTAX_WINDOW));   // upstream
+ tp->window_clamp = dst_metric(dst, RTAX_WINDOW);              // backport (unchanged)
```

I verified this is *not* something the backport dropped — the parent commit in this tree already had the plain assignment (no `WRITE_ONCE`), confirmed by `git show 488f5e59f0d9c^:net/ipv4/tcp_output.c` and a fruitless `git log -S"WRITE_ONCE(tp->window_clamp"` search. The `WRITE_ONCE` wrapping was introduced by a separate, unrelated upstream commit that was never backported to this tree — it's pure surrounding context that happened to collide with this commit's 3-line hunk during the 3-way merge, not part of this fix's payload. The resolver correctly recognized this and preserved the tree's existing form while applying only the actual substitution (`dst_metric_advmss(dst)` → `tcp_dst_advmss(dst)`) on that line.

**3. Potential concerns — none identified.**
- `TCP_MIN_MSS` (88U) already exists in this tree's `tcp.h`, so the helper compiles as-is.
- No other in-tree callers of `dst_metric_advmss()` were missed; the one remaining hit (`xfrm_policy.c`) is out of scope for this commit both upstream and here.
- Commit metadata (author, `Cc: stable`, `Fixes:`, `[ Upstream commit ... ]` line, dual SoB) is all intact and properly formatted.
- Build gate is green with no new warnings attributable to this change.

**4. Overall assessment: correct, minimal, faithful backport.** The single conflict was a genuine context mismatch from an unrelated, un-backported commit, and it was resolved with the minimum necessary divergence — no scope creep, no missed call sites, no incorrect logic. No issues found.
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This is an empty commit for documentation purposes only.
-- 
2.53.0