[netif] verify unicast netlink ACKs and gate cache updates

[binarylogic]

Summary

This PR hardens Linux unicast netif handling by tying cache mutation to verified netlink outcomes.

Problem

UpdateIp6UnicastAddresses() previously updated mIp6UnicastAddresses optimistically, without confirming whether the kernel accepted each unicast add/remove request.
When RTNETLINK rejected a request, OTBR cache could diverge from kernel state.

Changes

• Unicast address updates now verify the matching netlink ACK by request sequence.
• ACK wait is bounded by an absolute timeout deadline.
• ACK parsing validates nlmsgerr payload length before dereference.
• Transient recv() interruptions (EINTR, EAGAIN, EWOULDBLOCK) are retried while waiting.
• Idempotent outcomes are treated as success:
  • add: EEXIST
  • remove: ENOENT, EADDRNOTAVAIL
• UpdateIp6UnicastAddresses() now mutates local cache only for operations that succeeded.

Why this is safe

• Successful paths preserve existing behavior.
• Failure handling is stricter and prevents cache/kernel drift.
• Timeout handling avoids indefinite wait on unrelated netlink traffic.

Validation

• clang-format check (LLVM 19) on modified files.
• git diff --check clean.
• Linux CI/runtime validation is still required for full end-to-end netlink path verification.

Fixes #3244

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical issue where the OpenThread Border Router (OTBR) cache could diverge from the Linux kernel's network interface state. Previously, unicast IPv6 address updates were optimistically applied to the local cache without confirming the kernel's acceptance via netlink responses. This change hardens the netif handling by introducing a robust mechanism to verify netlink ACKs for unicast address additions and removals, ensuring that the local cache accurately reflects the kernel's state and preventing inconsistencies.

Highlights

• Netlink ACK Verification: Implemented robust verification of netlink ACKs for unicast IPv6 address additions and removals to ensure kernel operations are confirmed.
• Timeout Mechanism: Introduced a bounded timeout for waiting on netlink ACKs, preventing indefinite waits and improving system responsiveness.
• Error Handling and Idempotency: Enhanced netlink error parsing by validating payload length and treating idempotent outcomes (e.g., EEXIST for add, ENOENT/EADDRNOTAVAIL for remove) as successful operations.
• Cache Synchronization: Modified the local cache (mIp6UnicastAddresses) to only reflect changes that have been successfully acknowledged by the kernel, preventing divergence between OTBR's state and the actual network interface configuration.
• Transient Error Retries: Added retry logic for transient recv() interruptions (EINTR, EAGAIN, EWOULDBLOCK) during netlink communication, improving reliability.

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Changelog

• src/host/posix/netif.cpp
  • Refactored UpdateIp6UnicastAddresses to use a temporary updatedUnicastAddresses vector to manage pending changes.
  • Modified UpdateIp6UnicastAddresses to conditionally update the mIp6UnicastAddresses member based on the otbrError returned by ProcessUnicastAddressChange.
  • Added warning logs for failed unicast address additions or removals, providing better diagnostic information.
• src/host/posix/netif.hpp
  • Updated the signature of ProcessUnicastAddressChange to return otbrError, allowing for explicit error propagation.
• src/host/posix/netif_linux.cpp
  • Included <poll> and <chrono> headers to support timeout and polling functionality for netlink operations.
  • Changed ProcessUnicastAddressChange to return otbrError, aligning with the updated header definition.
  • Implemented a netlink ACK verification loop with a kNetlinkAckTimeoutMs (1000ms) deadline to ensure timely responses.
  • Utilized poll() to wait for netlink responses, incorporating retry logic for EINTR to handle signal interruptions gracefully.
  • Processed received netlink messages, verifying sequence numbers and message types to match the original request.
  • Validated nlmsgerr payload length before dereferencing to prevent potential memory access issues.
  • Interpreted specific kernel errors (EEXIST for add, ENOENT, EADDRNOTAVAIL for remove) as successful idempotent operations.
  • Added detailed error logging for netlink failures, including the specific errno.
• src/host/posix/netif_unix.cpp
  • Modified ProcessUnicastAddressChange to return otbrError, aligning with the updated header definition.
  • Provided a default success return (OTBR_ERROR_NONE) for ProcessUnicastAddressChange on non-Linux platforms, maintaining compatibility.

Activity

• No human activity has been recorded on this pull request since its creation.

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[gemini-code-assist]

Code Review

This pull request significantly improves the robustness of unicast address management on Linux by verifying netlink acknowledgements before mutating the internal address cache. This prevents state divergence between the border router and the kernel. The implementation is well-done, with careful handling of timeouts, transient errors, and idempotent operations. I have one suggestion to improve the efficiency of the address removal logic in netif.cpp.

[codecov]

Codecov Report

❌ Patch coverage is 65.30612% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 34.95%. Comparing base (2b41187) to head (7af0fe0).
⚠️ Report is 1387 commits behind head on main.

Files with missing lines	Patch %	Lines
src/host/posix/netif_linux.cpp	44.68%	21 Missing and 5 partials ⚠️
tests/gtest/test_netif.cpp	72.72%	6 Missing ⚠️
src/host/posix/netif.cpp	93.10%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3245       +/-   ##
===========================================
- Coverage   55.77%   34.95%   -20.83%     
===========================================
  Files          87      145       +58     
  Lines        6890    17859    +10969     
  Branches        0     1483     +1483     
===========================================
+ Hits         3843     6242     +2399     
- Misses       3047    11286     +8239     
- Partials        0      331      +331     

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