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Yuyang Huang says: ==================== ipv6: report why a route was deleted in RTM_DELROUTE When the kernel deletes an IPv6 route on its own, the RTM_DELROUTE notification does not say why. User space cannot tell a route that expired from one the router explicitly withdrew, yet the two call for different reactions: an expired RA route means the router failed to refresh it in time, which points at a misconfigured or unreliable router and may warrant action such as disabling IPv6 on that network, while a zero-lifetime withdrawal is normal, RFC-compliant operation. This is a general problem for any consumer device running Linux, especially on Wi-Fi networks, where multicast delivery is not guaranteed (e.g. frames can be lost around DTIM for clients in power save mode). The motivating case is Android: the userspace NetworkStack process listens on RTMGRP_IPV6_ROUTE and today treats any loss of the IPv6 default route as "router lost". To avoid the device repeatedly gaining and losing IPv6 connectivity on a badly configured network, when it detects the device is on a dual-stack network with working IPv4 connectivity, it defensively clears accept_ra_defrtr and restarts IPv6, so user space apps stop using broken global IPv6 connectivity while link-local IPv6 keeps working. That reaction is wrong if the route was withdrawn by a zero-lifetime RA (some ISPs do this intentionally for reconfiguration) - with accept_ra_defrtr off, IPv6 never recovers once the router advertises again. It is the right reaction if the route genuinely expired, since the router failed to refresh it in time. Fixing this in user space is not practical: RTM_NEWROUTE carries the initial route lifetime (in rta_cacheinfo), but the kernel does not resend it when a later RA refreshes the lifetime. So distinguishing the cause of an RTM_DELROUTE from user space would mean opening a raw socket, listening to RAs, and tracking lifetimes independently, duplicating logic the kernel already has. Sending RTM_NEWROUTE on every RA lifetime refresh was also considered, but that would be spammy and is technically wrong, since a lifetime update does not add a new route. This series proposes RTA_DEL_REASON instead: it tells user space why the route was deleted so it can react accordingly. In the Android case, NetworkStack would defensively disable global IPv6 only on RT_DEL_REASON_EXPIRED, and take no action on RT_DEL_REASON_RA_WITHDRAWN, since that is RFC-compliant behavior. Patches 1 to 6 add RTA_DEL_REASON and enum rt_del_reason to the rtnetlink uAPI, thread the reason from the kernel-initiated IPv6 deletion paths down to the RTM_DELROUTE notification, and record the cause: RT_DEL_REASON_EXPIRED for routes garbage collected after their RTF_EXPIRES lifetime ran out, and RT_DEL_REASON_RA_WITHDRAWN for default routes, prefix routes and RFC 4191 route information routes withdrawn by Router Advertisements. Patches 1 to 5 are no-ops on the wire; the attribute first appears in patch 6. The route addition path is not touched. Patches 7 to 9 extend the rt-route Netlink spec with the route notifications and their multicast groups, split the newroute and delroute request attribute lists out of the shared getroute reply list, and add the new attribute and its enum. Only kernel-initiated deletions that user space cannot otherwise explain are attributed. User-requested deletions are self-explanatory to the requester, so they carry no reason; the UAPI documents that absence and RT_DEL_REASON_UNSPEC must be treated identically, which keeps the door open for attributing more paths (nexthop removal cascades, device removal) later. Patch 10 adds selftests covering all three producer paths: a GC-expired route, and a default route + PIO prefix route + RIO route advertised and then withdrawn by hand-crafted RAs over a raw ICMPv6 socket (no external RA tool needed), plus a check that user-requested deletions carry no attribute. The notifications are decoded with YNL, which also exercises the rt-route spec additions. ==================== Link: https://patch.msgid.link/20260808005642.26901-1-sigefriedhyy@gmail.com Signed-off-by: Paolo Abeni <pabeni@redhat.com> |
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer - Getting started with kernel development * Academic Researcher - Studying kernel internals and architecture * Security Expert - Hardening and vulnerability analysis * Backport/Maintenance Engineer - Maintaining stable kernels * System Administrator - Configuring and troubleshooting * Maintainer - Leading subsystems and reviewing patches * Hardware Vendor - Writing drivers for new hardware * Distribution Maintainer - Packaging kernels for distros * AI Coding Assistant - LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst