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Kuniyuki Iwashima dff39930ad net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().
If a netlink socket sends RTM_GETCHAIN requests repeatedly
without recv()ing the responses, tc_ctl_chain() hogs CPU and
triggers Hung Task splat. [0]

As caught in the stack trace, netlink_attachskb() could confuse
tc_ctl_chain() by returning -EAGAIN when the userspace netlink
socket's receive buffer is full.

The replay: label exists since commit 32a4f5ecd7 ("net: sched:
introduce chain object to uapi") but was not used initially.

Since commit 9f407f1768 ("net: sched: introduce chain templates"),
the label is needed for RTM_NEWCHAIN because tcf_proto_lookup_ops()
may release RTNL to call request_module().

However, the replay logic is unnecessary for RTM_GETCHAIN.

Let's apply the replay logic only for RTM_NEWCHAIN.

[0]:
INFO: task repro:1018 is blocked on a mutex likely owned by task repro:1022.
task:repro           state:R  running task     stack:14096 pid:1022  tgid:1014  ppid:961    task_flags:0x400040 flags:0x00080000
Call Trace:
 <TASK>
 ? clockevents_program_event (kernel/time/clockevents.c:372)
 ? pskb_expand_head (net/core/skbuff.c:615)
 ? skb_release_data (net/core/skbuff.c:1122)
 ? netlink_attachskb (./include/linux/skbuff.h:1323 ./include/linux/skbuff.h:1332 net/netlink/af_netlink.c:1232)
 ? __netlink_lookup (./include/linux/rcupdate.h:882 ./include/linux/rhashtable.h:711 net/netlink/af_netlink.c:499)
 ? tc_chain_notify (net/sched/cls_api.c:3045)
 ? tc_chain_notify (./include/linux/skbuff.h:1384 net/sched/cls_api.c:3041)
 ? netlink_unicast (net/netlink/af_netlink.c:1335)
 ? rtnl_unicast (./include/net/netlink.h:1198 net/core/rtnetlink.c:985)
 ? tc_ctl_chain (net/sched/cls_api.c:3242)
 ? rtnetlink_rcv_msg (net/core/rtnetlink.c:7146)
 ? netlink_unicast (net/netlink/af_netlink.c:1354)
 ? __pfx_rtnetlink_rcv_msg (net/core/rtnetlink.c:7177)
 ? netlink_rcv_skb (net/netlink/af_netlink.c:2556)
 ? netlink_unicast (net/netlink/af_netlink.c:1319)
 ? netlink_sendmsg (net/netlink/af_netlink.c:1900)
 ? __sock_sendmsg (net/socket.c:800)
 ? __sys_sendto (net/socket.c:2281)
 ? __x64_sys_sendto (net/socket.c:2288 net/socket.c:2284 net/socket.c:2284)
 ? do_syscall_64 (arch/x86/entry/syscall_64.c:61 arch/x86/entry/syscall_64.c:84)
 ? entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
 </TASK>

Fixes: 2ed9db3074 ("net: sched: cls_api: fix dead code in switch")
Reported-by: Taras Madan <tarasmadan@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Tested-by: hybris@mojatatu.ai
Link: https://patch.msgid.link/20260908205537.863484-1-kuniyu@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
History 2026-09-10 12:06:23 +02:00
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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