Jakub Kicinski 6c00e85cd4 Merge branch 'net-sched-finish-the-qdisc_dequeue_peeked-conversion-taprio-multiq'
Bryam Vargas says:

====================
net/sched: finish the qdisc_dequeue_peeked conversion (taprio, multiq)

Commit 77be155cba added peek emulation: a non-work-conserving qdisc's
->peek dequeues one skb and stashes it in the child's gso_skb. A parent
that peeks such a child must then take the packet with
qdisc_dequeue_peeked(), not a direct ->dequeue(), or the stashed skb is
bypassed and the child's qlen/backlog desync. sch_red and sch_sfb were
just fixed for this; taprio and multiq still take the direct path.

With a qfq child the desync re-enters qfq_dequeue on an emptied aggregate
list and dereferences NULL, panicking from softirq on ordinary egress.
taprio reaches it on its own (root-only software path, all gates open);
multiq reaches it when a peeking parent such as tbf wraps it over a
non-work-conserving grandchild. Both need only CAP_NET_ADMIN.

Confirmed under KASAN: the unpatched arm panics, the patched arm is
clean, and a work-conserving-child control is clean. The reproducers and
splats for both are below; the per-patch changes are one line each.

taprio reproducer (self-triggering, no parent qdisc needed):

  ip link add dummy0 numtxqueues 4 type dummy; ip link set dummy0 up
  ip addr add 10.10.11.10/24 dev dummy0
  tc qdisc add dev dummy0 root handle 1: taprio num_tc 2 \
     map 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 queues 1@0 1@1 \
     base-time 9000000000000000000 sched-entry S 03 200000 flags 0x0 clockid CLOCK_TAI
  tc qdisc replace dev dummy0 parent 1:1 handle 3: qfq
  tc class  add dev dummy0 classid 3:1 parent 3: qfq maxpkt 512 weight 1
  tc filter add dev dummy0 parent 3: protocol ip prio 1 matchall classid 3:1
  ping -c1 10.10.11.99 -I dummy0

[  903.769174] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000009: 0000 [#1] SMP KASAN NOPTI
[  903.769953] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[  903.770456] CPU: 7 UID: 0 PID: 16162 Comm: ping Not tainted 7.1.0-rc5 #1 PREEMPT(lazy)
[  903.771725] RIP: 0010:qfq_dequeue+0x362/0x1580 [sch_qfq]
[  903.777452] Call Trace:
[  903.778311]  taprio_dequeue_from_txq+0x383/0x680 [sch_taprio]
[  903.778685]  taprio_dequeue_tc_priority+0x19a/0x330 [sch_taprio]
[  903.779645]  taprio_dequeue+0xa6/0x330 [sch_taprio]
[  903.780299]  __qdisc_run+0x16c/0x1890
[  903.780854]  __dev_queue_xmit+0x1ece/0x3390
[  903.784109]  ip_finish_output2+0x571/0x1da0
[  903.785996]  ip_output+0x26c/0x4d0
[  903.789572]  ping_v4_sendmsg+0xd22/0x12b0
[  903.796118]  __x64_sys_sendto+0xe0/0x1c0
[  903.796612]  do_syscall_64+0xee/0x590
[  903.818669] Kernel panic - not syncing: Fatal exception in interrupt

multiq reproducer (needs a peeking parent over a stashing child; tbf
values chosen to force it to throttle):

  ip link add dummy0 numtxqueues 2 type dummy; ip link set dummy0 up
  ip addr add 10.10.11.10/24 dev dummy0
  tc qdisc add dev dummy0 root handle 1: tbf rate 88bit burst 1661b \
     peakrate 2257333 minburst 1024 limit 7b
  tc qdisc add dev dummy0 parent 1: handle 2: multiq
  for b in 1 2; do                          # qfq on every band
    tc qdisc  add dev dummy0 parent 2:$b handle 3$b: qfq
    tc class  add dev dummy0 classid 3$b:1 parent 3$b: qfq maxpkt 512 weight 1
    tc filter add dev dummy0 parent 3$b: protocol ip prio 1 matchall classid 3$b:1
  done
  ping -c12 10.10.11.99 -I dummy0

[ 1066.385097] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000009: 0000 [#1] SMP KASAN NOPTI
[ 1066.386385] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[ 1066.387227] CPU: 1 UID: 0 PID: 5357 Comm: ping Not tainted 7.1.0-rc5 #1 PREEMPT(lazy)
[ 1066.389183] RIP: 0010:qfq_dequeue+0x362/0x1580 [sch_qfq]
[ 1066.396316] Call Trace:
[ 1066.396768]  multiq_dequeue+0x163/0x360 [sch_multiq]
[ 1066.397885]  tbf_dequeue+0x6b9/0xf17 [sch_tbf]
[ 1066.398269]  __qdisc_run+0x16c/0x1890
[ 1066.399315]  __dev_queue_xmit+0x1ece/0x3390
[ 1066.403276]  ip_finish_output2+0x571/0x1da0
[ 1066.404818]  ip_output+0x26c/0x4d0
[ 1066.408620]  ping_v4_sendmsg+0xd22/0x12b0
[ 1066.415264]  __x64_sys_sendto+0xe0/0x1c0
[ 1066.416251]  do_syscall_64+0xee/0x590
[ 1066.441210] Kernel panic - not syncing: Fatal exception in interrupt
====================

Link: https://patch.msgid.link/20260625-b4-disp-31bcb279-v1-0-85c40b83c529@proton.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2026-06-26 18:52:31 -07:00
2026-01-26 19:07:09 -08:00
2022-09-28 09:02:20 +02:00
2025-02-19 14:53:27 -07:00

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/


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============

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* New Kernel Developer - Getting started with kernel development
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For Specific Users
==================

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--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
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* 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
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---------------

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* 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

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--------------------

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
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* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

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* 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.


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=========================

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