Re: [PATCH v6 0/8] livepatch: Introduce replace set support

On Mon, Aug 24, 2026 at 11:27 AM Yafang Shao [off-list ref] wrote:

We previously proposed a BPF+livepatch method to enable rapid
experimentation with new kernel features without interrupting production
workloads:

  https://lore.kernel.org/live-patching/20260402092607.96430-1-laoar.shao@gmail.com/ (local)

In the resulting discussion, Song and Petr suggested adding a "replace set"
to support scenarios where specific livepatches can be selectively replaced
or skipped.

This patchset introduces a more flexible model using two new fields in
struct klp_patch:

- provides: an unsigned int id identifying the patch replace set.
  By default (provides=0), any livepatch replaces any other livepatch.

- obsoletes: an optional array of unsigned int ids specifying
  additional provides ids to be replaced. This allows a new patch
  to explicitly obsolete patches from different replace sets.

A new livepatch atomically replaces any existing livepatch whose
provides id matches either:
  1. The new patch provides id (same replace set), or
  2. Any id in the new patch obsoletes list

Additionally, this design deprecates the traditional non-atomic-replace
model. Previously, setting 'replace' to 0 was the only way to keep
certain livepatches persistent on the system, forcing developers to
disable atomic replacement entirely. With the introduction of replace set,
developers now have a selective option to keep specific livepatches
persistent while maintaining atomic replacement capabilities elsewhere.

At present, KLP state, shadow variables, and callbacks are not integrated
with the new replace_set mechanism in this patchset. Support for these
features is deferred until Petr's klp-state-transfer infrastructure is
completed and merged:

  https://github.com/pmladek/linux/tree/klp-state-transfer-v1-iter12

Future Work
===========

- Allow `provides` and `obsoletes` to be configured dynamically at
  module load time, rather than being fixed at build time.

Notes for sashiko-bot
=====================

In your review of v5, you found the following issues:

- A malformed livepatch module with a missing `old_name` triggers a NULL
  pointer dereference in `klp_find_func()`.

  This has already been addressed by commit 1a921fd13c31e
  ("livepatch: Fix NULL pointer dereference in klp_find_func()").

- When CONFIG_DEBUG_KOBJECT_RELEASE is enabled, an error during patch
  initialization causes a use-after-free during module unload due to the
  delayed kobject release.

  This is addressed by the patch posted at
  https://lore.kernel.org/live-patching/20260821031648.48195-1-laoar.shao@gmail.com/ (local)

- Other review issues are addressed in this version; details in the changes
  section.

Oops.

It seems the sashiko-bot failed to apply the patch because of this change:

  https://lore.kernel.org/live-patching/20260816090442.18128-2-laoar.shao@gmail.com/ (local)

I based this series on that change.
I'll wait for others' reviews and then rebase it onto the livepatch
tree's for-next branch.

Changes
=======

v5->v6:
- Check `--provides` argument in `klp-build (sashiko)
- Fix the 'replace' feature detection for OOT kernel builds (sashiko)
- Fix race condition in sysfs polling (sashiko)

v5: https://lore.kernel.org/live-patching/20260809091954.22930-1-laoar.shao@gmail.com (local)

[...]

-- 
Regards
Yafang