[RFC PATCH v4 0/4] livepatch: Introduce replace set support

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/[email protected]/

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.

v3->v4 (RFC):
- Allow a livepatch to replace livepatches with different provides IDs.
  Replace the single `replace_set` field with two separate fields,
  `provides` and `obsoletes`, for more flexible replacement semantics.
  (Petr, Joe)

v3: https://lore.kernel.org/live-patching/[email protected]/

v2->v3:
- Address the feedback from Sachiko AI
 - Fix the pre-existing NULL pointer dereference issue
 - Move klp_find_func into core.h
 - Don't deprecate stack_order completely

v2: https://lore.kernel.org/live-patching/[email protected]/

v1->v2:
- Incorporate feedback from Petr:
  - Initialize replace_set to 0 by default
  - Improve documentation
  - Enforce that livepatches in different replace_sets cannot use the same
    state->id.
  - Enforce that livepatches in different replace_sets cannot modify the
    same function.
  - Ensure consistent capitalization and naming usage of KLP_REPLACE_SET.
- Incorporate feedback from Sachiko AI:
  - Skip the klp_transition patch during klp_force_transition().

v1 (RFC): https://lore.kernel.org/live-patching/[email protected]/

Yafang Shao (4):
  livepatch: Make klp_find_func() non static
  livepatch: Call klp_init_patch_early() earlier
  livepatch: Implement replace set for scoped atomic replace
  livepatch: Deprecate stack_order

 .../ABI/removed/sysfs-kernel-livepatch        |  16 +++
 .../ABI/testing/sysfs-kernel-livepatch        |  27 ++---
 .../livepatch/cumulative-patches.rst          |  93 +++++++++++-----
 Documentation/livepatch/livepatch.rst         |  23 ++--
 include/linux/livepatch.h                     |   7 +-
 kernel/livepatch/core.c                       | 101 +++++++++++-------
 kernel/livepatch/core.h                       |   2 +
 kernel/livepatch/state.c                      |  56 ++++++++--
 kernel/livepatch/transition.c                 |  11 +-
 scripts/livepatch/init.c                      |  71 +++++++++++-
 scripts/livepatch/klp-build                   |  87 +++++++++++++--
 11 files changed, 380 insertions(+), 114 deletions(-)
 create mode 100644 Documentation/ABI/removed/sysfs-kernel-livepatch

-- 
2.52.0