(Un)patching Callbacks

Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
to execute callback functions when a kernel object is (un)patched.  They
can be considered a **power feature** that **extends livepatching abilities**
to include:

  - Safe updates to global data

  - "Patches" to init and probe functions

  - Patching otherwise unpatchable code (i.e. assembly)

In most cases, (un)patch callbacks will need to be used in conjunction
with memory barriers and kernel synchronization primitives, like
mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.

1. Motivation

Callbacks differ from existing kernel facilities:

  - Module init/exit code doesn't run when disabling and re-enabling a

  - A module notifier can't stop a to-be-patched module from loading.

Callbacks are part of the klp_object structure and their implementation
is specific to that klp_object.  Other livepatch objects may or may not
be patched, irrespective of the target klp_object's current state.

2. Callback types

Callbacks can be registered for the following livepatch actions:

  * Pre-patch
                 - before a klp_object is patched

  * Post-patch
                 - after a klp_object has been patched and is active
                   across all tasks

  * Pre-unpatch
                 - before a klp_object is unpatched (ie, patched code is
                   active), used to clean up post-patch callback

  * Post-unpatch
                 - after a klp_object has been patched, all code has
                   been restored and no tasks are running patched code,
                   used to cleanup pre-patch callback resources

3. How it works

Each callback is optional, omitting one does not preclude specifying any
other.  However, the livepatching core executes the handlers in
symmetry: pre-patch callbacks have a post-unpatch counterpart and
post-patch callbacks have a pre-unpatch counterpart.  An unpatch
callback will only be executed if its corresponding patch callback was
executed.  Typical use cases pair a patch handler that acquires and
configures resources with an unpatch handler tears down and releases
those same resources.

A callback is only executed if its host klp_object is loaded.  For
in-kernel vmlinux targets, this means that callbacks will always execute
when a livepatch is enabled/disabled.  For patch target kernel modules,
callbacks will only execute if the target module is loaded.  When a
module target is (un)loaded, its callbacks will execute only if the
livepatch module is enabled.

The pre-patch callback, if specified, is expected to return a status
code (0 for success, -ERRNO on error).  An error status code indicates
to the livepatching core that patching of the current klp_object is not
safe and to stop the current patching request.  (When no pre-patch
callback is provided, the transition is assumed to be safe.)  If a
pre-patch callback returns failure, the kernel's module loader will:

  - Refuse to load a livepatch, if the livepatch is loaded after
    targeted code.


  - Refuse to load a module, if the livepatch was already successfully

No post-patch, pre-unpatch, or post-unpatch callbacks will be executed
for a given klp_object if the object failed to patch, due to a failed
pre_patch callback or for any other reason.

If a patch transition is reversed, no pre-unpatch handlers will be run
(this follows the previously mentioned symmetry -- pre-unpatch callbacks
will only occur if their corresponding post-patch callback executed).

If the object did successfully patch, but the patch transition never
started for some reason (e.g., if another object failed to patch),
only the post-unpatch callback will be called.

4. Use cases

Sample livepatch modules demonstrating the callback API can be found in
samples/livepatch/ directory.  These samples were modified for use in
kselftests and can be found in the lib/livepatch directory.

Global data update

A pre-patch callback can be useful to update a global variable.  For
example, 75ff39ccc1bd ("tcp: make challenge acks less predictable")
changes a global sysctl, as well as patches the tcp_send_challenge_ack()

In this case, if we're being super paranoid, it might make sense to
patch the data *after* patching is complete with a post-patch callback,
so that tcp_send_challenge_ack() could first be changed to read
sysctl_tcp_challenge_ack_limit with READ_ONCE.

__init and probe function patches support

Although __init and probe functions are not directly livepatch-able, it
may be possible to implement similar updates via pre/post-patch

The commit ``48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST")`` change the way that
virtnet_probe() initialized its driver's net_device features.  A
pre/post-patch callback could iterate over all such devices, making a
similar change to their hw_features value.  (Client functions of the
value may need to be updated accordingly.)