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(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 patch.

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 resources

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.

or:

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

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, commit 75ff39ccc1bd (“tcp: make challenge acks less predictable”) changes a global sysctl, as well as patches the tcp_send_challenge_ack() function.

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 callbacks.

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.)

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