===================================
Atomic Replace & Cumulative Patches
===================================

There might be dependencies between livepatches. If multiple patches need
to do different changes to the same function(s) then we need to define
an order in which the patches will be installed. And function implementations
from any newer livepatch must be done on top of the older ones.

This might become a maintenance nightmare. Especially when more patches
modified the same function in different ways.

An elegant solution comes with the feature called "Atomic Replace". It allows
creation of so called "Cumulative Patches". They include all wanted changes
from all older livepatches and completely replace them in one transition.

Usage
-----

The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
for example::

	static struct klp_patch patch = {
		.mod = THIS_MODULE,
		.objs = objs,
		.replace = true,
	};

All processes are then migrated to use the code only from the new patch.
Once the transition is finished, all older patches are automatically
disabled.

Ftrace handlers are transparently removed from functions that are no
longer modified by the new cumulative patch.

As a result, the livepatch authors might maintain sources only for one
cumulative patch. It helps to keep the patch consistent while adding or
removing various fixes or features.

Users could keep only the last patch installed on the system after
the transition to has finished. It helps to clearly see what code is
actually in use. Also the livepatch might then be seen as a "normal"
module that modifies the kernel behavior. The only difference is that
it can be updated at runtime without breaking its functionality.


Features
--------

The atomic replace allows:

  - Atomically revert some functions in a previous patch while
    upgrading other functions.

  - Remove eventual performance impact caused by core redirection
    for functions that are no longer patched.

  - Decrease user confusion about dependencies between livepatches.


Limitations:
------------

  - Once the operation finishes, there is no straightforward way
    to reverse it and restore the replaced patches atomically.

    A good practice is to set .replace flag in any released livepatch.
    Then re-adding an older livepatch is equivalent to downgrading
    to that patch. This is safe as long as the livepatches do _not_ do
    extra modifications in (un)patching callbacks or in the module_init()
    or module_exit() functions, see below.

    Also note that the replaced patch can be removed and loaded again
    only when the transition was not forced.


  - Only the (un)patching callbacks from the _new_ cumulative livepatch are
    executed. Any callbacks from the replaced patches are ignored.

    In other words, the cumulative patch is responsible for doing any actions
    that are necessary to properly replace any older patch.

    As a result, it might be dangerous to replace newer cumulative patches by
    older ones. The old livepatches might not provide the necessary callbacks.

    This might be seen as a limitation in some scenarios. But it makes life
    easier in many others. Only the new cumulative livepatch knows what
    fixes/features are added/removed and what special actions are necessary
    for a smooth transition.

    In any case, it would be a nightmare to think about the order of
    the various callbacks and their interactions if the callbacks from all
    enabled patches were called.


  - There is no special handling of shadow variables. Livepatch authors
    must create their own rules how to pass them from one cumulative
    patch to the other. Especially that they should not blindly remove
    them in module_exit() functions.

    A good practice might be to remove shadow variables in the post-unpatch
    callback. It is called only when the livepatch is properly disabled.