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- ===================================
- 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.
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