103 lines
3.8 KiB
ReStructuredText
103 lines
3.8 KiB
ReStructuredText
===================================
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Atomic Replace & Cumulative Patches
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===================================
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There might be dependencies between livepatches. If multiple patches need
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to do different changes to the same function(s) then we need to define
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an order in which the patches will be installed. And function implementations
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from any newer livepatch must be done on top of the older ones.
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This might become a maintenance nightmare. Especially when more patches
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modified the same function in different ways.
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An elegant solution comes with the feature called "Atomic Replace". It allows
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creation of so called "Cumulative Patches". They include all wanted changes
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from all older livepatches and completely replace them in one transition.
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Usage
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-----
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The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
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for example::
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static struct klp_patch patch = {
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.mod = THIS_MODULE,
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.objs = objs,
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.replace = true,
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};
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All processes are then migrated to use the code only from the new patch.
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Once the transition is finished, all older patches are automatically
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disabled.
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Ftrace handlers are transparently removed from functions that are no
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longer modified by the new cumulative patch.
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As a result, the livepatch authors might maintain sources only for one
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cumulative patch. It helps to keep the patch consistent while adding or
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removing various fixes or features.
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Users could keep only the last patch installed on the system after
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the transition to has finished. It helps to clearly see what code is
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actually in use. Also the livepatch might then be seen as a "normal"
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module that modifies the kernel behavior. The only difference is that
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it can be updated at runtime without breaking its functionality.
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Features
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--------
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The atomic replace allows:
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- Atomically revert some functions in a previous patch while
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upgrading other functions.
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- Remove eventual performance impact caused by core redirection
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for functions that are no longer patched.
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- Decrease user confusion about dependencies between livepatches.
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Limitations:
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------------
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- Once the operation finishes, there is no straightforward way
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to reverse it and restore the replaced patches atomically.
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A good practice is to set .replace flag in any released livepatch.
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Then re-adding an older livepatch is equivalent to downgrading
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to that patch. This is safe as long as the livepatches do _not_ do
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extra modifications in (un)patching callbacks or in the module_init()
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or module_exit() functions, see below.
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Also note that the replaced patch can be removed and loaded again
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only when the transition was not forced.
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- Only the (un)patching callbacks from the _new_ cumulative livepatch are
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executed. Any callbacks from the replaced patches are ignored.
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In other words, the cumulative patch is responsible for doing any actions
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that are necessary to properly replace any older patch.
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As a result, it might be dangerous to replace newer cumulative patches by
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older ones. The old livepatches might not provide the necessary callbacks.
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This might be seen as a limitation in some scenarios. But it makes life
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easier in many others. Only the new cumulative livepatch knows what
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fixes/features are added/removed and what special actions are necessary
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for a smooth transition.
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In any case, it would be a nightmare to think about the order of
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the various callbacks and their interactions if the callbacks from all
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enabled patches were called.
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- There is no special handling of shadow variables. Livepatch authors
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must create their own rules how to pass them from one cumulative
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patch to the other. Especially that they should not blindly remove
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them in module_exit() functions.
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A good practice might be to remove shadow variables in the post-unpatch
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callback. It is called only when the livepatch is properly disabled.
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