aboutsummaryrefslogtreecommitdiff
path: root/Documentation/livepatch/cumulative-patches.txt
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/livepatch/cumulative-patches.txt')
-rw-r--r--Documentation/livepatch/cumulative-patches.txt102
1 files changed, 0 insertions, 102 deletions
diff --git a/Documentation/livepatch/cumulative-patches.txt b/Documentation/livepatch/cumulative-patches.txt
deleted file mode 100644
index 0012808e8d44..000000000000
--- a/Documentation/livepatch/cumulative-patches.txt
+++ /dev/null
@@ -1,102 +0,0 @@
-===================================
-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.