[PATCH v12 10/12] livepatch: Atomic replace and cumulative patches documentation

From: Petr Mladek
Date: Tue Aug 28 2018 - 10:37:02 EST

User documentation for the atomic replace feature. It makes it easier
to maintain livepatches using so-called cumulative patches.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Documentation/livepatch/cumulative-patches.txt | 105 +++++++++++++++++++++++++
1 file changed, 105 insertions(+)
create mode 100644 Documentation/livepatch/cumulative-patches.txt

diff --git a/Documentation/livepatch/cumulative-patches.txt b/Documentation/livepatch/cumulative-patches.txt
new file mode 100644
index 000000000000..206b7f98d270
--- /dev/null
+++ b/Documentation/livepatch/cumulative-patches.txt
@@ -0,0 +1,105 @@
+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 if anyone would want
+to remove a patch that is in the middle of the stack.
+An elegant solution comes with the feature called "Atomic Replace". It allows
+to create so called "Cumulative Patches". They include all wanted changes
+from all older livepatches and completely replace them in one transition.
+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,
+ };
+Such a patch is added on top of the livepatch stack when registered. It can
+be enabled even when some earlier patches have not been enabled yet.
+All processes are then migrated to use the code only from the new patch.
+Once the transition is finished, all older patches are removed from the stack
+of patches. Even the older not-enabled patches mentioned above. They can
+even be unregistered and the related modules unloaded.
+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.
+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 stacking order and what patches are
+ currently in effect.
+ + Replaced patches can no longer be enabled. But if the transition
+ to the cumulative patch was not forced, the kernel modules with
+ the older livepatches can be removed and eventually added again.
+ 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.
+ + Only the (un)patching callbacks from the _new_ cumulative livepatch are
+ executed. Any callbacks from the replaced patches are ignored.
+ By 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 the 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 each 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 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.