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

[Miroslav Benes <mbenes@suse.cz>]

On Tue, 28 Aug 2018, Petr Mladek wrote:

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

I think the documentation should be updated due to API changes.
 
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
>  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.
> +
> +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,
> +	};
> +
> +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.

Here.

> +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.

Here.

> +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 stacking order and what patches are
> +    currently in effect.
> +
> +
> +Limitations:
> +------------
> +
> +  + 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.

I'd rewrite even this.

> +    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.

s/By other words/In other words/

> +    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.

s/In each case/In any case/ ?

> +  + 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.

Miroslav