1===================================
2Atomic Replace & Cumulative Patches
3===================================
4
5There might be dependencies between livepatches. If multiple patches need
6to do different changes to the same function(s) then we need to define
7an order in which the patches will be installed. And function implementations
8from any newer livepatch must be done on top of the older ones.
9
10This might become a maintenance nightmare. Especially when more patches
11modified the same function in different ways.
12
13An elegant solution comes with the feature called "Atomic Replace". It allows
14creation of so called "Cumulative Patches". They include all wanted changes
15from all older livepatches and completely replace them in one transition.
16
17Usage
18-----
19
20The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
21for example::
22
23	static struct klp_patch patch = {
24		.mod = THIS_MODULE,
25		.objs = objs,
26		.replace = true,
27	};
28
29All processes are then migrated to use the code only from the new patch.
30Once the transition is finished, all older patches are automatically
31disabled.
32
33Ftrace handlers are transparently removed from functions that are no
34longer modified by the new cumulative patch.
35
36As a result, the livepatch authors might maintain sources only for one
37cumulative patch. It helps to keep the patch consistent while adding or
38removing various fixes or features.
39
40Users could keep only the last patch installed on the system after
41the transition to has finished. It helps to clearly see what code is
42actually in use. Also the livepatch might then be seen as a "normal"
43module that modifies the kernel behavior. The only difference is that
44it can be updated at runtime without breaking its functionality.
45
46
47Features
48--------
49
50The atomic replace allows:
51
52  - Atomically revert some functions in a previous patch while
53    upgrading other functions.
54
55  - Remove eventual performance impact caused by core redirection
56    for functions that are no longer patched.
57
58  - Decrease user confusion about dependencies between livepatches.
59
60
61Limitations:
62------------
63
64  - Once the operation finishes, there is no straightforward way
65    to reverse it and restore the replaced patches atomically.
66
67    A good practice is to set .replace flag in any released livepatch.
68    Then re-adding an older livepatch is equivalent to downgrading
69    to that patch. This is safe as long as the livepatches do _not_ do
70    extra modifications in (un)patching callbacks or in the module_init()
71    or module_exit() functions, see below.
72
73    Also note that the replaced patch can be removed and loaded again
74    only when the transition was not forced.
75
76
77  - Only the (un)patching callbacks from the _new_ cumulative livepatch are
78    executed. Any callbacks from the replaced patches are ignored.
79
80    In other words, the cumulative patch is responsible for doing any actions
81    that are necessary to properly replace any older patch.
82
83    As a result, it might be dangerous to replace newer cumulative patches by
84    older ones. The old livepatches might not provide the necessary callbacks.
85
86    This might be seen as a limitation in some scenarios. But it makes life
87    easier in many others. Only the new cumulative livepatch knows what
88    fixes/features are added/removed and what special actions are necessary
89    for a smooth transition.
90
91    In any case, it would be a nightmare to think about the order of
92    the various callbacks and their interactions if the callbacks from all
93    enabled patches were called.
94
95
96  - There is no special handling of shadow variables. Livepatch authors
97    must create their own rules how to pass them from one cumulative
98    patch to the other. Especially that they should not blindly remove
99    them in module_exit() functions.
100
101    A good practice might be to remove shadow variables in the post-unpatch
102    callback. It is called only when the livepatch is properly disabled.
103