1==================== 2System State Changes 3==================== 4 5Some users are really reluctant to reboot a system. This brings the need 6to provide more livepatches and maintain some compatibility between them. 7 8Maintaining more livepatches is much easier with cumulative livepatches. 9Each new livepatch completely replaces any older one. It can keep, 10add, and even remove fixes. And it is typically safe to replace any version 11of the livepatch with any other one thanks to the atomic replace feature. 12 13The problems might come with shadow variables and callbacks. They might 14change the system behavior or state so that it is no longer safe to 15go back and use an older livepatch or the original kernel code. Also 16any new livepatch must be able to detect what changes have already been 17done by the already installed livepatches. 18 19This is where the livepatch system state tracking gets useful. It 20allows to: 21 22 - store data needed to manipulate and restore the system state 23 24 - define compatibility between livepatches using a change id 25 and version 26 27 281. Livepatch system state API 29============================= 30 31The state of the system might get modified either by several livepatch callbacks 32or by the newly used code. Also it must be possible to find changes done by 33already installed livepatches. 34 35Each modified state is described by struct klp_state, see 36include/linux/livepatch.h. 37 38Each livepatch defines an array of struct klp_states. They mention 39all states that the livepatch modifies. 40 41The livepatch author must define the following two fields for each 42struct klp_state: 43 44 - *id* 45 46 - Non-zero number used to identify the affected system state. 47 48 - *version* 49 50 - Number describing the variant of the system state change that 51 is supported by the given livepatch. 52 53The state can be manipulated using two functions: 54 55 - *klp_get_state(patch, id)* 56 57 - Get struct klp_state associated with the given livepatch 58 and state id. 59 60 - *klp_get_prev_state(id)* 61 62 - Get struct klp_state associated with the given feature id and 63 already installed livepatches. 64 652. Livepatch compatibility 66========================== 67 68The system state version is used to prevent loading incompatible livepatches. 69The check is done when the livepatch is enabled. The rules are: 70 71 - Any completely new system state modification is allowed. 72 73 - System state modifications with the same or higher version are allowed 74 for already modified system states. 75 76 - Cumulative livepatches must handle all system state modifications from 77 already installed livepatches. 78 79 - Non-cumulative livepatches are allowed to touch already modified 80 system states. 81 823. Supported scenarios 83====================== 84 85Livepatches have their life-cycle and the same is true for the system 86state changes. Every compatible livepatch has to support the following 87scenarios: 88 89 - Modify the system state when the livepatch gets enabled and the state 90 has not been already modified by a livepatches that are being 91 replaced. 92 93 - Take over or update the system state modification when is has already 94 been done by a livepatch that is being replaced. 95 96 - Restore the original state when the livepatch is disabled. 97 98 - Restore the previous state when the transition is reverted. 99 It might be the original system state or the state modification 100 done by livepatches that were being replaced. 101 102 - Remove any already made changes when error occurs and the livepatch 103 cannot get enabled. 104 1054. Expected usage 106================= 107 108System states are usually modified by livepatch callbacks. The expected 109role of each callback is as follows: 110 111*pre_patch()* 112 113 - Allocate *state->data* when necessary. The allocation might fail 114 and *pre_patch()* is the only callback that could stop loading 115 of the livepatch. The allocation is not needed when the data 116 are already provided by previously installed livepatches. 117 118 - Do any other preparatory action that is needed by 119 the new code even before the transition gets finished. 120 For example, initialize *state->data*. 121 122 The system state itself is typically modified in *post_patch()* 123 when the entire system is able to handle it. 124 125 - Clean up its own mess in case of error. It might be done by a custom 126 code or by calling *post_unpatch()* explicitly. 127 128*post_patch()* 129 130 - Copy *state->data* from the previous livepatch when they are 131 compatible. 132 133 - Do the actual system state modification. Eventually allow 134 the new code to use it. 135 136 - Make sure that *state->data* has all necessary information. 137 138 - Free *state->data* from replaces livepatches when they are 139 not longer needed. 140 141*pre_unpatch()* 142 143 - Prevent the code, added by the livepatch, relying on the system 144 state change. 145 146 - Revert the system state modification.. 147 148*post_unpatch()* 149 150 - Distinguish transition reverse and livepatch disabling by 151 checking *klp_get_prev_state()*. 152 153 - In case of transition reverse, restore the previous system 154 state. It might mean doing nothing. 155 156 - Remove any not longer needed setting or data. 157 158.. note:: 159 160 *pre_unpatch()* typically does symmetric operations to *post_patch()*. 161 Except that it is called only when the livepatch is being disabled. 162 Therefore it does not need to care about any previously installed 163 livepatch. 164 165 *post_unpatch()* typically does symmetric operations to *pre_patch()*. 166 It might be called also during the transition reverse. Therefore it 167 has to handle the state of the previously installed livepatches. 168