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Diffstat (limited to 'Documentation/livepatch/livepatch.txt')
| -rw-r--r-- | Documentation/livepatch/livepatch.txt | 135 |
1 files changed, 53 insertions, 82 deletions
diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt index 2d7ed09dbd59a..8f56490a4bb66 100644 --- a/Documentation/livepatch/livepatch.txt +++ b/Documentation/livepatch/livepatch.txt @@ -12,12 +12,11 @@ Table of Contents: 4. Livepatch module 4.1. New functions 4.2. Metadata - 4.3. Livepatch module handling 5. Livepatch life-cycle - 5.1. Registration + 5.1. Loading 5.2. Enabling 5.3. Disabling - 5.4. Unregistration + 5.4. Removing 6. Sysfs 7. Limitations @@ -298,117 +297,89 @@ into three levels: see the "Consistency model" section. -4.3. Livepatch module handling ------------------------------- - -The usual behavior is that the new functions will get used when -the livepatch module is loaded. For this, the module init() function -has to register the patch (struct klp_patch) and enable it. See the -section "Livepatch life-cycle" below for more details about these -two operations. - -Module removal is only safe when there are no users of the underlying -functions. This is the reason why the force feature permanently disables -the removal. The forced tasks entered the functions but we cannot say -that they returned back. Therefore it cannot be decided when the -livepatch module can be safely removed. When the system is successfully -transitioned to a new patch state (patched/unpatched) without being -forced it is guaranteed that no task sleeps or runs in the old code. - - 5. Livepatch life-cycle ======================= -Livepatching defines four basic operations that define the life cycle of each -live patch: registration, enabling, disabling and unregistration. There are -several reasons why it is done this way. - -First, the patch is applied only when all patched symbols for already -loaded objects are found. The error handling is much easier if this -check is done before particular functions get redirected. +Livepatching can be described by four basic operations: +loading, enabling, disabling, removing. -Second, it might take some time until the entire system is migrated with -the hybrid consistency model being used. The patch revert might block -the livepatch module removal for too long. Therefore it is useful to -revert the patch using a separate operation that might be called -explicitly. But it does not make sense to remove all information until -the livepatch module is really removed. +5.1. Loading +------------ -5.1. Registration ------------------ +The only reasonable way is to enable the patch when the livepatch kernel +module is being loaded. For this, klp_enable_patch() has to be called +in the module_init() callback. There are two main reasons: -Each patch first has to be registered using klp_register_patch(). This makes -the patch known to the livepatch framework. Also it does some preliminary -computing and checks. +First, only the module has an easy access to the related struct klp_patch. -In particular, the patch is added into the list of known patches. The -addresses of the patched functions are found according to their names. -The special relocations, mentioned in the section "New functions", are -applied. The relevant entries are created under -/sys/kernel/livepatch/<name>. The patch is rejected when any operation -fails. +Second, the error code might be used to refuse loading the module when +the patch cannot get enabled. 5.2. Enabling ------------- -Registered patches might be enabled either by calling klp_enable_patch() or -by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will -start using the new implementation of the patched functions at this stage. +The livepatch gets enabled by calling klp_enable_patch() from +the module_init() callback. The system will start using the new +implementation of the patched functions at this stage. -When a patch is enabled, livepatch enters into a transition state where -tasks are converging to the patched state. This is indicated by a value -of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have -been patched, the 'transition' value changes to '0'. For more -information about this process, see the "Consistency model" section. +First, the addresses of the patched functions are found according to their +names. The special relocations, mentioned in the section "New functions", +are applied. The relevant entries are created under +/sys/kernel/livepatch/<name>. The patch is rejected when any above +operation fails. -If an original function is patched for the first time, a function -specific struct klp_ops is created and an universal ftrace handler is -registered. +Second, livepatch enters into a transition state where tasks are converging +to the patched state. If an original function is patched for the first +time, a function specific struct klp_ops is created and an universal +ftrace handler is registered[*]. This stage is indicated by a value of '1' +in /sys/kernel/livepatch/<name>/transition. For more information about +this process, see the "Consistency model" section. -Functions might be patched multiple times. The ftrace handler is registered -only once for the given function. Further patches just add an entry to the -list (see field `func_stack`) of the struct klp_ops. The last added -entry is chosen by the ftrace handler and becomes the active function -replacement. +Finally, once all tasks have been patched, the 'transition' value changes +to '0'. -Note that the patches might be enabled in a different order than they were -registered. +[*] Note that functions might be patched multiple times. The ftrace handler + is registered only once for a given function. Further patches just add + an entry to the list (see field `func_stack`) of the struct klp_ops. + The right implementation is selected by the ftrace handler, see + the "Consistency model" section. 5.3. Disabling -------------- -Enabled patches might get disabled either by calling klp_disable_patch() or -by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage -either the code from the previously enabled patch or even the original -code gets used. +Enabled patches might get disabled by writing '0' to +/sys/kernel/livepatch/<name>/enabled. -When a patch is disabled, livepatch enters into a transition state where -tasks are converging to the unpatched state. This is indicated by a -value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks -have been unpatched, the 'transition' value changes to '0'. For more -information about this process, see the "Consistency model" section. +First, livepatch enters into a transition state where tasks are converging +to the unpatched state. The system starts using either the code from +the previously enabled patch or even the original one. This stage is +indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition. +For more information about this process, see the "Consistency model" +section. -Here all the functions (struct klp_func) associated with the to-be-disabled +Second, once all tasks have been unpatched, the 'transition' value changes +to '0'. All the functions (struct klp_func) associated with the to-be-disabled patch are removed from the corresponding struct klp_ops. The ftrace handler is unregistered and the struct klp_ops is freed when the func_stack list becomes empty. -Patches must be disabled in exactly the reverse order in which they were -enabled. It makes the problem and the implementation much easier. +Third, the sysfs interface is destroyed. +Note that patches must be disabled in exactly the reverse order in which +they were enabled. It makes the problem and the implementation much easier. -5.4. Unregistration -------------------- -Disabled patches might be unregistered by calling klp_unregister_patch(). -This can be done only when the patch is disabled and the code is no longer -used. It must be called before the livepatch module gets unloaded. +5.4. Removing +------------- -At this stage, all the relevant sys-fs entries are removed and the patch -is removed from the list of known patches. +Module removal is only safe when there are no users of functions provided +by the module. This is the reason why the force feature permanently +disables the removal. Only when the system is successfully transitioned +to a new patch state (patched/unpatched) without being forced it is +guaranteed that no task sleeps or runs in the old code. 6. Sysfs |