[PATCH v6 1/3] livepatch: add (un)patch callbacks

[Joe Lawrence]

Provide livepatch modules a klp_object (un)patching notification
mechanism.  Pre and post-(un)patch callbacks allow livepatch modules to
setup or synchronize changes that would be difficult to support in only
patched-or-unpatched code contexts.

Callbacks can be registered for target module or vmlinux klp_objects,
but each implementation is klp_object specific.

  - Pre-(un)patch callbacks run before any (un)patching transition
    starts.

  - Post-(un)patch callbacks run once an object has been (un)patched and
    the klp_patch fully transitioned to its target state.

Example use cases include modification of global data and registration
of newly available services/handlers.

See Documentation/livepatch/callbacks.txt for details and
samples/livepatch/ for examples.

Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
---
 Documentation/livepatch/callbacks.txt           | 605 ++++++++++++++++++++++++
 include/linux/livepatch.h                       |  26 +
 kernel/livepatch/core.c                         |  51 +-
 kernel/livepatch/core.h                         |  38 ++
 kernel/livepatch/patch.c                        |   1 +
 kernel/livepatch/transition.c                   |  21 +-
 samples/livepatch/Makefile                      |   3 +
 samples/livepatch/livepatch-callbacks-busymod.c |  72 +++
 samples/livepatch/livepatch-callbacks-demo.c    | 234 +++++++++
 samples/livepatch/livepatch-callbacks-mod.c     |  53 +++
 10 files changed, 1091 insertions(+), 13 deletions(-)
 create mode 100644 Documentation/livepatch/callbacks.txt
 create mode 100644 samples/livepatch/livepatch-callbacks-busymod.c
 create mode 100644 samples/livepatch/livepatch-callbacks-demo.c
 create mode 100644 samples/livepatch/livepatch-callbacks-mod.c

diff --git a/Documentation/livepatch/callbacks.txt b/Documentation/livepatch/callbacks.txt
new file mode 100644
index 000000000000..c9776f48e458
--- /dev/null
+++ b/Documentation/livepatch/callbacks.txt
@@ -0,0 +1,605 @@
+======================
+(Un)patching Callbacks
+======================
+
+Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
+to execute callback functions when a kernel object is (un)patched.  They
+can be considered a "power feature" that extends livepatching abilities
+to include:
+
+  - Safe updates to global data
+
+  - "Patches" to init and probe functions
+
+  - Patching otherwise unpatchable code (i.e. assembly)
+
+In most cases, (un)patch callbacks will need to be used in conjunction
+with memory barriers and kernel synchronization primitives, like
+mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.
+
+Callbacks differ from existing kernel facilities:
+
+  - Module init/exit code doesn't run when disabling and re-enabling a
+    patch.
+
+  - A module notifier can't stop a to-be-patched module from loading.
+
+Callbacks are part of the klp_object structure and their implementation
+is specific to that klp_object.  Other livepatch objects may or may not
+be patched, irrespective of the target klp_object's current state.
+
+Callbacks can be registered for the following livepatch actions:
+
+  * Pre-patch    - before a klp_object is patched
+
+  * Post-patch   - after a klp_object has been patched and is active
+                   across all tasks
+
+  * Pre-unpatch  - before a klp_object is unpatched (ie, patched code is
+                   active), used to clean up post-patch callback
+                   resources
+
+  * Post-unpatch - after a klp_object has been patched, all code has
+                   been restored and no tasks are running patched code,
+                   used to cleanup pre-patch callback resources
+
+Each callback is optional, omitting one does not preclude specifying any
+other.  However, the livepatching core executes the handlers in
+symmetry: pre-patch callbacks have a post-unpatch counterpart and
+post-patch callbacks have a pre-unpatch counterpart.  An unpatch
+callback will only be executed if its corresponding patch callback was
+executed.  Typical use cases pair a patch handler that acquires and
+configures resources with an unpatch handler tears down and releases
+those same resources.
+
+A callback is only executed if its host klp_object is loaded.  For
+in-kernel vmlinux targets, this means that callbacks will always execute
+when a livepatch is enabled/disabled.  For patch target kernel modules,
+callbacks will only execute if the target module is loaded.  When a
+module target is (un)loaded, its callbacks will execute only if the
+livepatch module is enabled.
+
+The pre-patch callback, if specified, is expected to return a status
+code (0 for success, -ERRNO on error).  An error status code indicates
+to the livepatching core that patching of the current klp_object is not
+safe and to stop the current patching request.  (When no pre-patch
+callback is provided, the transition is assumed to be safe.)  If a
+pre-patch callback returns failure, the kernel's module loader will:
+
+  - Refuse to load a livepatch, if the livepatch is loaded after
+    targeted code.
+
+    or:
+
+  - Refuse to load a module, if the livepatch was already successfully
+    loaded.
+
+No post-patch, pre-unpatch, or post-unpatch callbacks will be executed
+for a given klp_object if the object failed to patch, due to a failed
+pre_patch callback or for any other reason.
+
+If a patch transition is reversed, no pre-unpatch handlers will be run
+(this follows the previously mentioned symmetry -- pre-unpatch callbacks
+will only occur if their corresponding post-patch callback executed).
+
+If the object did successfully patch, but the patch transition never
+started for some reason (e.g., if another object failed to patch),
+only the post-unpatch callback will be called.
+
+
+Example Use-cases
+=================
+
+Update global data
+------------------
+
+A pre-patch callback can be useful to update a global variable.  For
+example, 75ff39ccc1bd ("tcp: make challenge acks less predictable")
+changes a global sysctl, as well as patches the tcp_send_challenge_ack()
+function.
+
+In this case, if we're being super paranoid, it might make sense to
+patch the data *after* patching is complete with a post-patch callback,
+so that tcp_send_challenge_ack() could first be changed to read
+sysctl_tcp_challenge_ack_limit with READ_ONCE.
+
+
+Support __init and probe function patches
+-----------------------------------------
+
+Although __init and probe functions are not directly livepatch-able, it
+may be possible to implement similar updates via pre/post-patch
+callbacks.
+
+48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that
+virtnet_probe() initialized its driver's net_device features.  A
+pre/post-patch callback could iterate over all such devices, making a
+similar change to their hw_features value.  (Client functions of the
+value may need to be updated accordingly.)
+
+
+Test cases
+==========
+
+What follows is not an exhaustive test suite of every possible livepatch
+pre/post-(un)patch combination, but a selection that demonstrates a few
+important concepts.  Each test case uses the kernel modules located in
+the samples/livepatch/ and assumes that no livepatches are loaded at the
+beginning of the test.
+
+
+Test 1
+------
+
+Test a combination of loading a kernel module and a livepatch that
+patches a function in the first module.  (Un)load the target module
+before the livepatch module:
+
+- load target module
+- load livepatch
+- disable livepatch
+- unload target module
+- unload livepatch
+
+First load a target module:
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   34.475708] livepatch_callbacks_mod: livepatch_callbacks_mod_init
+
+On livepatch enable, before the livepatch transition starts, pre-patch
+callbacks are executed for vmlinux and livepatch_callbacks_mod (those
+klp_objects currently loaded).  After klp_objects are patched according
+to the klp_patch, their post-patch callbacks run and the transition
+completes:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [   36.503719] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   36.504213] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   36.504238] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   36.504721] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   36.505849] livepatch: 'livepatch_callbacks_demo': starting patching transition
+  [   37.727133] livepatch: 'livepatch_callbacks_demo': completing patching transition
+  [   37.727232] livepatch_callbacks_demo: post_patch_callback: vmlinux
+  [   37.727860] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   37.728792] livepatch: 'livepatch_callbacks_demo': patching complete
+
+Similarly, on livepatch disable, pre-patch callbacks run before the
+unpatching transition starts.  klp_objects are reverted, post-patch
+callbacks execute and the transition completes:
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [   38.510209] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition
+  [   38.510234] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux
+  [   38.510982] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   38.512209] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [   39.711132] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [   39.711210] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [   39.711779] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   39.712735] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+  % rmmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   42.534183] livepatch_callbacks_mod: livepatch_callbacks_mod_exit
+
+
+Test 2
+------
+
+This test is similar to the previous test, but (un)load the livepatch
+module before the target kernel module.  This tests the livepatch core's
+module_coming handler:
+
+- load livepatch
+- load target module
+- disable livepatch
+- unload livepatch
+- unload target module
+
+
+On livepatch enable, only pre/post-patch callbacks are executed for
+currently loaded klp_objects, in this case, vmlinux:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [   44.553328] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   44.553997] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   44.554049] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   44.554845] livepatch: 'livepatch_callbacks_demo': starting patching transition
+  [   45.727128] livepatch: 'livepatch_callbacks_demo': completing patching transition
+  [   45.727212] livepatch_callbacks_demo: post_patch_callback: vmlinux
+  [   45.727961] livepatch: 'livepatch_callbacks_demo': patching complete
+
+When a targeted module is subsequently loaded, only its pre/post-patch
+callbacks are executed:
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   46.560845] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod'
+  [   46.561988] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [   46.563452] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [   46.565495] livepatch_callbacks_mod: livepatch_callbacks_mod_init
+
+On livepatch disable, all currently loaded klp_objects' (vmlinux and
+livepatch_callbacks_mod) pre/post-unpatch callbacks are executed:
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [   48.568885] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition
+  [   48.568910] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux
+  [   48.569441] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   48.570502] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [   49.759091] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [   49.759171] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [   49.759742] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   49.760690] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+  % rmmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   52.592283] livepatch_callbacks_mod: livepatch_callbacks_mod_exit
+
+
+Test 3
+------
+
+Test loading the livepatch after a targeted kernel module, then unload
+the kernel module before disabling the livepatch.  This tests the
+livepatch core's module_going handler:
+
+- load target module
+- load livepatch
+- unload target module
+- disable livepatch
+- unload livepatch
+
+First load a target module, then the livepatch:
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   54.607948] livepatch_callbacks_mod: livepatch_callbacks_mod_init
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [   56.613919] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   56.614411] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   56.614436] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   56.614818] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   56.615656] livepatch: 'livepatch_callbacks_demo': starting patching transition
+  [   57.759070] livepatch: 'livepatch_callbacks_demo': completing patching transition
+  [   57.759147] livepatch_callbacks_demo: post_patch_callback: vmlinux
+  [   57.759621] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state
+  [   57.760307] livepatch: 'livepatch_callbacks_demo': patching complete
+
+When a target module is unloaded, the livepatch is only reverted from
+that klp_object (livepatch_callbacks_mod).  As such, only its pre and
+post-unpatch callbacks are executed when this occurs:
+
+  % rmmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   58.623409] livepatch_callbacks_mod: livepatch_callbacks_mod_exit
+  [   58.623903] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away
+  [   58.624658] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod'
+  [   58.625305] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away
+
+When the livepatch is disabled, pre and post-unpatch callbacks are run
+for the remaining klp_object, vmlinux:
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [   60.638420] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition
+  [   60.638444] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux
+  [   60.638996] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [   61.727088] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [   61.727165] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [   61.727985] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+
+
+Test 4
+------
+
+This test is similar to the previous test, however the livepatch is
+loaded first.  This tests the livepatch core's module_coming and
+module_going handlers:
+
+- load livepatch
+- load target module
+- unload target module
+- disable livepatch
+- unload livepatch
+
+First load the livepatch:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [   64.661552] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   64.662147] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   64.662175] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   64.662850] livepatch: 'livepatch_callbacks_demo': starting patching transition
+  [   65.695056] livepatch: 'livepatch_callbacks_demo': completing patching transition
+  [   65.695147] livepatch_callbacks_demo: post_patch_callback: vmlinux
+  [   65.695561] livepatch: 'livepatch_callbacks_demo': patching complete
+
+When a targeted kernel module is subsequently loaded, only its
+pre/post-patch callbacks are executed:
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   66.669196] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod'
+  [   66.669882] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [   66.670744] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [   66.672873] livepatch_callbacks_mod: livepatch_callbacks_mod_init
+
+When the target module is unloaded, the livepatch is only reverted from
+the livepatch_callbacks_mod klp_object.  As such, only pre and
+post-unpatch callbacks are executed when this occurs:
+
+  % rmmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   68.680065] livepatch_callbacks_mod: livepatch_callbacks_mod_exit
+  [   68.680688] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away
+  [   68.681452] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod'
+  [   68.682094] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [   70.689225] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition
+  [   70.689256] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux
+  [   70.689882] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [   71.711080] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [   71.711481] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [   71.711988] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+
+
+Test 5
+------
+
+A simple test of loading a livepatch without one of its patch target
+klp_objects ever loaded (livepatch_callbacks_mod):
+
+- load livepatch
+- disable livepatch
+- unload livepatch
+
+Load the livepatch:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [   74.711081] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   74.711595] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   74.711639] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   74.712272] livepatch: 'livepatch_callbacks_demo': starting patching transition
+  [   75.743137] livepatch: 'livepatch_callbacks_demo': completing patching transition
+  [   75.743219] livepatch_callbacks_demo: post_patch_callback: vmlinux
+  [   75.743867] livepatch: 'livepatch_callbacks_demo': patching complete
+
+As expected, only pre/post-(un)patch handlers are executed for vmlinux:
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [   76.716254] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition
+  [   76.716278] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux
+  [   76.716666] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [   77.727089] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [   77.727194] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [   77.727907] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+
+
+Test 6
+------
+
+Test a scenario where a vmlinux pre-patch callback returns a non-zero
+status (ie, failure):
+
+- load target module
+- load livepatch -ENODEV
+- unload target module
+
+First load a target module:
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   80.740520] livepatch_callbacks_mod: livepatch_callbacks_mod_init
+
+Load the livepatch module, setting its 'pre_patch_ret' value to -19
+(-ENODEV).  When its vmlinux pre-patch callback executed, this status
+code will propagate back to the module-loading subsystem.  The result is
+that the insmod command refuses to load the livepatch module:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko pre_patch_ret=-19
+  [   82.747326] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   82.747743] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   82.747767] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   82.748237] livepatch: pre-patch callback failed for object 'vmlinux'
+  [   82.748637] livepatch: failed to enable patch 'livepatch_callbacks_demo'
+  [   82.749059] livepatch: 'livepatch_callbacks_demo': canceling transition, going to unpatch
+  [   82.749060] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [   82.749868] livepatch: 'livepatch_callbacks_demo': unpatching complete
+  [   82.765809] insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-demo.ko: No such device
+
+  % rmmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   84.774238] livepatch_callbacks_mod: livepatch_callbacks_mod_exit
+
+
+Test 7
+------
+
+Similar to the previous test, setup a livepatch such that its vmlinux
+pre-patch callback returns success.  However, when a targeted kernel
+module is later loaded, have the livepatch return a failing status code:
+
+- load livepatch
+- setup -ENODEV
+- load target module
+- disable livepatch
+- unload livepatch
+
+Load the livepatch, notice vmlinux pre-patch callback succeeds:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [   86.787845] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   86.788325] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   86.788427] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   86.788821] livepatch: 'livepatch_callbacks_demo': starting patching transition
+  [   87.711069] livepatch: 'livepatch_callbacks_demo': completing patching transition
+  [   87.711143] livepatch_callbacks_demo: post_patch_callback: vmlinux
+  [   87.711886] livepatch: 'livepatch_callbacks_demo': patching complete
+
+Set a trap so subsequent pre-patch callbacks to this livepatch will
+return -ENODEV:
+
+  % echo -19 > /sys/module/livepatch_callbacks_demo/parameters/pre_patch_ret
+
+The livepatch pre-patch callback for subsequently loaded target modules
+will return failure, so the module loader refuses to load the kernel
+module.  Notice that no post-patch or pre/post-unpatch callbacks are
+executed for this klp_object:
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [   90.796976] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod'
+  [   90.797834] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [   90.798900] livepatch: pre-patch callback failed for object 'livepatch_callbacks_mod'
+  [   90.799652] livepatch: patch 'livepatch_callbacks_demo' failed for module 'livepatch_callbacks_mod', refusing to load module 'livepatch_callbacks_mod'
+  [   90.819737] insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-mod.ko: No such device
+
+However, pre/post-unpatch callbacks run for the vmlinux klp_object:
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [   92.823547] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition
+  [   92.823573] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux
+  [   92.824331] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [   93.727128] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [   93.727327] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [   93.727861] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+
+
+Test 8
+------
+
+Test loading multiple targeted kernel modules.  This test-case is
+mainly for comparing with the next test-case.
+
+- load busy target module (0s sleep),
+- load livepatch
+- load target module
+- unload target module
+- disable livepatch
+- unload livepatch
+- unload busy target module
+
+
+Load a target "busy" kernel module which kicks off a worker function
+that immediately exits:
+
+  % insmod samples/livepatch/livepatch-callbacks-busymod.ko sleep_secs=0
+  [   96.910107] livepatch_callbacks_busymod: livepatch_callbacks_mod_init
+  [   96.910600] livepatch_callbacks_busymod: busymod_work_func, sleeping 0 seconds ...
+  [   96.913024] livepatch_callbacks_busymod: busymod_work_func exit
+
+Proceed with loading the livepatch and another ordinary target module,
+notice that the post-patch callbacks are executed and the transition
+completes quickly:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [   98.917892] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [   98.918426] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [   98.918453] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [   98.918955] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state
+  [   98.923835] livepatch: 'livepatch_callbacks_demo': starting patching transition
+  [   99.743104] livepatch: 'livepatch_callbacks_demo': completing patching transition
+  [   99.743156] livepatch_callbacks_demo: post_patch_callback: vmlinux
+  [   99.743679] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state
+  [   99.744616] livepatch: 'livepatch_callbacks_demo': patching complete
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [  100.930955] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod'
+  [  100.931668] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [  100.932645] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [  100.934125] livepatch_callbacks_mod: livepatch_callbacks_mod_init
+
+  % rmmod samples/livepatch/livepatch-callbacks-mod.ko
+  [  102.942805] livepatch_callbacks_mod: livepatch_callbacks_mod_exit
+  [  102.943640] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away
+  [  102.944585] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod'
+  [  102.945455] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [  104.953815] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition
+  [  104.953838] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux
+  [  104.954431] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state
+  [  104.955426] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [  106.719073] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [  106.722633] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [  106.723282] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state
+  [  106.724279] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+  % rmmod samples/livepatch/livepatch-callbacks-busymod.ko
+  [  108.975660] livepatch_callbacks_busymod: livepatch_callbacks_mod_exit
+
+
+Test 9
+------
+
+A similar test as the previous one, but force the "busy" kernel module
+to do longer work.
+
+The livepatching core will refuse to patch a task that is currently
+executing a to-be-patched function -- the consistency model stalls the
+current patch transition until this safety-check is met.  Test a
+scenario where one of a livepatch's target klp_objects sits on such a
+function for a long time.  Meanwhile, load and unload other target
+kernel modules while the livepatch transition is in progress.
+
+- load busy target module (30s sleep)
+- load livepatch
+- load target module
+- unload target module
+- disable livepatch
+- unload livepatch
+- unload busy target module
+
+
+Load the "busy" kernel module, this time make it do 30 seconds worth of
+work:
+
+  % insmod samples/livepatch/livepatch-callbacks-busymod.ko sleep_secs=30
+  [  110.993362] livepatch_callbacks_busymod: livepatch_callbacks_mod_init
+  [  110.994059] livepatch_callbacks_busymod: busymod_work_func, sleeping 30 seconds ...
+
+Meanwhile, the livepatch is loaded.  Notice that the patch transition
+does not complete as the targeted "busy" module is sitting on a
+to-be-patched function:
+
+  % insmod samples/livepatch/livepatch-callbacks-demo.ko
+  [  113.000309] livepatch: enabling patch 'livepatch_callbacks_demo'
+  [  113.000764] livepatch: 'livepatch_callbacks_demo': initializing patching transition
+  [  113.000791] livepatch_callbacks_demo: pre_patch_callback: vmlinux
+  [  113.001289] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state
+  [  113.005208] livepatch: 'livepatch_callbacks_demo': starting patching transition
+
+Load a second target module (this one is an ordinary idle kernel
+module).  Note that *no* post-patch callbacks will be executed while the
+livepatch is still in transition:
+
+  % insmod samples/livepatch/livepatch-callbacks-mod.ko
+  [  115.012740] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod'
+  [  115.013406] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init
+  [  115.015315] livepatch_callbacks_mod: livepatch_callbacks_mod_init
+
+Request an unload of the simple kernel module.  The patch is still
+transitioning, so its pre-unpatch callbacks are skipped:
+
+  % rmmod samples/livepatch/livepatch-callbacks-mod.ko
+  [  117.022626] livepatch_callbacks_mod: livepatch_callbacks_mod_exit
+  [  117.023376] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod'
+  [  117.024533] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away
+
+Finally the livepatch is disabled.  Since none of the patch's
+klp_object's post-patch callbacks executed, the remaining klp_object's
+pre-unpatch callbacks are skipped:
+
+  % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+  [  119.035408] livepatch: 'livepatch_callbacks_demo': reversing transition from patching to unpatching
+  [  119.035485] livepatch: 'livepatch_callbacks_demo': starting unpatching transition
+  [  119.711166] livepatch: 'livepatch_callbacks_demo': completing unpatching transition
+  [  119.714179] livepatch_callbacks_demo: post_unpatch_callback: vmlinux
+  [  119.714653] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state
+  [  119.715437] livepatch: 'livepatch_callbacks_demo': unpatching complete
+
+  % rmmod samples/livepatch/livepatch-callbacks-demo.ko
+  % rmmod samples/livepatch/livepatch-callbacks-busymod.ko
+  [  141.279111] livepatch_callbacks_busymod: busymod_work_func exit
+  [  141.279760] livepatch_callbacks_busymod: livepatch_callbacks_mod_exit
diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index d08eddc00497..fc5c1be3f6f4 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -87,10 +87,35 @@ struct klp_func {
 	bool transition;
 };
 
+struct klp_object;
+
+/**
+ * struct klp_callbacks - pre/post live-(un)patch callback structure
+ * @pre_patch:		executed before code patching
+ * @post_patch:		executed after code patching
+ * @pre_unpatch:	executed before code unpatching
+ * @post_unpatch:	executed after code unpatching
+ * @post_unpatch_enabled:	flag indicating if post-unpatch callback
+ * 				should run
+ *
+ * All callbacks are optional.  Only the pre-patch callback, if provided,
+ * will be unconditionally executed.  If the parent klp_object fails to
+ * patch for any reason, including a non-zero error status returned from
+ * the pre-patch callback, no further callbacks will be executed.
+ */
+struct klp_callbacks {
+	int (*pre_patch)(struct klp_object *obj);
+	void (*post_patch)(struct klp_object *obj);
+	void (*pre_unpatch)(struct klp_object *obj);
+	void (*post_unpatch)(struct klp_object *obj);
+	bool post_unpatch_enabled;
+};
+
 /**
  * struct klp_object - kernel object structure for live patching
  * @name:	module name (or NULL for vmlinux)
  * @funcs:	function entries for functions to be patched in the object
+ * @callbacks:	functions to be executed pre/post (un)patching
  * @kobj:	kobject for sysfs resources
  * @mod:	kernel module associated with the patched object
  *		(NULL for vmlinux)
@@ -100,6 +125,7 @@ struct klp_object {
 	/* external */
 	const char *name;
 	struct klp_func *funcs;
+	struct klp_callbacks callbacks;
 
 	/* internal */
 	struct kobject kobj;
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index bf8c8fd72589..6f3274203b6b 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -54,11 +54,6 @@ static bool klp_is_module(struct klp_object *obj)
 	return obj->name;
 }
 
-static bool klp_is_object_loaded(struct klp_object *obj)
-{
-	return !obj->name || obj->mod;
-}
-
 /* sets obj->mod if object is not vmlinux and module is found */
 static void klp_find_object_module(struct klp_object *obj)
 {
@@ -285,6 +280,8 @@ static int klp_write_object_relocations(struct module *pmod,
 
 static int __klp_disable_patch(struct klp_patch *patch)
 {
+	struct klp_object *obj;
+
 	if (klp_transition_patch)
 		return -EBUSY;
 
@@ -295,6 +292,10 @@ static int __klp_disable_patch(struct klp_patch *patch)
 
 	klp_init_transition(patch, KLP_UNPATCHED);
 
+	klp_for_each_object(patch, obj)
+		if (patch->enabled && obj->patched)
+			klp_pre_unpatch_callback(obj);
+
 	/*
 	 * Enforce the order of the func->transition writes in
 	 * klp_init_transition() and the TIF_PATCH_PENDING writes in
@@ -388,13 +389,18 @@ static int __klp_enable_patch(struct klp_patch *patch)
 		if (!klp_is_object_loaded(obj))
 			continue;
 
-		ret = klp_patch_object(obj);
+		ret = klp_pre_patch_callback(obj);
 		if (ret) {
-			pr_warn("failed to enable patch '%s'\n",
-				patch->mod->name);
+			pr_warn("pre-patch callback failed for object '%s'\n",
+				klp_is_module(obj) ? obj->name : "vmlinux");
+			goto err;
+		}
 
-			klp_cancel_transition();
-			return ret;
+		ret = klp_patch_object(obj);
+		if (ret) {
+			pr_warn("failed to patch object '%s'\n",
+				klp_is_module(obj) ? obj->name : "vmlinux");
+			goto err;
 		}
 	}
 
@@ -403,6 +409,11 @@ static int __klp_enable_patch(struct klp_patch *patch)
 	patch->enabled = true;
 
 	return 0;
+err:
+	pr_warn("failed to enable patch '%s'\n", patch->mod->name);
+
+	klp_cancel_transition();
+	return ret;
 }
 
 /**
@@ -854,9 +865,15 @@ static void klp_cleanup_module_patches_limited(struct module *mod,
 			 * is in transition.
 			 */
 			if (patch->enabled || patch == klp_transition_patch) {
+
+				if (patch != klp_transition_patch)
+					klp_pre_unpatch_callback(obj);
+
 				pr_notice("reverting patch '%s' on unloading module '%s'\n",
 					  patch->mod->name, obj->mod->name);
 				klp_unpatch_object(obj);
+
+				klp_post_unpatch_callback(obj);
 			}
 
 			klp_free_object_loaded(obj);
@@ -906,13 +923,27 @@ int klp_module_coming(struct module *mod)
 			pr_notice("applying patch '%s' to loading module '%s'\n",
 				  patch->mod->name, obj->mod->name);
 
+			ret = klp_pre_patch_callback(obj);
+			if (ret) {
+				pr_warn("pre-patch callback failed for object '%s'\n",
+					obj->name);
+				goto err;
+			}
+
 			ret = klp_patch_object(obj);
 			if (ret) {
 				pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
 					patch->mod->name, obj->mod->name, ret);
+
+				if (patch != klp_transition_patch)
+					klp_post_unpatch_callback(obj);
+
 				goto err;
 			}
 
+			if (patch != klp_transition_patch)
+				klp_post_patch_callback(obj);
+
 			break;
 		}
 	}
diff --git a/kernel/livepatch/core.h b/kernel/livepatch/core.h
index c74f24c47837..6fc907b54e71 100644
--- a/kernel/livepatch/core.h
+++ b/kernel/livepatch/core.h
@@ -1,6 +1,44 @@
 #ifndef _LIVEPATCH_CORE_H
 #define _LIVEPATCH_CORE_H
 
+#include <linux/livepatch.h>
+
 extern struct mutex klp_mutex;
 
+static inline bool klp_is_object_loaded(struct klp_object *obj)
+{
+	return !obj->name || obj->mod;
+}
+
+static inline int klp_pre_patch_callback(struct klp_object *obj)
+{
+	int ret;
+
+	ret = (obj->callbacks.pre_patch) ?
+		(*obj->callbacks.pre_patch)(obj) : 0;
+
+	obj->callbacks.post_unpatch_enabled = !ret;
+
+	return ret;
+}
+
+static inline void klp_post_patch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.post_patch)
+		(*obj->callbacks.post_patch)(obj);
+}
+
+static inline void klp_pre_unpatch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.pre_unpatch)
+		(*obj->callbacks.pre_unpatch)(obj);
+}
+
+static inline void klp_post_unpatch_callback(struct klp_object *obj)
+{
+	if (obj->callbacks.post_unpatch_enabled &&
+	    obj->callbacks.post_unpatch)
+		(*obj->callbacks.post_unpatch)(obj);
+}
+
 #endif /* _LIVEPATCH_CORE_H */
diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index 52c4e907c14b..82d584225dc6 100644
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c
@@ -28,6 +28,7 @@
 #include <linux/slab.h>
 #include <linux/bug.h>
 #include <linux/printk.h>
+#include "core.h"
 #include "patch.h"
 #include "transition.h"
 
diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index b004a1fb6032..7bf55b7f3687 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -109,9 +109,6 @@ static void klp_complete_transition(void)
 		}
 	}
 
-	if (klp_target_state == KLP_UNPATCHED && !immediate_func)
-		module_put(klp_transition_patch->mod);
-
 	/* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
 	if (klp_target_state == KLP_PATCHED)
 		klp_synchronize_transition();
@@ -130,6 +127,24 @@ static void klp_complete_transition(void)
 	}
 
 done:
+	klp_for_each_object(klp_transition_patch, obj) {
+		if (!klp_is_object_loaded(obj))
+			continue;
+		if (klp_target_state == KLP_PATCHED)
+			klp_post_patch_callback(obj);
+		else if (klp_target_state == KLP_UNPATCHED)
+			klp_post_unpatch_callback(obj);
+	}
+
+	/*
+	 * See complementary comment in __klp_enable_patch() for why we
+	 * keep the module reference for immediate patches.
+	 */
+	if (!klp_transition_patch->immediate && !immediate_func &&
+	    klp_target_state == KLP_UNPATCHED) {
+		module_put(klp_transition_patch->mod);
+	}
+
 	klp_target_state = KLP_UNDEFINED;
 	klp_transition_patch = NULL;
 }
diff --git a/samples/livepatch/Makefile b/samples/livepatch/Makefile
index 539e81d433cd..2472ce39a18d 100644
--- a/samples/livepatch/Makefile
+++ b/samples/livepatch/Makefile
@@ -2,3 +2,6 @@ obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-sample.o
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-shadow-mod.o
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-shadow-fix1.o
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-shadow-fix2.o
+obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-demo.o
+obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-mod.o
+obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-busymod.o
diff --git a/samples/livepatch/livepatch-callbacks-busymod.c b/samples/livepatch/livepatch-callbacks-busymod.c
new file mode 100644
index 000000000000..80d06e103f1b
--- /dev/null
+++ b/samples/livepatch/livepatch-callbacks-busymod.c
@@ -0,0 +1,72 @@
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * livepatch-callbacks-busymod.c - (un)patching callbacks demo support module
+ *
+ *
+ * Purpose
+ * -------
+ *
+ * Simple module to demonstrate livepatch (un)patching callbacks.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * This module is not intended to be standalone.  See the "Usage"
+ * section of livepatch-callbacks-mod.c.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/workqueue.h>
+#include <linux/delay.h>
+
+static int sleep_secs;
+module_param(sleep_secs, int, 0644);
+MODULE_PARM_DESC(sleep_secs, "sleep_secs (default=0)");
+
+static void busymod_work_func(struct work_struct *work);
+static DECLARE_DELAYED_WORK(work, busymod_work_func);
+
+static void busymod_work_func(struct work_struct *work)
+{
+	pr_info("%s, sleeping %d seconds ...\n", __func__, sleep_secs);
+	msleep(sleep_secs * 1000);
+	pr_info("%s exit\n", __func__);
+}
+
+static int livepatch_callbacks_mod_init(void)
+{
+	pr_info("%s\n", __func__);
+	schedule_delayed_work(&work,
+		msecs_to_jiffies(1000 * 0));
+	return 0;
+}
+
+static void livepatch_callbacks_mod_exit(void)
+{
+	cancel_delayed_work_sync(&work);
+	pr_info("%s\n", __func__);
+}
+
+module_init(livepatch_callbacks_mod_init);
+module_exit(livepatch_callbacks_mod_exit);
+MODULE_LICENSE("GPL");
diff --git a/samples/livepatch/livepatch-callbacks-demo.c b/samples/livepatch/livepatch-callbacks-demo.c
new file mode 100644
index 000000000000..3d115bd68442
--- /dev/null
+++ b/samples/livepatch/livepatch-callbacks-demo.c
@@ -0,0 +1,234 @@
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * livepatch-callbacks-demo.c - (un)patching callbacks livepatch demo
+ *
+ *
+ * Purpose
+ * -------
+ *
+ * Demonstration of registering livepatch (un)patching callbacks.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * Step 1 - load the simple module
+ *
+ *   insmod samples/livepatch/livepatch-callbacks-mod.ko
+ *
+ *
+ * Step 2 - load the demonstration livepatch (with callbacks)
+ *
+ *   insmod samples/livepatch/livepatch-callbacks-demo.ko
+ *
+ *
+ * Step 3 - cleanup
+ *
+ *   echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+ *   rmmod livepatch_callbacks_demo
+ *   rmmod livepatch_callbacks_mod
+ *
+ * Watch dmesg output to see livepatch enablement, callback execution
+ * and patching operations for both vmlinux and module targets.
+ *
+ * NOTE: swap the insmod order of livepatch-callbacks-mod.ko and
+ *       livepatch-callbacks-demo.ko to observe what happens when a
+ *       target module is loaded after a livepatch with callbacks.
+ *
+ * NOTE: 'pre_patch_ret' is a module parameter that sets the pre-patch
+ *       callback return status.  Try setting up a non-zero status
+ *       such as -19 (-ENODEV):
+ *
+ *       # Load demo livepatch, vmlinux is patched
+ *       insmod samples/livepatch/livepatch-callbacks-demo.ko
+ *
+ *       # Setup next pre-patch callback to return -ENODEV
+ *       echo -19 > /sys/module/livepatch_callbacks_demo/parameters/pre_patch_ret
+ *
+ *       # Module loader refuses to load the target module
+ *       insmod samples/livepatch/livepatch-callbacks-mod.ko
+ *       insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-mod.ko: No such device
+ *
+ * NOTE: There is a second target module,
+ *       livepatch-callbacks-busymod.ko, available for experimenting
+ *       with livepatch (un)patch callbacks.  This module contains
+ *       a 'sleep_secs' parameter that parks the module on one of the
+ *       functions that the livepatch demo module wants to patch.
+ *       Modifying this value and tweaking the order of module loads can
+ *       effectively demonstrate stalled patch transitions:
+ *
+ *       # Load a target module, let it park on 'busymod_work_func' for
+ *       # thirty seconds
+ *       insmod samples/livepatch/livepatch-callbacks-busymod.ko sleep_secs=30
+ *
+ *       # Meanwhile load the livepatch
+ *       insmod samples/livepatch/livepatch-callbacks-demo.ko
+ *
+ *       # ... then load and unload another target module while the
+ *       # transition is in progress
+ *       insmod samples/livepatch/livepatch-callbacks-mod.ko
+ *       rmmod samples/livepatch/livepatch-callbacks-mod.ko
+ *
+ *       # Finally cleanup
+ *       echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled
+ *       rmmod samples/livepatch/livepatch-callbacks-demo.ko
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/livepatch.h>
+
+static int pre_patch_ret;
+module_param(pre_patch_ret, int, 0644);
+MODULE_PARM_DESC(pre_patch_ret, "pre_patch_ret (default=0)");
+
+static const char *const module_state[] = {
+	[MODULE_STATE_LIVE]	= "[MODULE_STATE_LIVE] Normal state",
+	[MODULE_STATE_COMING]	= "[MODULE_STATE_COMING] Full formed, running module_init",
+	[MODULE_STATE_GOING]	= "[MODULE_STATE_GOING] Going away",
+	[MODULE_STATE_UNFORMED]	= "[MODULE_STATE_UNFORMED] Still setting it up",
+};
+
+static void callback_info(const char *callback, struct klp_object *obj)
+{
+	if (obj->mod)
+		pr_info("%s: %s -> %s\n", callback, obj->mod->name,
+			module_state[obj->mod->state]);
+	else
+		pr_info("%s: vmlinux\n", callback);
+}
+
+/* Executed on object patching (ie, patch enablement) */
+static int pre_patch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+	return pre_patch_ret;
+}
+
+/* Executed on object unpatching (ie, patch disablement) */
+static void post_patch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+}
+
+/* Executed on object unpatching (ie, patch disablement) */
+static void pre_unpatch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+}
+
+/* Executed on object unpatching (ie, patch disablement) */
+static void post_unpatch_callback(struct klp_object *obj)
+{
+	callback_info(__func__, obj);
+}
+
+static void patched_work_func(struct work_struct *work)
+{
+	pr_info("%s\n", __func__);
+}
+
+static struct klp_func no_funcs[] = {
+	{ }
+};
+
+static struct klp_func busymod_funcs[] = {
+	{
+		.old_name = "busymod_work_func",
+		.new_func = patched_work_func,
+	}, { }
+};
+
+static struct klp_object objs[] = {
+	{
+		.name = NULL,	/* vmlinux */
+		.funcs = no_funcs,
+		.callbacks = {
+			.pre_patch = pre_patch_callback,
+			.post_patch = post_patch_callback,
+			.pre_unpatch = pre_unpatch_callback,
+			.post_unpatch = post_unpatch_callback,
+		},
+	},	{
+		.name = "livepatch_callbacks_mod",
+		.funcs = no_funcs,
+		.callbacks = {
+			.pre_patch = pre_patch_callback,
+			.post_patch = post_patch_callback,
+			.pre_unpatch = pre_unpatch_callback,
+			.post_unpatch = post_unpatch_callback,
+		},
+	},	{
+		.name = "livepatch_callbacks_busymod",
+		.funcs = busymod_funcs,
+		.callbacks = {
+			.pre_patch = pre_patch_callback,
+			.post_patch = post_patch_callback,
+			.pre_unpatch = pre_unpatch_callback,
+			.post_unpatch = post_unpatch_callback,
+		},
+	}, { }
+};
+
+static struct klp_patch patch = {
+	.mod = THIS_MODULE,
+	.objs = objs,
+};
+
+static int livepatch_callbacks_demo_init(void)
+{
+	int ret;
+
+	if (!klp_have_reliable_stack() && !patch.immediate) {
+		/*
+		 * WARNING: Be very careful when using 'patch.immediate' in
+		 * your patches.  It's ok to use it for simple patches like
+		 * this, but for more complex patches which change function
+		 * semantics, locking semantics, or data structures, it may not
+		 * be safe.  Use of this option will also prevent removal of
+		 * the patch.
+		 *
+		 * See Documentation/livepatch/livepatch.txt for more details.
+		 */
+		patch.immediate = true;
+		pr_notice("The consistency model isn't supported for your architecture.  Bypassing safety mechanisms and applying the patch immediately.\n");
+	}
+
+	ret = klp_register_patch(&patch);
+	if (ret)
+		return ret;
+	ret = klp_enable_patch(&patch);
+	if (ret) {
+		WARN_ON(klp_unregister_patch(&patch));
+		return ret;
+	}
+	return 0;
+}
+
+static void livepatch_callbacks_demo_exit(void)
+{
+	WARN_ON(klp_unregister_patch(&patch));
+}
+
+module_init(livepatch_callbacks_demo_init);
+module_exit(livepatch_callbacks_demo_exit);
+MODULE_LICENSE("GPL");
+MODULE_INFO(livepatch, "Y");
diff --git a/samples/livepatch/livepatch-callbacks-mod.c b/samples/livepatch/livepatch-callbacks-mod.c
new file mode 100644
index 000000000000..e610ce29ba44
--- /dev/null
+++ b/samples/livepatch/livepatch-callbacks-mod.c
@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * livepatch-callbacks-mod.c - (un)patching callbacks demo support module
+ *
+ *
+ * Purpose
+ * -------
+ *
+ * Simple module to demonstrate livepatch (un)patching callbacks.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * This module is not intended to be standalone.  See the "Usage"
+ * section of livepatch-callbacks-demo.c.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+static int livepatch_callbacks_mod_init(void)
+{
+	pr_info("%s\n", __func__);
+	return 0;
+}
+
+static void livepatch_callbacks_mod_exit(void)
+{
+	pr_info("%s\n", __func__);
+}
+
+module_init(livepatch_callbacks_mod_init);
+module_exit(livepatch_callbacks_mod_exit);
+MODULE_LICENSE("GPL");
-- 
1.8.3.1