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Initial commit: Linux kernel 4.19 for Unplugged OS

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Documentation/livepatch/callbacks.txt
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======================
(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
Documentation/livepatch/livepatch.txt
+467
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@@ -0,0 +1,467 @@
=========
Livepatch
=========
This document outlines basic information about kernel livepatching.
Table of Contents:
1. Motivation
2. Kprobes, Ftrace, Livepatching
3. Consistency model
4. Livepatch module
4.1. New functions
4.2. Metadata
4.3. Livepatch module handling
5. Livepatch life-cycle
5.1. Registration
5.2. Enabling
5.3. Disabling
5.4. Unregistration
6. Sysfs
7. Limitations
1. Motivation
=============
There are many situations where users are reluctant to reboot a system. It may
be because their system is performing complex scientific computations or under
heavy load during peak usage. In addition to keeping systems up and running,
users want to also have a stable and secure system. Livepatching gives users
both by allowing for function calls to be redirected; thus, fixing critical
functions without a system reboot.
2. Kprobes, Ftrace, Livepatching
================================
There are multiple mechanisms in the Linux kernel that are directly related
to redirection of code execution; namely: kernel probes, function tracing,
and livepatching:
+ The kernel probes are the most generic. The code can be redirected by
putting a breakpoint instruction instead of any instruction.
+ The function tracer calls the code from a predefined location that is
close to the function entry point. This location is generated by the
compiler using the '-pg' gcc option.
+ Livepatching typically needs to redirect the code at the very beginning
of the function entry before the function parameters or the stack
are in any way modified.
All three approaches need to modify the existing code at runtime. Therefore
they need to be aware of each other and not step over each other's toes.
Most of these problems are solved by using the dynamic ftrace framework as
a base. A Kprobe is registered as a ftrace handler when the function entry
is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
a live patch is called with the help of a custom ftrace handler. But there are
some limitations, see below.
3. Consistency model
====================
Functions are there for a reason. They take some input parameters, get or
release locks, read, process, and even write some data in a defined way,
have return values. In other words, each function has a defined semantic.
Many fixes do not change the semantic of the modified functions. For
example, they add a NULL pointer or a boundary check, fix a race by adding
a missing memory barrier, or add some locking around a critical section.
Most of these changes are self contained and the function presents itself
the same way to the rest of the system. In this case, the functions might
be updated independently one by one.
But there are more complex fixes. For example, a patch might change
ordering of locking in multiple functions at the same time. Or a patch
might exchange meaning of some temporary structures and update
all the relevant functions. In this case, the affected unit
(thread, whole kernel) need to start using all new versions of
the functions at the same time. Also the switch must happen only
when it is safe to do so, e.g. when the affected locks are released
or no data are stored in the modified structures at the moment.
The theory about how to apply functions a safe way is rather complex.
The aim is to define a so-called consistency model. It attempts to define
conditions when the new implementation could be used so that the system
stays consistent.
Livepatch has a consistency model which is a hybrid of kGraft and
kpatch: it uses kGraft's per-task consistency and syscall barrier
switching combined with kpatch's stack trace switching. There are also
a number of fallback options which make it quite flexible.
Patches are applied on a per-task basis, when the task is deemed safe to
switch over. When a patch is enabled, livepatch enters into a
transition state where tasks are converging to the patched state.
Usually this transition state can complete in a few seconds. The same
sequence occurs when a patch is disabled, except the tasks converge from
the patched state to the unpatched state.
An interrupt handler inherits the patched state of the task it
interrupts. The same is true for forked tasks: the child inherits the
patched state of the parent.
Livepatch uses several complementary approaches to determine when it's
safe to patch tasks:
1. The first and most effective approach is stack checking of sleeping
tasks. If no affected functions are on the stack of a given task,
the task is patched. In most cases this will patch most or all of
the tasks on the first try. Otherwise it'll keep trying
periodically. This option is only available if the architecture has
reliable stacks (HAVE_RELIABLE_STACKTRACE).
2. The second approach, if needed, is kernel exit switching. A
task is switched when it returns to user space from a system call, a
user space IRQ, or a signal. It's useful in the following cases:
a) Patching I/O-bound user tasks which are sleeping on an affected
function. In this case you have to send SIGSTOP and SIGCONT to
force it to exit the kernel and be patched.
b) Patching CPU-bound user tasks. If the task is highly CPU-bound
then it will get patched the next time it gets interrupted by an
IRQ.
3. For idle "swapper" tasks, since they don't ever exit the kernel, they
instead have a klp_update_patch_state() call in the idle loop which
allows them to be patched before the CPU enters the idle state.
(Note there's not yet such an approach for kthreads.)
Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
the second approach. It's highly likely that some tasks may still be
running with an old version of the function, until that function
returns. In this case you would have to signal the tasks. This
especially applies to kthreads. They may not be woken up and would need
to be forced. See below for more information.
Unless we can come up with another way to patch kthreads, architectures
without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
the kernel livepatching.
The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
is in transition. Only a single patch (the topmost patch on the stack)
can be in transition at a given time. A patch can remain in transition
indefinitely, if any of the tasks are stuck in the initial patch state.
A transition can be reversed and effectively canceled by writing the
opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
the transition is in progress. Then all the tasks will attempt to
converge back to the original patch state.
There's also a /proc/<pid>/patch_state file which can be used to
determine which tasks are blocking completion of a patching operation.
If a patch is in transition, this file shows 0 to indicate the task is
unpatched and 1 to indicate it's patched. Otherwise, if no patch is in
transition, it shows -1. Any tasks which are blocking the transition
can be signaled with SIGSTOP and SIGCONT to force them to change their
patched state. This may be harmful to the system though.
/sys/kernel/livepatch/<patch>/signal attribute provides a better alternative.
Writing 1 to the attribute sends a fake signal to all remaining blocking
tasks. No proper signal is actually delivered (there is no data in signal
pending structures). Tasks are interrupted or woken up, and forced to change
their patched state.
Administrator can also affect a transition through
/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
state. Important note! The force attribute is intended for cases when the
transition gets stuck for a long time because of a blocking task. Administrator
is expected to collect all necessary data (namely stack traces of such blocking
tasks) and request a clearance from a patch distributor to force the transition.
Unauthorized usage may cause harm to the system. It depends on the nature of the
patch, which functions are (un)patched, and which functions the blocking tasks
are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
modules is permanently disabled when the force feature is used. It cannot be
guaranteed there is no task sleeping in such module. It implies unbounded
reference count if a patch module is disabled and enabled in a loop.
Moreover, the usage of force may also affect future applications of live
patches and cause even more harm to the system. Administrator should first
consider to simply cancel a transition (see above). If force is used, reboot
should be planned and no more live patches applied.
3.1 Adding consistency model support to new architectures
---------------------------------------------------------
For adding consistency model support to new architectures, there are a
few options:
1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
for non-DWARF unwinders, also making sure there's a way for the stack
tracing code to detect interrupts on the stack.
2) Alternatively, ensure that every kthread has a call to
klp_update_patch_state() in a safe location. Kthreads are typically
in an infinite loop which does some action repeatedly. The safe
location to switch the kthread's patch state would be at a designated
point in the loop where there are no locks taken and all data
structures are in a well-defined state.
The location is clear when using workqueues or the kthread worker
API. These kthreads process independent actions in a generic loop.
It's much more complicated with kthreads which have a custom loop.
There the safe location must be carefully selected on a case-by-case
basis.
In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
able to use the non-stack-checking parts of the consistency model:
a) patching user tasks when they cross the kernel/user space
boundary; and
b) patching kthreads and idle tasks at their designated patch points.
This option isn't as good as option 1 because it requires signaling
user tasks and waking kthreads to patch them. But it could still be
a good backup option for those architectures which don't have
reliable stack traces yet.
4. Livepatch module
===================
Livepatches are distributed using kernel modules, see
samples/livepatch/livepatch-sample.c.
The module includes a new implementation of functions that we want
to replace. In addition, it defines some structures describing the
relation between the original and the new implementation. Then there
is code that makes the kernel start using the new code when the livepatch
module is loaded. Also there is code that cleans up before the
livepatch module is removed. All this is explained in more details in
the next sections.
4.1. New functions
------------------
New versions of functions are typically just copied from the original
sources. A good practice is to add a prefix to the names so that they
can be distinguished from the original ones, e.g. in a backtrace. Also
they can be declared as static because they are not called directly
and do not need the global visibility.
The patch contains only functions that are really modified. But they
might want to access functions or data from the original source file
that may only be locally accessible. This can be solved by a special
relocation section in the generated livepatch module, see
Documentation/livepatch/module-elf-format.txt for more details.
4.2. Metadata
-------------
The patch is described by several structures that split the information
into three levels:
+ struct klp_func is defined for each patched function. It describes
the relation between the original and the new implementation of a
particular function.
The structure includes the name, as a string, of the original function.
The function address is found via kallsyms at runtime.
Then it includes the address of the new function. It is defined
directly by assigning the function pointer. Note that the new
function is typically defined in the same source file.
As an optional parameter, the symbol position in the kallsyms database can
be used to disambiguate functions of the same name. This is not the
absolute position in the database, but rather the order it has been found
only for a particular object ( vmlinux or a kernel module ). Note that
kallsyms allows for searching symbols according to the object name.
+ struct klp_object defines an array of patched functions (struct
klp_func) in the same object. Where the object is either vmlinux
(NULL) or a module name.
The structure helps to group and handle functions for each object
together. Note that patched modules might be loaded later than
the patch itself and the relevant functions might be patched
only when they are available.
+ struct klp_patch defines an array of patched objects (struct
klp_object).
This structure handles all patched functions consistently and eventually,
synchronously. The whole patch is applied only when all patched
symbols are found. The only exception are symbols from objects
(kernel modules) that have not been loaded yet.
For more details on how the patch is applied on a per-task basis,
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.
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. Registration
-----------------
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.
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.
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.
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.
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.
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.
Note that the patches might be enabled in a different order than they were
registered.
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.
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.
Here 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.
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.
At this stage, all the relevant sys-fs entries are removed and the patch
is removed from the list of known patches.
6. Sysfs
========
Information about the registered patches can be found under
/sys/kernel/livepatch. The patches could be enabled and disabled
by writing there.
/sys/kernel/livepatch/<patch>/signal and /sys/kernel/livepatch/<patch>/force
attributes allow administrator to affect a patching operation.
See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
7. Limitations
==============
The current Livepatch implementation has several limitations:
+ Only functions that can be traced could be patched.
Livepatch is based on the dynamic ftrace. In particular, functions
implementing ftrace or the livepatch ftrace handler could not be
patched. Otherwise, the code would end up in an infinite loop. A
potential mistake is prevented by marking the problematic functions
by "notrace".
+ Livepatch works reliably only when the dynamic ftrace is located at
the very beginning of the function.
The function need to be redirected before the stack or the function
parameters are modified in any way. For example, livepatch requires
using -fentry gcc compiler option on x86_64.
One exception is the PPC port. It uses relative addressing and TOC.
Each function has to handle TOC and save LR before it could call
the ftrace handler. This operation has to be reverted on return.
Fortunately, the generic ftrace code has the same problem and all
this is handled on the ftrace level.
+ Kretprobes using the ftrace framework conflict with the patched
functions.
Both kretprobes and livepatches use a ftrace handler that modifies
the return address. The first user wins. Either the probe or the patch
is rejected when the handler is already in use by the other.
+ Kprobes in the original function are ignored when the code is
redirected to the new implementation.
There is a work in progress to add warnings about this situation.
Documentation/livepatch/module-elf-format.txt
+323
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@@ -0,0 +1,323 @@
===========================
Livepatch module Elf format
===========================
This document outlines the Elf format requirements that livepatch modules must follow.
-----------------
Table of Contents
-----------------
0. Background and motivation
1. Livepatch modinfo field
2. Livepatch relocation sections
2.1 What are livepatch relocation sections?
2.2 Livepatch relocation section format
2.2.1 Required flags
2.2.2 Required name format
2.2.3 Example livepatch relocation section names
2.2.4 Example `readelf --sections` output
2.2.5 Example `readelf --relocs` output
3. Livepatch symbols
3.1 What are livepatch symbols?
3.2 A livepatch module's symbol table
3.3 Livepatch symbol format
3.3.1 Required flags
3.3.2 Required name format
3.3.3 Example livepatch symbol names
3.3.4 Example `readelf --symbols` output
4. Architecture-specific sections
5. Symbol table and Elf section access
----------------------------
0. Background and motivation
----------------------------
Formerly, livepatch required separate architecture-specific code to write
relocations. However, arch-specific code to write relocations already
exists in the module loader, so this former approach produced redundant
code. So, instead of duplicating code and re-implementing what the module
loader can already do, livepatch leverages existing code in the module
loader to perform the all the arch-specific relocation work. Specifically,
livepatch reuses the apply_relocate_add() function in the module loader to
write relocations. The patch module Elf format described in this document
enables livepatch to be able to do this. The hope is that this will make
livepatch more easily portable to other architectures and reduce the amount
of arch-specific code required to port livepatch to a particular
architecture.
Since apply_relocate_add() requires access to a module's section header
table, symbol table, and relocation section indices, Elf information is
preserved for livepatch modules (see section 5). Livepatch manages its own
relocation sections and symbols, which are described in this document. The
Elf constants used to mark livepatch symbols and relocation sections were
selected from OS-specific ranges according to the definitions from glibc.
0.1 Why does livepatch need to write its own relocations?
---------------------------------------------------------
A typical livepatch module contains patched versions of functions that can
reference non-exported global symbols and non-included local symbols.
Relocations referencing these types of symbols cannot be left in as-is
since the kernel module loader cannot resolve them and will therefore
reject the livepatch module. Furthermore, we cannot apply relocations that
affect modules not yet loaded at patch module load time (e.g. a patch to a
driver that is not loaded). Formerly, livepatch solved this problem by
embedding special "dynrela" (dynamic rela) sections in the resulting patch
module Elf output. Using these dynrela sections, livepatch could resolve
symbols while taking into account its scope and what module the symbol
belongs to, and then manually apply the dynamic relocations. However this
approach required livepatch to supply arch-specific code in order to write
these relocations. In the new format, livepatch manages its own SHT_RELA
relocation sections in place of dynrela sections, and the symbols that the
relas reference are special livepatch symbols (see section 2 and 3). The
arch-specific livepatch relocation code is replaced by a call to
apply_relocate_add().
================================
PATCH MODULE FORMAT REQUIREMENTS
================================
--------------------------
1. Livepatch modinfo field
--------------------------
Livepatch modules are required to have the "livepatch" modinfo attribute.
See the sample livepatch module in samples/livepatch/ for how this is done.
Livepatch modules can be identified by users by using the 'modinfo' command
and looking for the presence of the "livepatch" field. This field is also
used by the kernel module loader to identify livepatch modules.
Example modinfo output:
-----------------------
% modinfo livepatch-meminfo.ko
filename: livepatch-meminfo.ko
livepatch: Y
license: GPL
depends:
vermagic: 4.3.0+ SMP mod_unload
--------------------------------
2. Livepatch relocation sections
--------------------------------
-------------------------------------------
2.1 What are livepatch relocation sections?
-------------------------------------------
A livepatch module manages its own Elf relocation sections to apply
relocations to modules as well as to the kernel (vmlinux) at the
appropriate time. For example, if a patch module patches a driver that is
not currently loaded, livepatch will apply the corresponding livepatch
relocation section(s) to the driver once it loads.
Each "object" (e.g. vmlinux, or a module) within a patch module may have
multiple livepatch relocation sections associated with it (e.g. patches to
multiple functions within the same object). There is a 1-1 correspondence
between a livepatch relocation section and the target section (usually the
text section of a function) to which the relocation(s) apply. It is
also possible for a livepatch module to have no livepatch relocation
sections, as in the case of the sample livepatch module (see
samples/livepatch).
Since Elf information is preserved for livepatch modules (see Section 5), a
livepatch relocation section can be applied simply by passing in the
appropriate section index to apply_relocate_add(), which then uses it to
access the relocation section and apply the relocations.
Every symbol referenced by a rela in a livepatch relocation section is a
livepatch symbol. These must be resolved before livepatch can call
apply_relocate_add(). See Section 3 for more information.
---------------------------------------
2.2 Livepatch relocation section format
---------------------------------------
2.2.1 Required flags
--------------------
Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
section flag. See include/uapi/linux/elf.h for the definition. The module
loader recognizes this flag and will avoid applying those relocation sections
at patch module load time. These sections must also be marked with SHF_ALLOC,
so that the module loader doesn't discard them on module load (i.e. they will
be copied into memory along with the other SHF_ALLOC sections).
2.2.2 Required name format
--------------------------
The name of a livepatch relocation section must conform to the following format:
.klp.rela.objname.section_name
^ ^^ ^ ^ ^
|________||_____| |__________|
[A] [B] [C]
[A] The relocation section name is prefixed with the string ".klp.rela."
[B] The name of the object (i.e. "vmlinux" or name of module) to
which the relocation section belongs follows immediately after the prefix.
[C] The actual name of the section to which this relocation section applies.
2.2.3 Example livepatch relocation section names:
-------------------------------------------------
.klp.rela.ext4.text.ext4_attr_store
.klp.rela.vmlinux.text.cmdline_proc_show
2.2.4 Example `readelf --sections` output for a patch
module that patches vmlinux and modules 9p, btrfs, ext4:
--------------------------------------------------------
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ snip ]
[29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
[30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
[ snip ]
[34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
[35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
[36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
[37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
[ snip ] ^ ^
| |
[*] [*]
[*] Livepatch relocation sections are SHT_RELA sections but with a few special
characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
not be discarded when the module is loaded into memory, as well as with the
SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).
2.2.5 Example `readelf --relocs` output for a patch module:
-----------------------------------------------------------
Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
[ snip ] ^
|
[*]
[*] Every symbol referenced by a relocation is a livepatch symbol.
--------------------
3. Livepatch symbols
--------------------
-------------------------------
3.1 What are livepatch symbols?
-------------------------------
Livepatch symbols are symbols referred to by livepatch relocation sections.
These are symbols accessed from new versions of functions for patched
objects, whose addresses cannot be resolved by the module loader (because
they are local or unexported global syms). Since the module loader only
resolves exported syms, and not every symbol referenced by the new patched
functions is exported, livepatch symbols were introduced. They are used
also in cases where we cannot immediately know the address of a symbol when
a patch module loads. For example, this is the case when livepatch patches
a module that is not loaded yet. In this case, the relevant livepatch
symbols are resolved simply when the target module loads. In any case, for
any livepatch relocation section, all livepatch symbols referenced by that
section must be resolved before livepatch can call apply_relocate_add() for
that reloc section.
Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
loader can identify and ignore them. Livepatch modules keep these symbols
in their symbol tables, and the symbol table is made accessible through
module->symtab.
-------------------------------------
3.2 A livepatch module's symbol table
-------------------------------------
Normally, a stripped down copy of a module's symbol table (containing only
"core" symbols) is made available through module->symtab (See layout_symtab()
in kernel/module.c). For livepatch modules, the symbol table copied into memory
on module load must be exactly the same as the symbol table produced when the
patch module was compiled. This is because the relocations in each livepatch
relocation section refer to their respective symbols with their symbol indices,
and the original symbol indices (and thus the symtab ordering) must be
preserved in order for apply_relocate_add() to find the right symbol.
For example, take this particular rela from a livepatch module:
Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
symbol index 94.
And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
[ snip ]
94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
[ snip ]
---------------------------
3.3 Livepatch symbol format
---------------------------
3.3.1 Required flags
--------------------
Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
that the module loader can identify them and not attempt to resolve them.
See include/uapi/linux/elf.h for the actual definitions.
3.3.2 Required name format
--------------------------
Livepatch symbol names must conform to the following format:
.klp.sym.objname.symbol_name,sympos
^ ^^ ^ ^ ^ ^
|_______||_____| |_________| |
[A] [B] [C] [D]
[A] The symbol name is prefixed with the string ".klp.sym."
[B] The name of the object (i.e. "vmlinux" or name of module) to
which the symbol belongs follows immediately after the prefix.
[C] The actual name of the symbol.
[D] The position of the symbol in the object (as according to kallsyms)
This is used to differentiate duplicate symbols within the same
object. The symbol position is expressed numerically (0, 1, 2...).
The symbol position of a unique symbol is 0.
3.3.3 Example livepatch symbol names:
-------------------------------------
.klp.sym.vmlinux.snprintf,0
.klp.sym.vmlinux.printk,0
.klp.sym.btrfs.btrfs_ktype,0
3.3.4 Example `readelf --symbols` output for a patch module:
------------------------------------------------------------
Symbol table '.symtab' contains 127 entries:
Num: Value Size Type Bind Vis Ndx Name
[ snip ]
73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
[ snip ] ^
|
[*]
[*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
"OS" means OS-specific.
---------------------------------
4. Architecture-specific sections
---------------------------------
Architectures may override arch_klp_init_object_loaded() to perform
additional arch-specific tasks when a target module loads, such as applying
arch-specific sections. On x86 for example, we must apply per-object
.altinstructions and .parainstructions sections when a target module loads.
These sections must be prefixed with ".klp.arch.$objname." so that they can
be easily identified when iterating through a patch module's Elf sections
(See arch/x86/kernel/livepatch.c for a complete example).
--------------------------------------
5. Symbol table and Elf section access
--------------------------------------
A livepatch module's symbol table is accessible through module->symtab.
Since apply_relocate_add() requires access to a module's section headers,
symbol table, and relocation section indices, Elf information is preserved for
livepatch modules and is made accessible by the module loader through
module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
this struct is filled in by the module loader. Its fields are documented below:
struct klp_modinfo {
Elf_Ehdr hdr; /* Elf header */
Elf_Shdr *sechdrs; /* Section header table */
char *secstrings; /* String table for the section headers */
unsigned int symndx; /* The symbol table section index */
};
Documentation/livepatch/shadow-vars.txt
+209
View File
@@ -0,0 +1,209 @@
================
Shadow Variables
================
Shadow variables are a simple way for livepatch modules to associate
additional "shadow" data with existing data structures. Shadow data is
allocated separately from parent data structures, which are left
unmodified. The shadow variable API described in this document is used
to allocate/add and remove/free shadow variables to/from their parents.
The implementation introduces a global, in-kernel hashtable that
associates pointers to parent objects and a numeric identifier of the
shadow data. The numeric identifier is a simple enumeration that may be
used to describe shadow variable version, class or type, etc. More
specifically, the parent pointer serves as the hashtable key while the
numeric id subsequently filters hashtable queries. Multiple shadow
variables may attach to the same parent object, but their numeric
identifier distinguishes between them.
1. Brief API summary
====================
(See the full API usage docbook notes in livepatch/shadow.c.)
A hashtable references all shadow variables. These references are
stored and retrieved through a <obj, id> pair.
* The klp_shadow variable data structure encapsulates both tracking
meta-data and shadow-data:
- meta-data
- obj - pointer to parent object
- id - data identifier
- data[] - storage for shadow data
It is important to note that the klp_shadow_alloc() and
klp_shadow_get_or_alloc() are zeroing the variable by default.
They also allow to call a custom constructor function when a non-zero
value is needed. Callers should provide whatever mutual exclusion
is required.
Note that the constructor is called under klp_shadow_lock spinlock. It allows
to do actions that can be done only once when a new variable is allocated.
* klp_shadow_get() - retrieve a shadow variable data pointer
- search hashtable for <obj, id> pair
* klp_shadow_alloc() - allocate and add a new shadow variable
- search hashtable for <obj, id> pair
- if exists
- WARN and return NULL
- if <obj, id> doesn't already exist
- allocate a new shadow variable
- initialize the variable using a custom constructor and data when provided
- add <obj, id> to the global hashtable
* klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable
- search hashtable for <obj, id> pair
- if exists
- return existing shadow variable
- if <obj, id> doesn't already exist
- allocate a new shadow variable
- initialize the variable using a custom constructor and data when provided
- add <obj, id> pair to the global hashtable
* klp_shadow_free() - detach and free a <obj, id> shadow variable
- find and remove a <obj, id> reference from global hashtable
- if found
- call destructor function if defined
- free shadow variable
* klp_shadow_free_all() - detach and free all <*, id> shadow variables
- find and remove any <*, id> references from global hashtable
- if found
- call destructor function if defined
- free shadow variable
2. Use cases
============
(See the example shadow variable livepatch modules in samples/livepatch/
for full working demonstrations.)
For the following use-case examples, consider commit 1d147bfa6429
("mac80211: fix AP powersave TX vs. wakeup race"), which added a
spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case
example can be considered a stand-alone livepatch implementation of this
fix.
Matching parent's lifecycle
---------------------------
If parent data structures are frequently created and destroyed, it may
be easiest to align their shadow variables lifetimes to the same
allocation and release functions. In this case, the parent data
structure is typically allocated, initialized, then registered in some
manner. Shadow variable allocation and setup can then be considered
part of the parent's initialization and should be completed before the
parent "goes live" (ie, any shadow variable get-API requests are made
for this <obj, id> pair.)
For commit 1d147bfa6429, when a parent sta_info structure is allocated,
allocate a shadow copy of the ps_lock pointer, then initialize it:
#define PS_LOCK 1
struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
const u8 *addr, gfp_t gfp)
{
struct sta_info *sta;
spinlock_t *ps_lock;
/* Parent structure is created */
sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp);
/* Attach a corresponding shadow variable, then initialize it */
ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp,
NULL, NULL);
if (!ps_lock)
goto shadow_fail;
spin_lock_init(ps_lock);
...
When requiring a ps_lock, query the shadow variable API to retrieve one
for a specific struct sta_info:
void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
{
spinlock_t *ps_lock;
/* sync with ieee80211_tx_h_unicast_ps_buf */
ps_lock = klp_shadow_get(sta, PS_LOCK);
if (ps_lock)
spin_lock(ps_lock);
...
When the parent sta_info structure is freed, first free the shadow
variable:
void sta_info_free(struct ieee80211_local *local, struct sta_info *sta)
{
klp_shadow_free(sta, PS_LOCK, NULL);
kfree(sta);
...
In-flight parent objects
------------------------
Sometimes it may not be convenient or possible to allocate shadow
variables alongside their parent objects. Or a livepatch fix may
require shadow varibles to only a subset of parent object instances. In
these cases, the klp_shadow_get_or_alloc() call can be used to attach
shadow variables to parents already in-flight.
For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
inside ieee80211_sta_ps_deliver_wakeup():
int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data)
{
spinlock_t *lock = shadow_data;
spin_lock_init(lock);
return 0;
}
#define PS_LOCK 1
void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
{
spinlock_t *ps_lock;
/* sync with ieee80211_tx_h_unicast_ps_buf */
ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
sizeof(*ps_lock), GFP_ATOMIC,
ps_lock_shadow_ctor, NULL);
if (ps_lock)
spin_lock(ps_lock);
...
This usage will create a shadow variable, only if needed, otherwise it
will use one that was already created for this <obj, id> pair.
Like the previous use-case, the shadow spinlock needs to be cleaned up.
A shadow variable can be freed just before its parent object is freed,
or even when the shadow variable itself is no longer required.
Other use-cases
---------------
Shadow variables can also be used as a flag indicating that a data
structure was allocated by new, livepatched code. In this case, it
doesn't matter what data value the shadow variable holds, its existence
suggests how to handle the parent object.
3. References
=============
* https://github.com/dynup/kpatch
The livepatch implementation is based on the kpatch version of shadow
variables.
* http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf
Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity
Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented
a datatype update technique called "shadow data structures".