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d9defe448f
Make the structure of "Livepatch module Elf format" document similar to the main "Livepatch" document. Also make the structure of "(Un)patching Callbacks" document similar to the "Shadow Variables" document. It fixes the most visible inconsistencies of the documentation generated from the ReST format. Signed-off-by: Petr Mladek <pmladek@suse.com> Acked-by: Joe Lawrence <joe.lawrence@redhat.com> Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> Acked-by: Miroslav Benes <mbenes@suse.cz> Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org> Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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5.1 KiB
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134 lines
5.1 KiB
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======================
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(Un)patching Callbacks
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======================
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Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
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to execute callback functions when a kernel object is (un)patched. They
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can be considered a **power feature** that **extends livepatching abilities**
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to include:
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- Safe updates to global data
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- "Patches" to init and probe functions
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- Patching otherwise unpatchable code (i.e. assembly)
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In most cases, (un)patch callbacks will need to be used in conjunction
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with memory barriers and kernel synchronization primitives, like
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mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.
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1. Motivation
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=============
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Callbacks differ from existing kernel facilities:
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- Module init/exit code doesn't run when disabling and re-enabling a
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patch.
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- A module notifier can't stop a to-be-patched module from loading.
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Callbacks are part of the klp_object structure and their implementation
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is specific to that klp_object. Other livepatch objects may or may not
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be patched, irrespective of the target klp_object's current state.
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2. Callback types
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=================
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Callbacks can be registered for the following livepatch actions:
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* Pre-patch
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- before a klp_object is patched
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* Post-patch
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- after a klp_object has been patched and is active
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across all tasks
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* Pre-unpatch
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- before a klp_object is unpatched (ie, patched code is
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active), used to clean up post-patch callback
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resources
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* Post-unpatch
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- after a klp_object has been patched, all code has
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been restored and no tasks are running patched code,
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used to cleanup pre-patch callback resources
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3. How it works
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===============
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Each callback is optional, omitting one does not preclude specifying any
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other. However, the livepatching core executes the handlers in
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symmetry: pre-patch callbacks have a post-unpatch counterpart and
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post-patch callbacks have a pre-unpatch counterpart. An unpatch
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callback will only be executed if its corresponding patch callback was
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executed. Typical use cases pair a patch handler that acquires and
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configures resources with an unpatch handler tears down and releases
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those same resources.
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A callback is only executed if its host klp_object is loaded. For
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in-kernel vmlinux targets, this means that callbacks will always execute
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when a livepatch is enabled/disabled. For patch target kernel modules,
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callbacks will only execute if the target module is loaded. When a
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module target is (un)loaded, its callbacks will execute only if the
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livepatch module is enabled.
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The pre-patch callback, if specified, is expected to return a status
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code (0 for success, -ERRNO on error). An error status code indicates
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to the livepatching core that patching of the current klp_object is not
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safe and to stop the current patching request. (When no pre-patch
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callback is provided, the transition is assumed to be safe.) If a
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pre-patch callback returns failure, the kernel's module loader will:
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- Refuse to load a livepatch, if the livepatch is loaded after
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targeted code.
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or:
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- Refuse to load a module, if the livepatch was already successfully
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loaded.
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No post-patch, pre-unpatch, or post-unpatch callbacks will be executed
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for a given klp_object if the object failed to patch, due to a failed
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pre_patch callback or for any other reason.
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If a patch transition is reversed, no pre-unpatch handlers will be run
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(this follows the previously mentioned symmetry -- pre-unpatch callbacks
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will only occur if their corresponding post-patch callback executed).
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If the object did successfully patch, but the patch transition never
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started for some reason (e.g., if another object failed to patch),
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only the post-unpatch callback will be called.
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4. Use cases
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============
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Sample livepatch modules demonstrating the callback API can be found in
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samples/livepatch/ directory. These samples were modified for use in
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kselftests and can be found in the lib/livepatch directory.
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Global data update
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------------------
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A pre-patch callback can be useful to update a global variable. For
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example, 75ff39ccc1bd ("tcp: make challenge acks less predictable")
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changes a global sysctl, as well as patches the tcp_send_challenge_ack()
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function.
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In this case, if we're being super paranoid, it might make sense to
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patch the data *after* patching is complete with a post-patch callback,
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so that tcp_send_challenge_ack() could first be changed to read
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sysctl_tcp_challenge_ack_limit with READ_ONCE.
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__init and probe function patches support
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-----------------------------------------
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Although __init and probe functions are not directly livepatch-able, it
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may be possible to implement similar updates via pre/post-patch
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callbacks.
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The commit ``48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST")`` change the way that
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virtnet_probe() initialized its driver's net_device features. A
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pre/post-patch callback could iterate over all such devices, making a
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similar change to their hw_features value. (Client functions of the
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value may need to be updated accordingly.)
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