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Diffstat (limited to 'Documentation/livepatch/callbacks.rst')
| -rw-r--r-- | Documentation/livepatch/callbacks.rst | 33 |
1 files changed, 18 insertions, 15 deletions
diff --git a/Documentation/livepatch/callbacks.rst b/Documentation/livepatch/callbacks.rst index d76d1f0d9fcf..470944aa8658 100644 --- a/Documentation/livepatch/callbacks.rst +++ b/Documentation/livepatch/callbacks.rst @@ -4,7 +4,7 @@ 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 +can be considered a **power feature** that **extends livepatching abilities** to include: - Safe updates to global data @@ -17,6 +17,9 @@ 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. +1. Motivation +============= + Callbacks differ from existing kernel facilities: - Module init/exit code doesn't run when disabling and re-enabling a @@ -28,6 +31,9 @@ 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. +2. Callback types +================= + Callbacks can be registered for the following livepatch actions: * Pre-patch @@ -47,6 +53,9 @@ Callbacks can be registered for the following livepatch actions: been restored and no tasks are running patched code, used to cleanup pre-patch callback resources +3. How it works +=============== + 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 @@ -90,11 +99,14 @@ 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. +4. Use cases +============ -Example Use-cases -================= +Sample livepatch modules demonstrating the callback API can be found in +samples/livepatch/ directory. These samples were modified for use in +kselftests and can be found in the lib/livepatch directory. -Update global data +Global data update ------------------ A pre-patch callback can be useful to update a global variable. For @@ -107,24 +119,15 @@ 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 +__init and probe function patches support ----------------------------------------- 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 +The commit ``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.) - - -Other Examples -============== - -Sample livepatch modules demonstrating the callback API can be found in -samples/livepatch/ directory. These samples were modified for use in -kselftests and can be found in the lib/livepatch directory. |