From 89e33ea73295327f22fd1594f97cc70a5381b74a Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Fri, 3 May 2019 16:30:23 +0200 Subject: docs: livepatch: convert docs to ReST and rename to *.rst Convert livepatch documentation to ReST format. The changes are mostly trivial, as the documents are already on a good shape. Just a few markup changes are needed for Sphinx to properly parse the docs. The conversion is actually: - add blank lines and identation in order to identify paragraphs; - fix tables markups; - add some lists markups; - mark literal blocks; - The in-file TOC becomes a comment, in order to skip it from the output, as Sphinx already generates an index there. - adjust title markups. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab Signed-off-by: Petr Mladek Acked-by: Miroslav Benes Acked-by: Josh Poimboeuf Acked-by: Joe Lawrence Reviewed-by: Kamalesh Babulal Signed-off-by: Jonathan Corbet --- Documentation/livepatch/callbacks.rst | 130 ++++++++++++++++++++++++++++++++++ 1 file changed, 130 insertions(+) create mode 100644 Documentation/livepatch/callbacks.rst (limited to 'Documentation/livepatch/callbacks.rst') diff --git a/Documentation/livepatch/callbacks.rst b/Documentation/livepatch/callbacks.rst new file mode 100644 index 000000000000..d76d1f0d9fcf --- /dev/null +++ b/Documentation/livepatch/callbacks.rst @@ -0,0 +1,130 @@ +====================== +(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.) + + +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. -- cgit v1.2.3