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 <mchehab+samsung@kernel.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 459 deletions

diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt
deleted file mode 100644
index 4627b41ff02e..000000000000
--- a/Documentation/livepatch/livepatch.txt
+++ /dev/null
@@ -1,459 +0,0 @@
-=========
-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
-5. Livepatch life-cycle
-   5.1. Loading
-   5.2. Enabling
-   5.3. Replacing
-   5.4. Disabling
-   5.5. Removing
-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 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. Sending a fake signal
-to all remaining blocking tasks is a better alternative. 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. The fake
-signal is automatically sent every 15 seconds.
-
-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.
-
-
-5. Livepatch life-cycle
-=======================
-
-Livepatching can be described by five basic operations:
-loading, enabling, replacing, disabling, removing.
-
-Where the replacing and the disabling operations are mutually
-exclusive. They have the same result for the given patch but
-not for the system.
-
-
-5.1. Loading
-------------
-
-The only reasonable way is to enable the patch when the livepatch kernel
-module is being loaded. For this, klp_enable_patch() has to be called
-in the module_init() callback. There are two main reasons:
-
-First, only the module has an easy access to the related struct klp_patch.
-
-Second, the error code might be used to refuse loading the module when
-the patch cannot get enabled.
-
-
-5.2. Enabling
--------------
-
-The livepatch gets enabled by calling klp_enable_patch() from
-the module_init() callback. The system will start using the new
-implementation of the patched functions at this stage.
-
-First, 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 above
-operation fails.
-
-Second, livepatch enters into a transition state where tasks are converging
-to the patched state. 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[*]. This stage is indicated by a value of '1'
-in /sys/kernel/livepatch/<name>/transition. For more information about
-this process, see the "Consistency model" section.
-
-Finally, once all tasks have been patched, the 'transition' value changes
-to '0'.
-
-[*] Note that functions might be patched multiple times. The ftrace handler
-    is registered only once for a given function. Further patches just add
-    an entry to the list (see field `func_stack`) of the struct klp_ops.
-    The right implementation is selected by the ftrace handler, see
-    the "Consistency model" section.
-
-    That said, it is highly recommended to use cumulative livepatches
-    because they help keeping the consistency of all changes. In this case,
-    functions might be patched two times only during the transition period.
-
-
-5.3. Replacing
---------------
-
-All enabled patches might get replaced by a cumulative patch that
-has the .replace flag set.
-
-Once the new patch is enabled and the 'transition' finishes then
-all the functions (struct klp_func) associated with the replaced
-patches are removed from the corresponding struct klp_ops. Also
-the ftrace handler is unregistered and the struct klp_ops is
-freed when the related function is not modified by the new patch
-and func_stack list becomes empty.
-
-See Documentation/livepatch/cumulative-patches.txt for more details.
-
-
-5.4. Disabling
---------------
-
-Enabled patches might get disabled by writing '0' to
-/sys/kernel/livepatch/<name>/enabled.
-
-First, livepatch enters into a transition state where tasks are converging
-to the unpatched state. The system starts using either the code from
-the previously enabled patch or even the original one. This stage is
-indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition.
-For more information about this process, see the "Consistency model"
-section.
-
-Second, once all tasks have been unpatched, the 'transition' value changes
-to '0'. 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.
-
-Third, the sysfs interface is destroyed.
-
-
-5.5. Removing
--------------
-
-Module removal is only safe when there are no users of functions provided
-by the module. This is the reason why the force feature permanently
-disables the removal. Only 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.
-
-
-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>/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.