livepatch: Add some basic livepatch documentation

livepatch framework deserves some documentation, definitely.
This is an attempt to provide some basic info. I hope that
it will be useful for both LivePatch producers and also
potential developers of the framework itself.

[jkosina@suse.cz:
 - incorporated feedback (grammar fixes) from
   Chris J Arges <chris.j.arges@canonical.com>
 - s/LivePatch/livepatch in changelog as pointed out by
   Josh Poimboeuf <jpoimboe@redhat.com>
 - incorporated part of feedback (grammar fixes / reformulations) from
   Balbir Singh <bsingharora@gmail.com>
]
Acked-by: Jessica Yu <jeyu@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/livepatch/livepatch.txt

+=========
+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. The theory is not yet finished. See the discussion at
+http://thread.gmane.org/gmane.linux.kernel/1823033/focus=1828189
+
+The current consistency model is very simple. It guarantees that either
+the old or the new function is called. But various functions get redirected
+one by one without any synchronization.
+
+In other words, the current implementation _never_ modifies the behavior
+in the middle of the call. It is because it does _not_ rewrite the entire
+function in the memory. Instead, the function gets redirected at the
+very beginning. But this redirection is used immediately even when
+some other functions from the same patch have not been redirected yet.
+
+See also the section "Limitations" below.
+
+
+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. Also if a more complex
+    consistency model is supported then a selected unit (thread,
+    kernel as a whole) will see the new code from the entire patch
+    only when it is in a safe state.
+
+
+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.  The immediate consistency model is not able to detect this;
+therefore livepatch modules cannot be removed. See "Limitations" below.
+
+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, the immediate consistency model does not guarantee that anyone is not
+sleeping in the new code after the patch is reverted. This means that the new
+code needs to stay around "forever". If the code is there, one could apply it
+again. Therefore it makes sense to separate the operations that might be done
+once and those that need to be repeated when the patch is enabled (applied)
+again.
+
+Third, it might take some time until the entire system is migrated
+when a more complex consistency model is 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.
+
+In particular, 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.
+
+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.
+
+See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
+
+
+7. Limitations
+==============
+
+The current Livepatch implementation has several limitations:
+
+
+  + The patch must not change the semantic of the patched functions.
+
+    The current implementation guarantees only that either the old
+    or the new function is called. The functions are patched one
+    by one. It means that the patch must _not_ change the semantic
+    of the function.
+
+
+  + Data structures can not be patched.
+
+    There is no support to version data structures or anyhow migrate
+    one structure into another. Also the simple consistency model does
+    not allow to switch more functions atomically.
+
+    Once there is more complex consistency mode, it will be possible to
+    use some workarounds. For example, it will be possible to use a hole
+    for a new member because the data structure is aligned. Or it will
+    be possible to use an existing member for something else.
+
+    There are no plans to add more generic support for modified structures
+    at the moment.
+
+
+  + 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".
+
+
+  + Anything inlined into __schedule() can not be patched.
+
+    The switch_to macro is inlined into __schedule(). It switches the
+    context between two processes in the middle of the macro. It does
+    not save RIP in x86_64 version (contrary to 32-bit version). Instead,
+    the currently used __schedule()/switch_to() handles both processes.
+
+    Now, let's have two different tasks. One calls the original
+    __schedule(), its registers are stored in a defined order and it
+    goes to sleep in the switch_to macro and some other task is restored
+    using the original __schedule(). Then there is the second task which
+    calls patched__schedule(), it goes to sleep there and the first task
+    is picked by the patched__schedule(). Its RSP is restored and now
+    the registers should be restored as well. But the order is different
+    in the new patched__schedule(), so...
+
+    There is work in progress to remove this limitation.
+
+
+  + Livepatch modules can not be removed.
+
+    The current implementation just redirects the functions at the very
+    beginning. It does not check if the functions are in use. In other
+    words, it knows when the functions get called but it does not
+    know when the functions return. Therefore it can not decide when
+    the livepatch module can be safely removed.
+
+    This will get most likely solved once a more complex consistency model
+    is supported. The idea is that a safe state for patching should also
+    mean a safe state for removing the patch.
+
+    Note that the patch itself might get disabled by writing zero
+    to /sys/kernel/livepatch/<patch>/enabled. It causes that the new
+    code will not longer get called. But it does not guarantee
+    that anyone is not sleeping anywhere in the new code.
+
+
+  + 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 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.

MAINTAINERS

@@ -6605,6 +6605,7 @@ F:	kernel/livepatch/
 F:	include/linux/livepatch.h
 F:	arch/x86/include/asm/livepatch.h
 F:	arch/x86/kernel/livepatch.c
+F:	Documentation/livepatch/
 F:	Documentation/ABI/testing/sysfs-kernel-livepatch
 F:	samples/livepatch/
 L:	live-patching@vger.kernel.org