Subject: Re: [PATCH] livepatch: Add some basic LivePatch documentation

From: Nikolay Borisov <[email-masked]>
Date: Wed, 9 Mar 2016 16:38:22 +0200

On 03/09/2016 04:01 PM, Petr Mladek wrote:
> 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.
> 
> Signed-off-by: Petr Mladek <[email-masked]>
> ---
> 
> This patch was motivated by the LivePatch port for PPC. The guys
> might want to document some PPC-specific limitations on top of it.
> 
> I am sure that it is far from perfect. But I hope that it is
> an acceptable start that can be improved later. I hope that
> I did not write that many factual mistakes.
> 
> I wrote only some generic info about the consistency model.
> I am not sure if we have agreed on some specification yet.
> 
> I am sorry for grammar mistakes. I hope that some hairs will
> stay on your head if you are sensitive.
> 
> 
>  Documentation/livepatch/livepatch.txt | 277 ++++++++++++++++++++++++++++++++++
>  MAINTAINERS                           |   1 +
>  2 files changed, 278 insertions(+)
>  create mode 100644 Documentation/livepatch/livepatch.txt
> 
> diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt
> new file mode 100644
> index 000000000000..28e8047abb61
> --- /dev/null
> +++ b/Documentation/livepatch/livepatch.txt
> @@ -0,0 +1,277 @@
> +=========
> +LivePatch
> +=========
> +
> +This document outlines basic information about kernel LivePatching.
> +
> +Table of Contents:
> +
> +1. Motivation
> +2. Kprobes, Ftrace, LivePatching
> +3. Consistency model
> +4. LivePatch life-cycle
> +   4.1. Registration
> +   4.2. Enabling
> +   4.3. Disabling
> +   4.4. Unregistration
> +5. Livepatch module
> +   5.1. New functions
> +   5.2. Metadata
> +   5.3. Module handling
> +6. Sysfs
> +7. Limitations
> +
> +
> +1. Motivation
> +=============
> +
> +There are situations when people are really reluctant to reboot a system.
> +It might be because the computer is in the middle of a complex scientific
> +computation. Or the system is busy handling customer requests in the high
> +season.
> +
> +On the other hand, people also want to keep the system stable and secure.
> +This is where LivePatch infrastructure comes handy. It allows to redirect
> +selected function calls to a fixed implementation without rebooting
> +the system.
> +
> +
> +2. Kprobes, Ftrace, LivePatching
> +================================
> +
> +Linux kernel has more ways how to redirect an existing code into a new one.
> +It happens with kernel probes, function tracing, and LivePatching:
> +
> +  + The kernel probes are the most generic way. The code can be redirected
> +    by putting an interrupt instruction instead of any instruction.
> +
> +  + The function tracer calls the code from a predefined location that is
> +    close the function entry. The location is generated by the compiler,
> +    see -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 anyhow muffled.
> +
> +All three approaches need to modify the existing code at runtime. Therefore
> +they need to be aware of each other and do not step over othres' toes. Most
do not step over each other's toes

> +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 from 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. By other words, each function has a defined semantic.
In other words

> +
> +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 about a critical section. Most of these
> +changes are self contained and the function present 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 more 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,
> +the data using the modified structures are empty.
> +
> +The theory about how to apply functions a safe way is rather complex.
> +The aim is to define a so-called consistency model. It means 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 implementation supports the easiest scenario that
> +is sufficeint for the most common fixes. See the limitations below.
> +
> +
> +4. LivePatch life-cycle
> +=======================
> +
> +LivePatching defines four basic operations that define the life cycle
> +of each live patch.
> +
> +4.1. Registration
> +-----------------
> +
> +Each patch has to be registered using klp_register_patch().
> +
> +Here the patch is added into the list of known patches. The addresses
> +of the patched functions are found according to their names.
> +Relocations are applied. The relevant entries are created under
> +/sys/kernel/livepatch/<name>.

Maybe expand a bit more on what type of relocations are applied. Or at
least defer the explanation to a later paragraph (I saw there is a FIXME
later in the file)

> +
> +
> +4.2. Enabling
> +-------------
> +
> +Registered patches might be enabled either by calling klp_enable_patch() or
> +by writing '1' to /sys/kernel/livepatch/<name>/enabled.
> +
> +At this stage, an universal ftrace handler is registered for all newly patched
> +functions with a function-specific ftrace_ops structure. The structure points
> +to a list of struct klp_func, see func_stack. This way the same function

see struct func_stack. It's not obvious what func_stack is. Maybe 1-2
sentences explaining what's the idea of func_stack

> +can be patched more times. The last variant from the func_stack is used.
> +
> +Note that we could enable patches in a different order than they are
> +registered. The actually used function is defined by the order in
> +the func_stack list.
> +
> +
> +4.3. Disabling
> +--------------
> +
> +Enabled patches might get disabled either by calling klp_disable_patch() or
> +by writing '0' to /sys/kernel/livepatch/<name>/enabled.
> +
> +Here all the struct klp_functions are removed from the appropriate
> +ftrace_ops. The ftrace handler is unregistered when the func_stack
> +list gets empty.
> +
> +Patches must be disabled in the exactly revese order in which they were
> +enabled. It makes the problem and the implementation easier.
> +
> +
> +4.4. Unregistration
> +-------------------
> +
> +Disabled patched might be unregistered by calling klp_unregister_patch().
> +
> +At this stage, all the relevant sys-fs entries are removed and the patch
> +is removed from the list of known patches.
> +
> +
> +5. Livepatch module
> +===================
> +
> +Live patches 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 what
> +functions are replaced. Finally, there is a code for registering,
> +enabling, and unregistering the patch.
> +
> +
> +5.1. New functions
> +------------------
> +
> +New versions of functions are typically just copied from the fixed 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 it
> +is usually enough to have a local visibility (static).

Maybe rephrase the last sentence of this paragraph. What does 'local
visibility' refer to - the new function implementations in the patch
module be static or something else?

> +
> +The patch contains only functions that are really modified. But they might
> +want to access functions or data with local visibility from the original
> +source.c file. This can be solved by relocation information. FIXME:
> +The support and documentation for relocations is still in progress.
> +
> +
> +5.2. Metadata
> +------------
> +
> +The patch is described by several structures that split the information
> +into three levels:
> +
> +  + struct klp_patch is defined for each patched function. It includes
> +    a name (string) of the original function, optionaly the position
> +    of the symbol within an object, and a name (pointer) to the new

within which object - the resulting to-be-patched kernel or some other
obejct?

> +    function implementation. The old function will be later found via
> +    kallsyms at runtime. The new function is defined in the same
> +    source file.

The last sentence is confusing, it seems it is not connected to the
earlier sentences in the paragraph.

> +
> +  + struct klp_object defines an array of patched functions (struct
> +    klp_patch) in the same object. Where object is either vmlinux (NULL)
> +    or a module name. It helps to group and handle functions for each

Here object is being explained, maybe move it to the first paragraph so
that readers can continue reading and have an idea what "object" is
referring to.

> +    object together. Note that patched modules might be loaded later
> +    then the patch itself and the relevant functions might be patched
s/then/than/
> +    only when they are available.
> +
> +  + struct klp_patch defines an array of patched objects (struct
> +    klp_object). It allows to handle all patched functions consistently
> +    and synchronously. The whole patch is applied only when all available
> +    symbols can be patched. 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 they are in a safe state.
> +
> +
> +5.3. Module handling
> +-------------------
> +
> +The live patch is typically registered and enabled when the module
> +is loaded. The reverse operations are called when the module
> +is being removed.
> +
> +IMPORTANT: Livepatch modules could not be removed at the moment.
s/could/can/
> +See the limitations below.
> +
> +
> +6. Sysfs
> +========
> +
> +Information about the registered patches might 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 initial Livepatch implementation has several limitations:
s/initial/current

Hopefully the documentation will be kept up to date as the livepatching
framework evolves
> +
> +  + The modules with LivePatches could not be removed without forcing
> +    at the moment.

Livepatching modules can only be removed by force unloading them e.g.
rmmod -f patch.ko . If that's what you meant.

> +
> +    The problem is how to detect if anyone is still using (sleeping inside)
> +    a code from the patch. It 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.

The impression I'm left with when reading this paragraph is that there
is no consistency model whatsoever at present times. Whereas I believe
there is a very crude one

> +
> +
> +  + 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, you 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 muffled any way. For example, LivePatch requires
> +    using -fentry on x86_64.
s/-fentry/-fentry compiler option/ if you are referring to
https://gcc.gnu.org/onlinedocs/gccint/Function-Entry.html

> +
> +
> +  + 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.
> +
> +
> +  + 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 situations.
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 4029c63d8a7d..0e7049688862 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6590,6 +6590,7 @@ 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:	[email-masked]
> 

It's a good starting point. Hopefully a bit of tweaking will make it
even more friendly to newcomers of livepatching (I have read the sources
but I'm in no way VERY familiar with it). I'm especially interested in
the relocation and current consistency model documentation.

Reviewed-by: Nikolay Borisov <[email-masked]>