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

From: Josh Poimboeuf <[email-masked]>
Date: Thu, 10 Mar 2016 15:08:41 -0600

On Wed, Mar 09, 2016 at 03:01:46PM +0100, 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]>

Hi Petr,

This is awesome.  Thank you for starting this!

Some minor nits below.

> 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
> +=========

At the risk of sounding like a broken record, I have to say that I
really don't like the CamelCase in LivePatch.  I don't have a good
reason other than it gives me flashbacks to being exposed to
EnterpriseApplicationObjectOrientedProgramming.

And there's no existing precedent...

  # git grep LivePatch
  #

...so can we please not create one now? :-)

</end grammar pedantry>

> +
> +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
> +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.
> +
> +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.

I can't grok "the data using the modified structures are empty".  Can you
clarify?

> +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>.
> +
> +
> +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
> +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).
> +
> +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

s/klp_patch/klp_func/

> +    a name (string) of the original function, optionaly the position
> +    of the symbol within an object, and a name (pointer) to the new
> +    function implementation. The old function will be later found via
> +    kallsyms at runtime. The new function is defined in the same
> +    source file.
> +
> +  + struct klp_object defines an array of patched functions (struct
> +    klp_patch) in the same object. Where object is either vmlinux (NULL)

s/klp_patch/klp_func/

> +    or a module name. It helps to group and handle functions for each
> +    object together. Note that patched modules might be loaded later
> +    then 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). It allows to handle all patched functions consistently
> +    and synchronously. The whole patch is applied only when all available
> +    symbols can be patched.

This last sentence confuses me.  What does "all available symbols" mean?
Does it imply a more complex consistency model?

> 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
> +-------------------

Similar to Jessica's comment, maybe change this header to "Livepatch
module handling" to distinguish livepatch modules from patched modules.

> +
> +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.
> +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:
> +
> +  + The modules with LivePatches could not be removed without forcing
> +    at the moment.
> +
> +    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.
> +
> +
> +  + 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.
> +
> +
> +  + 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]
> -- 
> 1.8.5.6
> 

-- 
Josh