Linux/Documentations: Kernel Livepatching
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Linux/Document: Livepatch_RToax-CSDN博客Livepatch — The Linux Kernel documentationhttps://www.kernel.org/doc/html/latest/livepatch/livepatch.htmlThis document outlines basic information about kernel livepatching.livepatch限制：只有可以跟踪的功能才能打补丁。只有当动态 ftrace 位于函数的最开始时，Livepatch 才能可靠地工作。
https://rtoax.blog.csdn.net/article/details/120393743
Livepatch — The Linux Kernel documentationLivepatching_RToax-CSDN博客Live patching - Gentoo Wikihttps://wiki.gentoo.org/wiki/Live_patchingKernel live patching is an 'update-and-coming' kernel feature being developed by a few corporate Linux companies. Several companies have open sourced their development efforts, making ihttps://rtoax.blog.csdn.net/article/details/120393665Kernel Livepatching — The Linux Kernel documentation
https://www.kernel.org/doc/html/v5.15-rc2/livepatch/index.html
目录
2. Kprobes, Ftrace, Livepatching
3.1 Adding consistency model support to new architectures
__init and probe function patches support
Atomic Replace & Cumulative Patches
Why does livepatch need to write its own relocations?
3. Livepatch relocation sections
3.1 Livepatch relocation section format
4.1 A livepatch module’s symbol table
5. Symbol table and Elf section access
4.1 Identifying successful termination
4.2 Identifying unwindable code
4.3 Unwinding across interrupts and exceptions
4.4 Rewriting of return addresses
4.5 Obscuring of return addresses
4.6 Link register unreliability
Kernel Livepatching
- Livepatch
- (Un)patching Callbacks
- Atomic Replace & Cumulative Patches
- Livepatch module Elf format
- Shadow Variables
- System State Changes
- Reliable Stacktrace
Livepatch
This document outlines basic information about kernel livepatching.
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:
-
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).
-
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:
-
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.
-
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.
-
-
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:
-
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.
-
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:
-
patching user tasks when they cross the kernel/user space boundary; and
-
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 Livepatch module Elf format 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 registered1. 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 Atomic Replace & Cumulative Patches 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.
(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.
1. Motivation
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.
2. Callback types
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
3. How it works
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.
4. Use cases
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.
Global data update
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.
__init and probe function patches support
Although __init and probe functions are not directly livepatch-able, it may be possible to implement similar updates via pre/post-patch callbacks.
The commit 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.)
Atomic Replace & Cumulative Patches
There might be dependencies between livepatches. If multiple patches need to do different changes to the same function(s) then we need to define an order in which the patches will be installed. And function implementations from any newer livepatch must be done on top of the older ones.
This might become a maintenance nightmare. Especially when more patches modified the same function in different ways.
An elegant solution comes with the feature called “Atomic Replace”. It allows creation of so called “Cumulative Patches”. They include all wanted changes from all older livepatches and completely replace them in one transition.
Usage
The atomic replace can be enabled by setting “replace” flag in struct klp_patch, for example:
static struct klp_patch patch = {
.mod = THIS_MODULE,
.objs = objs,
.replace = true,
};
All processes are then migrated to use the code only from the new patch. Once the transition is finished, all older patches are automatically disabled.
Ftrace handlers are transparently removed from functions that are no longer modified by the new cumulative patch.
As a result, the livepatch authors might maintain sources only for one cumulative patch. It helps to keep the patch consistent while adding or removing various fixes or features.
Users could keep only the last patch installed on the system after the transition to has finished. It helps to clearly see what code is actually in use. Also the livepatch might then be seen as a “normal” module that modifies the kernel behavior. The only difference is that it can be updated at runtime without breaking its functionality.
Features
The atomic replace allows:
Atomically revert some functions in a previous patch while upgrading other functions.
Remove eventual performance impact caused by core redirection for functions that are no longer patched.
Decrease user confusion about dependencies between livepatches.
Limitations:
Once the operation finishes, there is no straightforward way to reverse it and restore the replaced patches atomically.
A good practice is to set .replace flag in any released livepatch. Then re-adding an older livepatch is equivalent to downgrading to that patch. This is safe as long as the livepatches do _not_ do extra modifications in (un)patching callbacks or in the module_init() or module_exit() functions, see below.
Also note that the replaced patch can be removed and loaded again only when the transition was not forced.
Only the (un)patching callbacks from the _new_ cumulative livepatch are executed. Any callbacks from the replaced patches are ignored.
In other words, the cumulative patch is responsible for doing any actions that are necessary to properly replace any older patch.
As a result, it might be dangerous to replace newer cumulative patches by older ones. The old livepatches might not provide the necessary callbacks.
This might be seen as a limitation in some scenarios. But it makes life easier in many others. Only the new cumulative livepatch knows what fixes/features are added/removed and what special actions are necessary for a smooth transition.
In any case, it would be a nightmare to think about the order of the various callbacks and their interactions if the callbacks from all enabled patches were called.
There is no special handling of shadow variables. Livepatch authors must create their own rules how to pass them from one cumulative patch to the other. Especially that they should not blindly remove them in module_exit() functions.
A good practice might be to remove shadow variables in the post-unpatch callback. It is called only when the livepatch is properly disabled.
Livepatch module Elf format
This document outlines the Elf format requirements that livepatch modules must follow.
1. Background and motivation
Formerly, livepatch required separate architecture-specific code to write relocations. However, arch-specific code to write relocations already exists in the module loader, so this former approach produced redundant code. So, instead of duplicating code and re-implementing what the module loader can already do, livepatch leverages existing code in the module loader to perform the all the arch-specific relocation work. Specifically, livepatch reuses the apply_relocate_add() function in the module loader to write relocations. The patch module Elf format described in this document enables livepatch to be able to do this. The hope is that this will make livepatch more easily portable to other architectures and reduce the amount of arch-specific code required to port livepatch to a particular architecture.
Since apply_relocate_add() requires access to a module’s section header table, symbol table, and relocation section indices, Elf information is preserved for livepatch modules (see section 5). Livepatch manages its own relocation sections and symbols, which are described in this document. The Elf constants used to mark livepatch symbols and relocation sections were selected from OS-specific ranges according to the definitions from glibc.
Why does livepatch need to write its own relocations?
A typical livepatch module contains patched versions of functions that can reference non-exported global symbols and non-included local symbols. Relocations referencing these types of symbols cannot be left in as-is since the kernel module loader cannot resolve them and will therefore reject the livepatch module. Furthermore, we cannot apply relocations that affect modules not yet loaded at patch module load time (e.g. a patch to a driver that is not loaded). Formerly, livepatch solved this problem by embedding special “dynrela” (dynamic rela) sections in the resulting patch module Elf output. Using these dynrela sections, livepatch could resolve symbols while taking into account its scope and what module the symbol belongs to, and then manually apply the dynamic relocations. However this approach required livepatch to supply arch-specific code in order to write these relocations. In the new format, livepatch manages its own SHT_RELA relocation sections in place of dynrela sections, and the symbols that the relas reference are special livepatch symbols (see section 2 and 3). The arch-specific livepatch relocation code is replaced by a call to apply_relocate_add().
2. Livepatch modinfo field
Livepatch modules are required to have the “livepatch” modinfo attribute. See the sample livepatch module in samples/livepatch/ for how this is done.
Livepatch modules can be identified by users by using the ‘modinfo’ command and looking for the presence of the “livepatch” field. This field is also used by the kernel module loader to identify livepatch modules.
Example:
Modinfo output:
% modinfo livepatch-meminfo.ko filename: livepatch-meminfo.ko livepatch: Y license: GPL depends: vermagic: 4.3.0+ SMP mod_unload
3. Livepatch relocation sections
A livepatch module manages its own Elf relocation sections to apply relocations to modules as well as to the kernel (vmlinux) at the appropriate time. For example, if a patch module patches a driver that is not currently loaded, livepatch will apply the corresponding livepatch relocation section(s) to the driver once it loads.
Each “object” (e.g. vmlinux, or a module) within a patch module may have multiple livepatch relocation sections associated with it (e.g. patches to multiple functions within the same object). There is a 1-1 correspondence between a livepatch relocation section and the target section (usually the text section of a function) to which the relocation(s) apply. It is also possible for a livepatch module to have no livepatch relocation sections, as in the case of the sample livepatch module (see samples/livepatch).
Since Elf information is preserved for livepatch modules (see Section 5), a livepatch relocation section can be applied simply by passing in the appropriate section index to apply_relocate_add(), which then uses it to access the relocation section and apply the relocations.
Every symbol referenced by a rela in a livepatch relocation section is a livepatch symbol. These must be resolved before livepatch can call apply_relocate_add(). See Section 3 for more information.
3.1 Livepatch relocation section format
Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH section flag. See include/uapi/linux/elf.h for the definition. The module loader recognizes this flag and will avoid applying those relocation sections at patch module load time. These sections must also be marked with SHF_ALLOC, so that the module loader doesn’t discard them on module load (i.e. they will be copied into memory along with the other SHF_ALLOC sections).
The name of a livepatch relocation section must conform to the following format:
.klp.rela.objname.section_name ^ ^^ ^ ^ ^ |________||_____| |__________| [A] [B] [C]
[A]
The relocation section name is prefixed with the string “.klp.rela.”
[B]
The name of the object (i.e. “vmlinux” or name of module) to which the relocation section belongs follows immediately after the prefix.
[C]
The actual name of the section to which this relocation section applies.
Examples:
Livepatch relocation section names:
.klp.rela.ext4.text.ext4_attr_store .klp.rela.vmlinux.text.cmdline_proc_show
`readelf –sections` output for a patch module that patches vmlinux and modules 9p, btrfs, ext4:
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ snip ]
[29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
[30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
[ snip ]
[34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
[35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
[36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
[37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
[ snip ] ^ ^
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Livepatch relocation sections are SHT_RELA sections but with a few special characteristics. Notice that they are marked SHF_ALLOC (“A”) so that they will not be discarded when the module is loaded into memory, as well as with the SHF_RELA_LIVEPATCH flag (“o” - for OS-specific).
`readelf –relocs` output for a patch module:
Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
[ snip ] ^
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Every symbol referenced by a relocation is a livepatch symbol.
4. Livepatch symbols
Livepatch symbols are symbols referred to by livepatch relocation sections. These are symbols accessed from new versions of functions for patched objects, whose addresses cannot be resolved by the module loader (because they are local or unexported global syms). Since the module loader only resolves exported syms, and not every symbol referenced by the new patched functions is exported, livepatch symbols were introduced. They are used also in cases where we cannot immediately know the address of a symbol when a patch module loads. For example, this is the case when livepatch patches a module that is not loaded yet. In this case, the relevant livepatch symbols are resolved simply when the target module loads. In any case, for any livepatch relocation section, all livepatch symbols referenced by that section must be resolved before livepatch can call apply_relocate_add() for that reloc section.
Livepatch symbols must be marked with SHN_LIVEPATCH so that the module loader can identify and ignore them. Livepatch modules keep these symbols in their symbol tables, and the symbol table is made accessible through module->symtab.
4.1 A livepatch module’s symbol table
Normally, a stripped down copy of a module’s symbol table (containing only “core” symbols) is made available through module->symtab (See layout_symtab() in kernel/module.c). For livepatch modules, the symbol table copied into memory on module load must be exactly the same as the symbol table produced when the patch module was compiled. This is because the relocations in each livepatch relocation section refer to their respective symbols with their symbol indices, and the original symbol indices (and thus the symtab ordering) must be preserved in order for apply_relocate_add() to find the right symbol.
For example, take this particular rela from a livepatch module::
Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
symbol index 94.
And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
[ snip ]
94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
[ snip ]
4.2 Livepatch symbol format
Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so that the module loader can identify them and not attempt to resolve them. See include/uapi/linux/elf.h for the actual definitions.
Livepatch symbol names must conform to the following format:
.klp.sym.objname.symbol_name,sympos ^ ^^ ^ ^ ^ ^ |_______||_____| |_________| | [A] [B] [C] [D]
[A]
The symbol name is prefixed with the string “.klp.sym.”
[B]
The name of the object (i.e. “vmlinux” or name of module) to which the symbol belongs follows immediately after the prefix.
[C]
The actual name of the symbol.
[D]
The position of the symbol in the object (as according to kallsyms) This is used to differentiate duplicate symbols within the same object. The symbol position is expressed numerically (0, 1, 2…). The symbol position of a unique symbol is 0.
Examples:
Livepatch symbol names:
.klp.sym.vmlinux.snprintf,0 .klp.sym.vmlinux.printk,0 .klp.sym.btrfs.btrfs_ktype,0
`readelf –symbols` output for a patch module:
Symbol table '.symtab' contains 127 entries:
Num: Value Size Type Bind Vis Ndx Name
[ snip ]
73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
[ snip ] ^
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Note that the ‘Ndx’ (Section index) for these symbols is SHN_LIVEPATCH (0xff20). “OS” means OS-specific.
5. Symbol table and Elf section access
A livepatch module’s symbol table is accessible through module->symtab.
Since apply_relocate_add() require
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