Livepatching APIs

Livepatch Enablement

int klp_enable_patch(struct klp_patch *patch)

enable the livepatch

Parameters

struct klp_patch *patch
patch to be enabled

Description

Initializes the data structure associated with the patch, creates the sysfs interface, performs the needed symbol lookups and code relocations, registers the patched functions with ftrace.

This function is supposed to be called from the livepatch module_init() callback.

Return

0 on success, otherwise error

Shadow Variables

void * klp_shadow_get(void *obj, unsigned long id)

retrieve a shadow variable data pointer

Parameters

void *obj
pointer to parent object
unsigned long id
data identifier

Return

the shadow variable data element, NULL on failure.

void * klp_shadow_alloc(void *obj, unsigned long id, size_t size, gfp_t gfp_flags, klp_shadow_ctor_t ctor, void *ctor_data)

allocate and add a new shadow variable

Parameters

void *obj
pointer to parent object
unsigned long id
data identifier
size_t size
size of attached data
gfp_t gfp_flags
GFP mask for allocation
klp_shadow_ctor_t ctor
custom constructor to initialize the shadow data (optional)
void *ctor_data
pointer to any data needed by ctor (optional)

Description

Allocates size bytes for new shadow variable data using gfp_flags. The data are zeroed by default. They are further initialized by ctor function if it is not NULL. The new shadow variable is then added to the global hashtable.

If an existing <obj, id> shadow variable can be found, this routine will issue a WARN, exit early and return NULL.

This function guarantees that the constructor function is called only when the variable did not exist before. The cost is that ctor is called in atomic context under a spin lock.

Return

the shadow variable data element, NULL on duplicate or failure.

void * klp_shadow_get_or_alloc(void *obj, unsigned long id, size_t size, gfp_t gfp_flags, klp_shadow_ctor_t ctor, void *ctor_data)

get existing or allocate a new shadow variable

Parameters

void *obj
pointer to parent object
unsigned long id
data identifier
size_t size
size of attached data
gfp_t gfp_flags
GFP mask for allocation
klp_shadow_ctor_t ctor
custom constructor to initialize the shadow data (optional)
void *ctor_data
pointer to any data needed by ctor (optional)

Description

Returns a pointer to existing shadow data if an <obj, id> shadow variable is already present. Otherwise, it creates a new shadow variable like klp_shadow_alloc().

This function guarantees that only one shadow variable exists with the given id for the given obj. It also guarantees that the constructor function will be called only when the variable did not exist before. The cost is that ctor is called in atomic context under a spin lock.

Return

the shadow variable data element, NULL on failure.

void klp_shadow_free(void *obj, unsigned long id, klp_shadow_dtor_t dtor)

detach and free a <obj, id> shadow variable

Parameters

void *obj
pointer to parent object
unsigned long id
data identifier
klp_shadow_dtor_t dtor
custom callback that can be used to unregister the variable and/or free data that the shadow variable points to (optional)

Description

This function releases the memory for this <obj, id> shadow variable instance, callers should stop referencing it accordingly.

void klp_shadow_free_all(unsigned long id, klp_shadow_dtor_t dtor)

detach and free all <_, id> shadow variables

Parameters

unsigned long id
data identifier
klp_shadow_dtor_t dtor
custom callback that can be used to unregister the variable and/or free data that the shadow variable points to (optional)

Description

This function releases the memory for all <_, id> shadow variable instances, callers should stop referencing them accordingly.

System State Changes

struct klp_state * klp_get_state(struct klp_patch *patch, unsigned long id)

get information about system state modified by the given patch

Parameters

struct klp_patch *patch
livepatch that modifies the given system state
unsigned long id
custom identifier of the modified system state

Description

Checks whether the given patch modifies the given system state.

The function can be called either from pre/post (un)patch callbacks or from the kernel code added by the livepatch.

Return

pointer to struct klp_state when found, otherwise NULL.

struct klp_state * klp_get_prev_state(unsigned long id)

get information about system state modified by the already installed livepatches

Parameters

unsigned long id
custom identifier of the modified system state

Description

Checks whether already installed livepatches modify the given system state.

The same system state can be modified by more non-cumulative livepatches. It is expected that the latest livepatch has the most up-to-date information.

The function can be called only during transition when a new livepatch is being enabled or when such a transition is reverted. It is typically called only from pre/post (un)patch callbacks.

Return

pointer to the latest struct klp_state from already
installed livepatches, NULL when not found.

Object Types

struct klp_func

function structure for live patching

Definition

struct klp_func {
  const char *old_name;
  void *new_func;
  unsigned long old_sympos;
  void *old_func;
  struct kobject kobj;
  struct list_head node;
  struct list_head stack_node;
  unsigned long old_size, new_size;
  bool nop;
  bool patched;
  bool transition;
};

Members

old_name
name of the function to be patched
new_func
pointer to the patched function code
old_sympos
a hint indicating which symbol position the old function can be found (optional)
old_func
pointer to the function being patched
kobj
kobject for sysfs resources
node
list node for klp_object func_list
stack_node
list node for klp_ops func_stack list
old_size
size of the old function
new_size
size of the new function
nop
temporary patch to use the original code again; dyn. allocated
patched
the func has been added to the klp_ops list
transition
the func is currently being applied or reverted

Description

The patched and transition variables define the func’s patching state. When patching, a func is always in one of the following states:

patched=0 transition=0: unpatched patched=0 transition=1: unpatched, temporary starting state patched=1 transition=1: patched, may be visible to some tasks patched=1 transition=0: patched, visible to all tasks

And when unpatching, it goes in the reverse order:

patched=1 transition=0: patched, visible to all tasks patched=1 transition=1: patched, may be visible to some tasks patched=0 transition=1: unpatched, temporary ending state patched=0 transition=0: unpatched
struct klp_callbacks

pre/post live-(un)patch callback structure

Definition

struct klp_callbacks {
  int (*pre_patch)(struct klp_object *obj);
  void (*post_patch)(struct klp_object *obj);
  void (*pre_unpatch)(struct klp_object *obj);
  void (*post_unpatch)(struct klp_object *obj);
  bool post_unpatch_enabled;
};

Members

pre_patch
executed before code patching
post_patch
executed after code patching
pre_unpatch
executed before code unpatching
post_unpatch
executed after code unpatching
post_unpatch_enabled
flag indicating if post-unpatch callback should run

Description

All callbacks are optional. Only the pre-patch callback, if provided, will be unconditionally executed. If the parent klp_object fails to patch for any reason, including a non-zero error status returned from the pre-patch callback, no further callbacks will be executed.

struct klp_object

kernel object structure for live patching

Definition

struct klp_object {
  const char *name;
  struct klp_func *funcs;
  struct klp_callbacks callbacks;
  struct kobject kobj;
  struct list_head func_list;
  struct list_head node;
  struct module *mod;
  bool dynamic;
  bool patched;
};

Members

name
module name (or NULL for vmlinux)
funcs
function entries for functions to be patched in the object
callbacks
functions to be executed pre/post (un)patching
kobj
kobject for sysfs resources
func_list
dynamic list of the function entries
node
list node for klp_patch obj_list
mod
kernel module associated with the patched object (NULL for vmlinux)
dynamic
temporary object for nop functions; dynamically allocated
patched
the object’s funcs have been added to the klp_ops list
struct klp_state

state of the system modified by the livepatch

Definition

struct klp_state {
  unsigned long id;
  unsigned int version;
  void *data;
};

Members

id
system state identifier (non-zero)
version
version of the change
data
custom data
struct klp_patch

patch structure for live patching

Definition

struct klp_patch {
  struct module *mod;
  struct klp_object *objs;
  struct klp_state *states;
  bool replace;
  struct list_head list;
  struct kobject kobj;
  struct list_head obj_list;
  bool enabled;
  bool forced;
  struct work_struct free_work;
  struct completion finish;
};

Members

mod
reference to the live patch module
objs
object entries for kernel objects to be patched
states
system states that can get modified
replace
replace all actively used patches
list
list node for global list of actively used patches
kobj
kobject for sysfs resources
obj_list
dynamic list of the object entries
enabled
the patch is enabled (but operation may be incomplete)
forced
was involved in a forced transition
free_work
patch cleanup from workqueue-context
finish
for waiting till it is safe to remove the patch module