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

The Linux Kernel

Contents

This Page

Livepatching APIs¶

Livepatch Enablement¶

Shadow Variables¶

System State Changes¶

Object Types¶