The object-lifetime debugging infrastructure

Author:Thomas Gleixner

Introduction

debugobjects is a generic infrastructure to track the life time of kernel objects and validate the operations on those.

debugobjects is useful to check for the following error patterns:

  • Activation of uninitialized objects
  • Initialization of active objects
  • Usage of freed/destroyed objects

debugobjects is not changing the data structure of the real object so it can be compiled in with a minimal runtime impact and enabled on demand with a kernel command line option.

Howto use debugobjects

A kernel subsystem needs to provide a data structure which describes the object type and add calls into the debug code at appropriate places. The data structure to describe the object type needs at minimum the name of the object type. Optional functions can and should be provided to fixup detected problems so the kernel can continue to work and the debug information can be retrieved from a live system instead of hard core debugging with serial consoles and stack trace transcripts from the monitor.

The debug calls provided by debugobjects are:

  • debug_object_init
  • debug_object_init_on_stack
  • debug_object_activate
  • debug_object_deactivate
  • debug_object_destroy
  • debug_object_free
  • debug_object_assert_init

Each of these functions takes the address of the real object and a pointer to the object type specific debug description structure.

Each detected error is reported in the statistics and a limited number of errors are printk’ed including a full stack trace.

The statistics are available via /sys/kernel/debug/debug_objects/stats. They provide information about the number of warnings and the number of successful fixups along with information about the usage of the internal tracking objects and the state of the internal tracking objects pool.

Debug functions

void debug_object_init(void *addr, const struct debug_obj_descr *descr)

debug checks when an object is initialized

Parameters

void *addr
address of the object
const struct debug_obj_descr *descr
pointer to an object specific debug description structure

This function is called whenever the initialization function of a real object is called.

When the real object is already tracked by debugobjects it is checked, whether the object can be initialized. Initializing is not allowed for active and destroyed objects. When debugobjects detects an error, then it calls the fixup_init function of the object type description structure if provided by the caller. The fixup function can correct the problem before the real initialization of the object happens. E.g. it can deactivate an active object in order to prevent damage to the subsystem.

When the real object is not yet tracked by debugobjects, debugobjects allocates a tracker object for the real object and sets the tracker object state to ODEBUG_STATE_INIT. It verifies that the object is not on the callers stack. If it is on the callers stack then a limited number of warnings including a full stack trace is printk’ed. The calling code must use debug_object_init_on_stack() and remove the object before leaving the function which allocated it. See next section.

void debug_object_init_on_stack(void *addr, const struct debug_obj_descr *descr)

debug checks when an object on stack is initialized

Parameters

void *addr
address of the object
const struct debug_obj_descr *descr
pointer to an object specific debug description structure

This function is called whenever the initialization function of a real object which resides on the stack is called.

When the real object is already tracked by debugobjects it is checked, whether the object can be initialized. Initializing is not allowed for active and destroyed objects. When debugobjects detects an error, then it calls the fixup_init function of the object type description structure if provided by the caller. The fixup function can correct the problem before the real initialization of the object happens. E.g. it can deactivate an active object in order to prevent damage to the subsystem.

When the real object is not yet tracked by debugobjects debugobjects allocates a tracker object for the real object and sets the tracker object state to ODEBUG_STATE_INIT. It verifies that the object is on the callers stack.

An object which is on the stack must be removed from the tracker by calling debug_object_free() before the function which allocates the object returns. Otherwise we keep track of stale objects.

int debug_object_activate(void *addr, const struct debug_obj_descr *descr)

debug checks when an object is activated

Parameters

void *addr
address of the object
const struct debug_obj_descr *descr
pointer to an object specific debug description structure Returns 0 for success, -EINVAL for check failed.

This function is called whenever the activation function of a real object is called.

When the real object is already tracked by debugobjects it is checked, whether the object can be activated. Activating is not allowed for active and destroyed objects. When debugobjects detects an error, then it calls the fixup_activate function of the object type description structure if provided by the caller. The fixup function can correct the problem before the real activation of the object happens. E.g. it can deactivate an active object in order to prevent damage to the subsystem.

When the real object is not yet tracked by debugobjects then the fixup_activate function is called if available. This is necessary to allow the legitimate activation of statically allocated and initialized objects. The fixup function checks whether the object is valid and calls the debug_objects_init() function to initialize the tracking of this object.

When the activation is legitimate, then the state of the associated tracker object is set to ODEBUG_STATE_ACTIVE.

void debug_object_deactivate(void *addr, const struct debug_obj_descr *descr)

debug checks when an object is deactivated

Parameters

void *addr
address of the object
const struct debug_obj_descr *descr
pointer to an object specific debug description structure

This function is called whenever the deactivation function of a real object is called.

When the real object is tracked by debugobjects it is checked, whether the object can be deactivated. Deactivating is not allowed for untracked or destroyed objects.

When the deactivation is legitimate, then the state of the associated tracker object is set to ODEBUG_STATE_INACTIVE.

void debug_object_destroy(void *addr, const struct debug_obj_descr *descr)

debug checks when an object is destroyed

Parameters

void *addr
address of the object
const struct debug_obj_descr *descr
pointer to an object specific debug description structure

This function is called to mark an object destroyed. This is useful to prevent the usage of invalid objects, which are still available in memory: either statically allocated objects or objects which are freed later.

When the real object is tracked by debugobjects it is checked, whether the object can be destroyed. Destruction is not allowed for active and destroyed objects. When debugobjects detects an error, then it calls the fixup_destroy function of the object type description structure if provided by the caller. The fixup function can correct the problem before the real destruction of the object happens. E.g. it can deactivate an active object in order to prevent damage to the subsystem.

When the destruction is legitimate, then the state of the associated tracker object is set to ODEBUG_STATE_DESTROYED.

void debug_object_free(void *addr, const struct debug_obj_descr *descr)

debug checks when an object is freed

Parameters

void *addr
address of the object
const struct debug_obj_descr *descr
pointer to an object specific debug description structure

This function is called before an object is freed.

When the real object is tracked by debugobjects it is checked, whether the object can be freed. Free is not allowed for active objects. When debugobjects detects an error, then it calls the fixup_free function of the object type description structure if provided by the caller. The fixup function can correct the problem before the real free of the object happens. E.g. it can deactivate an active object in order to prevent damage to the subsystem.

Note that debug_object_free removes the object from the tracker. Later usage of the object is detected by the other debug checks.

void debug_object_assert_init(void *addr, const struct debug_obj_descr *descr)

debug checks when object should be init-ed

Parameters

void *addr
address of the object
const struct debug_obj_descr *descr
pointer to an object specific debug description structure

This function is called to assert that an object has been initialized.

When the real object is not tracked by debugobjects, it calls fixup_assert_init of the object type description structure provided by the caller, with the hardcoded object state ODEBUG_NOT_AVAILABLE. The fixup function can correct the problem by calling debug_object_init and other specific initializing functions.

When the real object is already tracked by debugobjects it is ignored.

Fixup functions

Debug object type description structure

struct debug_obj

representation of an tracked object

Definition

struct debug_obj {
  struct hlist_node       node;
  enum debug_obj_state    state;
  unsigned int            astate;
  void *object;
  const struct debug_obj_descr *descr;
};

Members

node
hlist node to link the object into the tracker list
state
tracked object state
astate
current active state
object
pointer to the real object
descr
pointer to an object type specific debug description structure
struct debug_obj_descr

object type specific debug description structure

Definition

struct debug_obj_descr {
  const char              *name;
  void *(*debug_hint)(void *addr);
  bool (*is_static_object)(void *addr);
  bool (*fixup_init)(void *addr, enum debug_obj_state state);
  bool (*fixup_activate)(void *addr, enum debug_obj_state state);
  bool (*fixup_destroy)(void *addr, enum debug_obj_state state);
  bool (*fixup_free)(void *addr, enum debug_obj_state state);
  bool (*fixup_assert_init)(void *addr, enum debug_obj_state state);
};

Members

name
name of the object typee
debug_hint
function returning address, which have associated kernel symbol, to allow identify the object
is_static_object
return true if the obj is static, otherwise return false
fixup_init
fixup function, which is called when the init check fails. All fixup functions must return true if fixup was successful, otherwise return false
fixup_activate
fixup function, which is called when the activate check fails
fixup_destroy
fixup function, which is called when the destroy check fails
fixup_free
fixup function, which is called when the free check fails
fixup_assert_init
fixup function, which is called when the assert_init check fails

fixup_init

This function is called from the debug code whenever a problem in debug_object_init is detected. The function takes the address of the object and the state which is currently recorded in the tracker.

Called from debug_object_init when the object state is:

  • ODEBUG_STATE_ACTIVE

The function returns true when the fixup was successful, otherwise false. The return value is used to update the statistics.

Note, that the function needs to call the debug_object_init() function again, after the damage has been repaired in order to keep the state consistent.

fixup_activate

This function is called from the debug code whenever a problem in debug_object_activate is detected.

Called from debug_object_activate when the object state is:

  • ODEBUG_STATE_NOTAVAILABLE
  • ODEBUG_STATE_ACTIVE

The function returns true when the fixup was successful, otherwise false. The return value is used to update the statistics.

Note that the function needs to call the debug_object_activate() function again after the damage has been repaired in order to keep the state consistent.

The activation of statically initialized objects is a special case. When debug_object_activate() has no tracked object for this object address then fixup_activate() is called with object state ODEBUG_STATE_NOTAVAILABLE. The fixup function needs to check whether this is a legitimate case of a statically initialized object or not. In case it is it calls debug_object_init() and debug_object_activate() to make the object known to the tracker and marked active. In this case the function should return false because this is not a real fixup.

fixup_destroy

This function is called from the debug code whenever a problem in debug_object_destroy is detected.

Called from debug_object_destroy when the object state is:

  • ODEBUG_STATE_ACTIVE

The function returns true when the fixup was successful, otherwise false. The return value is used to update the statistics.

fixup_free

This function is called from the debug code whenever a problem in debug_object_free is detected. Further it can be called from the debug checks in kfree/vfree, when an active object is detected from the debug_check_no_obj_freed() sanity checks.

Called from debug_object_free() or debug_check_no_obj_freed() when the object state is:

  • ODEBUG_STATE_ACTIVE

The function returns true when the fixup was successful, otherwise false. The return value is used to update the statistics.

fixup_assert_init

This function is called from the debug code whenever a problem in debug_object_assert_init is detected.

Called from debug_object_assert_init() with a hardcoded state ODEBUG_STATE_NOTAVAILABLE when the object is not found in the debug bucket.

The function returns true when the fixup was successful, otherwise false. The return value is used to update the statistics.

Note, this function should make sure debug_object_init() is called before returning.

The handling of statically initialized objects is a special case. The fixup function should check if this is a legitimate case of a statically initialized object or not. In this case only debug_object_init() should be called to make the object known to the tracker. Then the function should return false because this is not a real fixup.

Known Bugs And Assumptions

None (knock on wood).