Introduction¶

debugobjects is a generic infrastructure to track the life time ofkernel 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 itcan be compiled in with a minimal runtime impact and enabled on demandwith a kernel command line option.

Howto use debugobjects¶

A kernel subsystem needs to provide a data structure which describes theobject type and add calls into the debug code at appropriate places. Thedata structure to describe the object type needs at minimum the name ofthe object type. Optional functions can and should be provided to fixupdetected problems so the kernel can continue to work and the debuginformation can be retrieved from a live system instead of hard coredebugging with serial consoles and stack trace transcripts from themonitor.

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 apointer to the object type specific debug description structure.

Each detected error is reported in the statistics and a limited numberof 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 ofsuccessful fixups along with information about the usage of the internaltracking objects and the state of the internal tracking objects pool.

Debug functions¶

voiddebug_object_init(void*addr,conststructdebug_obj_descr*descr)¶

debug checks when an object is initialized

This function is called whenever the initialization function of a realobject is called.

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

When the real object is not yet tracked by debugobjects, debugobjectsallocates a tracker object for the real object and sets the trackerobject state to ODEBUG_STATE_INIT. It verifies that the object is noton the callers stack. If it is on the callers stack then a limitednumber of warnings including a full stack trace is printk’ed. Thecalling code must usedebug_object_init_on_stack() and remove theobject before leaving the function which allocated it. See next section.

voiddebug_object_init_on_stack(void*addr,conststructdebug_obj_descr*descr)¶

debug checks when an object on stack is initialized

This function is called whenever the initialization function of a realobject 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 foractive and destroyed objects. When debugobjects detects an error, thenit calls the fixup_init function of the object type descriptionstructure if provided by the caller. The fixup function can correct theproblem before the real initialization of the object happens. E.g. itcan deactivate an active object in order to prevent damage to thesubsystem.

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

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

intdebug_object_activate(void*addr,conststructdebug_obj_descr*descr)¶

debug checks when an object is activated

This function is called whenever the activation function of a realobject is called.

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

When the real object is not yet tracked by debugobjects then thefixup_activate function is called if available. This is necessary toallow the legitimate activation of statically allocated and initializedobjects. The fixup function checks whether the object is valid and callsthedebug_objects_init() function to initialize the tracking of thisobject.

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

voiddebug_object_deactivate(void*addr,conststructdebug_obj_descr*descr)¶

debug checks when an object is deactivated

This function is called whenever the deactivation function of a realobject is called.

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

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

voiddebug_object_destroy(void*addr,conststructdebug_obj_descr*descr)¶

debug checks when an object is destroyed

This function is called to mark an object destroyed. This is useful toprevent the usage of invalid objects, which are still available inmemory: either statically allocated objects or objects which are freedlater.

When the real object is tracked by debugobjects it is checked, whetherthe object can be destroyed. Destruction is not allowed for active anddestroyed objects. When debugobjects detects an error, then it calls thefixup_destroy function of the object type description structure ifprovided by the caller. The fixup function can correct the problembefore the real destruction of the object happens. E.g. it candeactivate an active object in order to prevent damage to the subsystem.

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

voiddebug_object_free(void*addr,conststructdebug_obj_descr*descr)¶

debug checks when an object is freed

This function is called before an object is freed.

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

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

voiddebug_object_assert_init(void*addr,conststructdebug_obj_descr*descr)¶

debug checks when object should be init-ed

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

When the real object is not tracked by debugobjects, it callsfixup_assert_init of the object type description structure provided bythe caller, with the hardcoded object state ODEBUG_NOT_AVAILABLE. Thefixup function can correct the problem by calling debug_object_initand other specific initializing functions.

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

Fixup functions¶

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

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);};

Known Bugs And Assumptions¶

None (knock on wood).