311 lines
11 KiB
ReStructuredText
311 lines
11 KiB
ReStructuredText
============================================
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The object-lifetime debugging infrastructure
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============================================
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:Author: Thomas Gleixner
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Introduction
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============
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debugobjects is a generic infrastructure to track the life time of
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kernel objects and validate the operations on those.
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debugobjects is useful to check for the following error patterns:
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- Activation of uninitialized objects
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- Initialization of active objects
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- Usage of freed/destroyed objects
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debugobjects is not changing the data structure of the real object so it
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can be compiled in with a minimal runtime impact and enabled on demand
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with a kernel command line option.
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Howto use debugobjects
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======================
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A kernel subsystem needs to provide a data structure which describes the
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object type and add calls into the debug code at appropriate places. The
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data structure to describe the object type needs at minimum the name of
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the object type. Optional functions can and should be provided to fixup
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detected problems so the kernel can continue to work and the debug
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information can be retrieved from a live system instead of hard core
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debugging with serial consoles and stack trace transcripts from the
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monitor.
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The debug calls provided by debugobjects are:
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- debug_object_init
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- debug_object_init_on_stack
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- debug_object_activate
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- debug_object_deactivate
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- debug_object_destroy
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- debug_object_free
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- debug_object_assert_init
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Each of these functions takes the address of the real object and a
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pointer to the object type specific debug description structure.
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Each detected error is reported in the statistics and a limited number
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of errors are printk'ed including a full stack trace.
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The statistics are available via /sys/kernel/debug/debug_objects/stats.
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They provide information about the number of warnings and the number of
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successful fixups along with information about the usage of the internal
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tracking objects and the state of the internal tracking objects pool.
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Debug functions
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===============
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.. kernel-doc:: lib/debugobjects.c
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:functions: debug_object_init
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This function is called whenever the initialization function of a real
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object is called.
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When the real object is already tracked by debugobjects it is checked,
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whether the object can be initialized. Initializing is not allowed for
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active and destroyed objects. When debugobjects detects an error, then
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it calls the fixup_init function of the object type description
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structure if provided by the caller. The fixup function can correct the
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problem before the real initialization of the object happens. E.g. it
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can deactivate an active object in order to prevent damage to the
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subsystem.
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When the real object is not yet tracked by debugobjects, debugobjects
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allocates a tracker object for the real object and sets the tracker
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object state to ODEBUG_STATE_INIT. It verifies that the object is not
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on the callers stack. If it is on the callers stack then a limited
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number of warnings including a full stack trace is printk'ed. The
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calling code must use debug_object_init_on_stack() and remove the
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object before leaving the function which allocated it. See next section.
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.. kernel-doc:: lib/debugobjects.c
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:functions: debug_object_init_on_stack
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This function is called whenever the initialization function of a real
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object which resides on the stack is called.
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When the real object is already tracked by debugobjects it is checked,
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whether the object can be initialized. Initializing is not allowed for
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active and destroyed objects. When debugobjects detects an error, then
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it calls the fixup_init function of the object type description
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structure if provided by the caller. The fixup function can correct the
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problem before the real initialization of the object happens. E.g. it
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can deactivate an active object in order to prevent damage to the
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subsystem.
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When the real object is not yet tracked by debugobjects debugobjects
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allocates a tracker object for the real object and sets the tracker
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object state to ODEBUG_STATE_INIT. It verifies that the object is on
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the callers stack.
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An object which is on the stack must be removed from the tracker by
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calling debug_object_free() before the function which allocates the
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object returns. Otherwise we keep track of stale objects.
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.. kernel-doc:: lib/debugobjects.c
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:functions: debug_object_activate
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This function is called whenever the activation function of a real
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object is called.
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When the real object is already tracked by debugobjects it is checked,
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whether the object can be activated. Activating is not allowed for
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active and destroyed objects. When debugobjects detects an error, then
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it calls the fixup_activate function of the object type description
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structure if provided by the caller. The fixup function can correct the
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problem before the real activation of the object happens. E.g. it can
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deactivate an active object in order to prevent damage to the subsystem.
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When the real object is not yet tracked by debugobjects then the
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fixup_activate function is called if available. This is necessary to
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allow the legitimate activation of statically allocated and initialized
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objects. The fixup function checks whether the object is valid and calls
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the debug_objects_init() function to initialize the tracking of this
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object.
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When the activation is legitimate, then the state of the associated
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tracker object is set to ODEBUG_STATE_ACTIVE.
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.. kernel-doc:: lib/debugobjects.c
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:functions: debug_object_deactivate
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This function is called whenever the deactivation function of a real
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object is called.
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When the real object is tracked by debugobjects it is checked, whether
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the object can be deactivated. Deactivating is not allowed for untracked
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or destroyed objects.
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When the deactivation is legitimate, then the state of the associated
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tracker object is set to ODEBUG_STATE_INACTIVE.
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.. kernel-doc:: lib/debugobjects.c
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:functions: debug_object_destroy
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This function is called to mark an object destroyed. This is useful to
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prevent the usage of invalid objects, which are still available in
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memory: either statically allocated objects or objects which are freed
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later.
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When the real object is tracked by debugobjects it is checked, whether
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the object can be destroyed. Destruction is not allowed for active and
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destroyed objects. When debugobjects detects an error, then it calls the
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fixup_destroy function of the object type description structure if
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provided by the caller. The fixup function can correct the problem
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before the real destruction of the object happens. E.g. it can
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deactivate an active object in order to prevent damage to the subsystem.
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When the destruction is legitimate, then the state of the associated
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tracker object is set to ODEBUG_STATE_DESTROYED.
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.. kernel-doc:: lib/debugobjects.c
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:functions: debug_object_free
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This function is called before an object is freed.
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When the real object is tracked by debugobjects it is checked, whether
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the object can be freed. Free is not allowed for active objects. When
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debugobjects detects an error, then it calls the fixup_free function of
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the object type description structure if provided by the caller. The
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fixup function can correct the problem before the real free of the
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object happens. E.g. it can deactivate an active object in order to
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prevent damage to the subsystem.
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Note that debug_object_free removes the object from the tracker. Later
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usage of the object is detected by the other debug checks.
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.. kernel-doc:: lib/debugobjects.c
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:functions: debug_object_assert_init
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This function is called to assert that an object has been initialized.
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When the real object is not tracked by debugobjects, it calls
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fixup_assert_init of the object type description structure provided by
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the caller, with the hardcoded object state ODEBUG_NOT_AVAILABLE. The
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fixup function can correct the problem by calling debug_object_init
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and other specific initializing functions.
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When the real object is already tracked by debugobjects it is ignored.
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Fixup functions
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===============
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Debug object type description structure
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---------------------------------------
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.. kernel-doc:: include/linux/debugobjects.h
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:internal:
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fixup_init
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-----------
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This function is called from the debug code whenever a problem in
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debug_object_init is detected. The function takes the address of the
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object and the state which is currently recorded in the tracker.
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Called from debug_object_init when the object state is:
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- ODEBUG_STATE_ACTIVE
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The function returns true when the fixup was successful, otherwise
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false. The return value is used to update the statistics.
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Note, that the function needs to call the debug_object_init() function
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again, after the damage has been repaired in order to keep the state
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consistent.
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fixup_activate
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---------------
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This function is called from the debug code whenever a problem in
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debug_object_activate is detected.
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Called from debug_object_activate when the object state is:
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- ODEBUG_STATE_NOTAVAILABLE
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- ODEBUG_STATE_ACTIVE
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The function returns true when the fixup was successful, otherwise
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false. The return value is used to update the statistics.
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Note that the function needs to call the debug_object_activate()
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function again after the damage has been repaired in order to keep the
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state consistent.
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The activation of statically initialized objects is a special case. When
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debug_object_activate() has no tracked object for this object address
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then fixup_activate() is called with object state
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ODEBUG_STATE_NOTAVAILABLE. The fixup function needs to check whether
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this is a legitimate case of a statically initialized object or not. In
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case it is it calls debug_object_init() and debug_object_activate()
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to make the object known to the tracker and marked active. In this case
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the function should return false because this is not a real fixup.
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fixup_destroy
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--------------
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This function is called from the debug code whenever a problem in
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debug_object_destroy is detected.
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Called from debug_object_destroy when the object state is:
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- ODEBUG_STATE_ACTIVE
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The function returns true when the fixup was successful, otherwise
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false. The return value is used to update the statistics.
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fixup_free
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-----------
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This function is called from the debug code whenever a problem in
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debug_object_free is detected. Further it can be called from the debug
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checks in kfree/vfree, when an active object is detected from the
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debug_check_no_obj_freed() sanity checks.
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Called from debug_object_free() or debug_check_no_obj_freed() when
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the object state is:
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- ODEBUG_STATE_ACTIVE
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The function returns true when the fixup was successful, otherwise
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false. The return value is used to update the statistics.
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fixup_assert_init
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-------------------
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This function is called from the debug code whenever a problem in
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debug_object_assert_init is detected.
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Called from debug_object_assert_init() with a hardcoded state
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ODEBUG_STATE_NOTAVAILABLE when the object is not found in the debug
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bucket.
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The function returns true when the fixup was successful, otherwise
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false. The return value is used to update the statistics.
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Note, this function should make sure debug_object_init() is called
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before returning.
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The handling of statically initialized objects is a special case. The
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fixup function should check if this is a legitimate case of a statically
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initialized object or not. In this case only debug_object_init()
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should be called to make the object known to the tracker. Then the
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function should return false because this is not a real fixup.
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Known Bugs And Assumptions
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==========================
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None (knock on wood).
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