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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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<book id="debug-objects-guide">
<bookinfo>
<title>Debug objects life time</title>
<authorgroup>
<author>
<firstname>Thomas</firstname>
<surname>Gleixner</surname>
<affiliation>
<address>
<email>tglx@linutronix.de</email>
</address>
</affiliation>
</author>
</authorgroup>
<copyright>
<year>2008</year>
<holder>Thomas Gleixner</holder>
</copyright>
<legalnotice>
<para>
This documentation is free software; you can redistribute
it and/or modify it under the terms of the GNU General Public
License version 2 as published by the Free Software Foundation.
</para>
<para>
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
</para>
<para>
You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA
</para>
<para>
For more details see the file COPYING in the source
distribution of Linux.
</para>
</legalnotice>
</bookinfo>
<toc></toc>
<chapter id="intro">
<title>Introduction</title>
<para>
debugobjects is a generic infrastructure to track the life time
of kernel objects and validate the operations on those.
</para>
<para>
debugobjects is useful to check for the following error patterns:
<itemizedlist>
<listitem><para>Activation of uninitialized objects</para></listitem>
<listitem><para>Initialization of active objects</para></listitem>
<listitem><para>Usage of freed/destroyed objects</para></listitem>
</itemizedlist>
</para>
<para>
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.
</para>
</chapter>
<chapter id="howto">
<title>Howto use debugobjects</title>
<para>
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.
</para>
<para>
The debug calls provided by debugobjects are:
<itemizedlist>
<listitem><para>debug_object_init</para></listitem>
<listitem><para>debug_object_init_on_stack</para></listitem>
<listitem><para>debug_object_activate</para></listitem>
<listitem><para>debug_object_deactivate</para></listitem>
<listitem><para>debug_object_destroy</para></listitem>
<listitem><para>debug_object_free</para></listitem>
<listitem><para>debug_object_assert_init</para></listitem>
</itemizedlist>
Each of these functions takes the address of the real object and
a pointer to the object type specific debug description
structure.
</para>
<para>
Each detected error is reported in the statistics and a limited
number of errors are printk'ed including a full stack trace.
</para>
<para>
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.
</para>
</chapter>
<chapter id="debugfunctions">
<title>Debug functions</title>
<sect1 id="prototypes">
<title>Debug object function reference</title>
!Elib/debugobjects.c
</sect1>
<sect1 id="debug_object_init">
<title>debug_object_init</title>
<para>
This function is called whenever the initialization function
of a real object is called.
</para>
<para>
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.
</para>
<para>
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.
</para>
</sect1>
<sect1 id="debug_object_init_on_stack">
<title>debug_object_init_on_stack</title>
<para>
This function is called whenever the initialization function
of a real object which resides on the stack is called.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
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.
</para>
</sect1>
<sect1 id="debug_object_activate">
<title>debug_object_activate</title>
<para>
This function is called whenever the activation function of a
real object is called.
</para>
<para>
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.
</para>
<para>
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.
</para>
<para>
When the activation is legitimate, then the state of the
associated tracker object is set to ODEBUG_STATE_ACTIVE.
</para>
</sect1>
<sect1 id="debug_object_deactivate">
<title>debug_object_deactivate</title>
<para>
This function is called whenever the deactivation function of
a real object is called.
</para>
<para>
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.
</para>
<para>
When the deactivation is legitimate, then the state of the
associated tracker object is set to ODEBUG_STATE_INACTIVE.
</para>
</sect1>
<sect1 id="debug_object_destroy">
<title>debug_object_destroy</title>
<para>
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.
</para>
<para>
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.
</para>
<para>
When the destruction is legitimate, then the state of the
associated tracker object is set to ODEBUG_STATE_DESTROYED.
</para>
</sect1>
<sect1 id="debug_object_free">
<title>debug_object_free</title>
<para>
This function is called before an object is freed.
</para>
<para>
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.
</para>
<para>
Note that debug_object_free removes the object from the
tracker. Later usage of the object is detected by the other
debug checks.
</para>
</sect1>
<sect1 id="debug_object_assert_init">
<title>debug_object_assert_init</title>
<para>
This function is called to assert that an object has been
initialized.
</para>
<para>
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.
</para>
<para>
When the real object is already tracked by debugobjects it is
ignored.
</para>
</sect1>
</chapter>
<chapter id="fixupfunctions">
<title>Fixup functions</title>
<sect1 id="debug_obj_descr">
<title>Debug object type description structure</title>
!Iinclude/linux/debugobjects.h
</sect1>
<sect1 id="fixup_init">
<title>fixup_init</title>
<para>
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.
</para>
<para>
Called from debug_object_init when the object state is:
<itemizedlist>
<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
</itemizedlist>
</para>
<para>
The function returns 1 when the fixup was successful,
otherwise 0. The return value is used to update the
statistics.
</para>
<para>
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.
</para>
</sect1>
<sect1 id="fixup_activate">
<title>fixup_activate</title>
<para>
This function is called from the debug code whenever a problem
in debug_object_activate is detected.
</para>
<para>
Called from debug_object_activate when the object state is:
<itemizedlist>
<listitem><para>ODEBUG_STATE_NOTAVAILABLE</para></listitem>
<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
</itemizedlist>
</para>
<para>
The function returns 1 when the fixup was successful,
otherwise 0. The return value is used to update the
statistics.
</para>
<para>
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.
</para>
<para>
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 0 because this is not a real fixup.
</para>
</sect1>
<sect1 id="fixup_destroy">
<title>fixup_destroy</title>
<para>
This function is called from the debug code whenever a problem
in debug_object_destroy is detected.
</para>
<para>
Called from debug_object_destroy when the object state is:
<itemizedlist>
<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
</itemizedlist>
</para>
<para>
The function returns 1 when the fixup was successful,
otherwise 0. The return value is used to update the
statistics.
</para>
</sect1>
<sect1 id="fixup_free">
<title>fixup_free</title>
<para>
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.
</para>
<para>
Called from debug_object_free() or debug_check_no_obj_freed()
when the object state is:
<itemizedlist>
<listitem><para>ODEBUG_STATE_ACTIVE</para></listitem>
</itemizedlist>
</para>
<para>
The function returns 1 when the fixup was successful,
otherwise 0. The return value is used to update the
statistics.
</para>
</sect1>
<sect1 id="fixup_assert_init">
<title>fixup_assert_init</title>
<para>
This function is called from the debug code whenever a problem
in debug_object_assert_init is detected.
</para>
<para>
Called from debug_object_assert_init() with a hardcoded state
ODEBUG_STATE_NOTAVAILABLE when the object is not found in the
debug bucket.
</para>
<para>
The function returns 1 when the fixup was successful,
otherwise 0. The return value is used to update the
statistics.
</para>
<para>
Note, this function should make sure debug_object_init() is
called before returning.
</para>
<para>
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 0 because this is not
a real fixup.
</para>
</sect1>
</chapter>
<chapter id="bugs">
<title>Known Bugs And Assumptions</title>
<para>
None (knock on wood).
</para>
</chapter>
</book>