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authorFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
committerFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+ "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<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>