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<!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="kgdbOnLinux">
<bookinfo>
<title>Using kgdb and the kgdb Internals</title>
<authorgroup>
<author>
<firstname>Jason</firstname>
<surname>Wessel</surname>
<affiliation>
<address>
<email>jason.wessel@windriver.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rini</surname>
<affiliation>
<address>
<email>trini@kernel.crashing.org</email>
</address>
</affiliation>
</author>
</authorgroup>
<authorgroup>
<author>
<firstname>Amit S.</firstname>
<surname>Kale</surname>
<affiliation>
<address>
<email>amitkale@linsyssoft.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<copyright>
<year>2008</year>
<holder>Wind River Systems, Inc.</holder>
</copyright>
<copyright>
<year>2004-2005</year>
<holder>MontaVista Software, Inc.</holder>
</copyright>
<copyright>
<year>2004</year>
<holder>Amit S. Kale</holder>
</copyright>
<legalnotice>
<para>
This file is licensed under the terms of the GNU General Public License
version 2. This program is licensed "as is" without any warranty of any
kind, whether express or implied.
</para>
</legalnotice>
</bookinfo>
<toc></toc>
<chapter id="Introduction">
<title>Introduction</title>
<para>
kgdb is a source level debugger for linux kernel. It is used along
with gdb to debug a linux kernel. The expectation is that gdb can
be used to "break in" to the kernel to inspect memory, variables
and look through call stack information similar to what an
application developer would use gdb for. It is possible to place
breakpoints in kernel code and perform some limited execution
stepping.
</para>
<para>
Two machines are required for using kgdb. One of these machines is a
development machine and the other is a test machine. The kernel
to be debugged runs on the test machine. The development machine
runs an instance of gdb against the vmlinux file which contains
the symbols (not boot image such as bzImage, zImage, uImage...).
In gdb the developer specifies the connection parameters and
connects to kgdb. The type of connection a developer makes with
gdb depends on the availability of kgdb I/O modules compiled as
builtin's or kernel modules in the test machine's kernel.
</para>
</chapter>
<chapter id="CompilingAKernel">
<title>Compiling a kernel</title>
<para>
To enable <symbol>CONFIG_KGDB</symbol> you should first turn on
"Prompt for development and/or incomplete code/drivers"
(CONFIG_EXPERIMENTAL) in "General setup", then under the
"Kernel debugging" select "KGDB: kernel debugging with remote gdb".
</para>
<para>
It is advised, but not required that you turn on the
CONFIG_FRAME_POINTER kernel option. This option inserts code to
into the compiled executable which saves the frame information in
registers or on the stack at different points which will allow a
debugger such as gdb to more accurately construct stack back traces
while debugging the kernel.
</para>
<para>
If the architecture that you are using supports the kernel option
CONFIG_DEBUG_RODATA, you should consider turning it off. This
option will prevent the use of software breakpoints because it
marks certain regions of the kernel's memory space as read-only.
If kgdb supports it for the architecture you are using, you can
use hardware breakpoints if you desire to run with the
CONFIG_DEBUG_RODATA option turned on, else you need to turn off
this option.
</para>
<para>
Next you should choose one of more I/O drivers to interconnect debugging
host and debugged target. Early boot debugging requires a KGDB
I/O driver that supports early debugging and the driver must be
built into the kernel directly. Kgdb I/O driver configuration
takes place via kernel or module parameters, see following
chapter.
</para>
<para>
The kgdb test compile options are described in the kgdb test suite chapter.
</para>
</chapter>
<chapter id="EnableKGDB">
<title>Enable kgdb for debugging</title>
<para>
In order to use kgdb you must activate it by passing configuration
information to one of the kgdb I/O drivers. If you do not pass any
configuration information kgdb will not do anything at all. Kgdb
will only actively hook up to the kernel trap hooks if a kgdb I/O
driver is loaded and configured. If you unconfigure a kgdb I/O
driver, kgdb will unregister all the kernel hook points.
</para>
<para>
All drivers can be reconfigured at run time, if
<symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
are enabled, by echo'ing a new config string to
<constant>/sys/module/<driver>/parameter/<option></constant>.
The driver can be unconfigured by passing an empty string. You cannot
change the configuration while the debugger is attached. Make sure
to detach the debugger with the <constant>detach</constant> command
prior to trying unconfigure a kgdb I/O driver.
</para>
<sect1 id="kgdbwait">
<title>Kernel parameter: kgdbwait</title>
<para>
The Kernel command line option <constant>kgdbwait</constant> makes
kgdb wait for a debugger connection during booting of a kernel. You
can only use this option you compiled a kgdb I/O driver into the
kernel and you specified the I/O driver configuration as a kernel
command line option. The kgdbwait parameter should always follow the
configuration parameter for the kgdb I/O driver in the kernel
command line else the I/O driver will not be configured prior to
asking the kernel to use it to wait.
</para>
<para>
The kernel will stop and wait as early as the I/O driver and
architecture will allow when you use this option. If you build the
kgdb I/O driver as a kernel module kgdbwait will not do anything.
</para>
</sect1>
<sect1 id="kgdboc">
<title>Kernel parameter: kgdboc</title>
<para>
The kgdboc driver was originally an abbreviation meant to stand for
"kgdb over console". Kgdboc is designed to work with a single
serial port. It was meant to cover the circumstance
where you wanted to use a serial console as your primary console as
well as using it to perform kernel debugging. Of course you can
also use kgdboc without assigning a console to the same port.
</para>
<sect2 id="UsingKgdboc">
<title>Using kgdboc</title>
<para>
You can configure kgdboc via sysfs or a module or kernel boot line
parameter depending on if you build with CONFIG_KGDBOC as a module
or built-in.
<orderedlist>
<listitem><para>From the module load or build-in</para>
<para><constant>kgdboc=<tty-device>,[baud]</constant></para>
<para>
The example here would be if your console port was typically ttyS0, you would use something like <constant>kgdboc=ttyS0,115200</constant> or on the ARM Versatile AB you would likely use <constant>kgdboc=ttyAMA0,115200</constant>
</para>
</listitem>
<listitem><para>From sysfs</para>
<para><constant>echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc</constant></para>
</listitem>
</orderedlist>
</para>
<para>
NOTE: Kgdboc does not support interrupting the target via the
gdb remote protocol. You must manually send a sysrq-g unless you
have a proxy that splits console output to a terminal problem and
has a separate port for the debugger to connect to that sends the
sysrq-g for you.
</para>
<para>When using kgdboc with no debugger proxy, you can end up
connecting the debugger for one of two entry points. If an
exception occurs after you have loaded kgdboc a message should print
on the console stating it is waiting for the debugger. In case you
disconnect your terminal program and then connect the debugger in
its place. If you want to interrupt the target system and forcibly
enter a debug session you have to issue a Sysrq sequence and then
type the letter <constant>g</constant>. Then you disconnect the
terminal session and connect gdb. Your options if you don't like
this are to hack gdb to send the sysrq-g for you as well as on the
initial connect, or to use a debugger proxy that allows an
unmodified gdb to do the debugging.
</para>
</sect2>
</sect1>
<sect1 id="kgdbcon">
<title>Kernel parameter: kgdbcon</title>
<para>
Kgdb supports using the gdb serial protocol to send console messages
to the debugger when the debugger is connected and running. There
are two ways to activate this feature.
<orderedlist>
<listitem><para>Activate with the kernel command line option:</para>
<para><constant>kgdbcon</constant></para>
</listitem>
<listitem><para>Use sysfs before configuring an io driver</para>
<para>
<constant>echo 1 > /sys/module/kgdb/parameters/kgdb_use_con</constant>
</para>
<para>
NOTE: If you do this after you configure the kgdb I/O driver, the
setting will not take effect until the next point the I/O is
reconfigured.
</para>
</listitem>
</orderedlist>
</para>
<para>
IMPORTANT NOTE: Using this option with kgdb over the console
(kgdboc) is not supported.
</para>
</sect1>
</chapter>
<chapter id="ConnectingGDB">
<title>Connecting gdb</title>
<para>
If you are using kgdboc, you need to have used kgdbwait as a boot
argument, issued a sysrq-g, or the system you are going to debug
has already taken an exception and is waiting for the debugger to
attach before you can connect gdb.
</para>
<para>
If you are not using different kgdb I/O driver other than kgdboc,
you should be able to connect and the target will automatically
respond.
</para>
<para>
Example (using a serial port):
</para>
<programlisting>
% gdb ./vmlinux
(gdb) set remotebaud 115200
(gdb) target remote /dev/ttyS0
</programlisting>
<para>
Example (kgdb to a terminal server on tcp port 2012):
</para>
<programlisting>
% gdb ./vmlinux
(gdb) target remote 192.168.2.2:2012
</programlisting>
<para>
Once connected, you can debug a kernel the way you would debug an
application program.
</para>
<para>
If you are having problems connecting or something is going
seriously wrong while debugging, it will most often be the case
that you want to enable gdb to be verbose about its target
communications. You do this prior to issuing the <constant>target
remote</constant> command by typing in: <constant>set remote debug 1</constant>
</para>
</chapter>
<chapter id="KGDBTestSuite">
<title>kgdb Test Suite</title>
<para>
When kgdb is enabled in the kernel config you can also elect to
enable the config parameter KGDB_TESTS. Turning this on will
enable a special kgdb I/O module which is designed to test the
kgdb internal functions.
</para>
<para>
The kgdb tests are mainly intended for developers to test the kgdb
internals as well as a tool for developing a new kgdb architecture
specific implementation. These tests are not really for end users
of the Linux kernel. The primary source of documentation would be
to look in the drivers/misc/kgdbts.c file.
</para>
<para>
The kgdb test suite can also be configured at compile time to run
the core set of tests by setting the kernel config parameter
KGDB_TESTS_ON_BOOT. This particular option is aimed at automated
regression testing and does not require modifying the kernel boot
config arguments. If this is turned on, the kgdb test suite can
be disabled by specifying "kgdbts=" as a kernel boot argument.
</para>
</chapter>
<chapter id="CommonBackEndReq">
<title>KGDB Internals</title>
<sect1 id="kgdbArchitecture">
<title>Architecture Specifics</title>
<para>
Kgdb is organized into three basic components:
<orderedlist>
<listitem><para>kgdb core</para>
<para>
The kgdb core is found in kernel/kgdb.c. It contains:
<itemizedlist>
<listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
<listitem><para>A generic OS exception handler which includes sync'ing the processors into a stopped state on an multi cpu system.</para></listitem>
<listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
<listitem><para>The API to make calls to the arch specific kgdb implementation</para></listitem>
<listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
<listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
</itemizedlist>
</para>
</listitem>
<listitem><para>kgdb arch specific implementation</para>
<para>
This implementation is generally found in arch/*/kernel/kgdb.c.
As an example, arch/x86/kernel/kgdb.c contains the specifics to
implement HW breakpoint as well as the initialization to
dynamically register and unregister for the trap handlers on
this architecture. The arch specific portion implements:
<itemizedlist>
<listitem><para>contains an arch specific trap catcher which
invokes kgdb_handle_exception() to start kgdb about doing its
work</para></listitem>
<listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
<listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
<listitem><para>Any special exception handling and cleanup</para></listitem>
<listitem><para>NMI exception handling and cleanup</para></listitem>
<listitem><para>(optional)HW breakpoints</para></listitem>
</itemizedlist>
</para>
</listitem>
<listitem><para>kgdb I/O driver</para>
<para>
Each kgdb I/O driver has to provide an implemenation for the following:
<itemizedlist>
<listitem><para>configuration via builtin or module</para></listitem>
<listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
<listitem><para>read and write character interface</para></listitem>
<listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
<listitem><para>(optional) Early debug methodology</para></listitem>
</itemizedlist>
Any given kgdb I/O driver has to operate very closely with the
hardware and must do it in such a way that does not enable
interrupts or change other parts of the system context without
completely restoring them. The kgdb core will repeatedly "poll"
a kgdb I/O driver for characters when it needs input. The I/O
driver is expected to return immediately if there is no data
available. Doing so allows for the future possibility to touch
watch dog hardware in such a way as to have a target system not
reset when these are enabled.
</para>
</listitem>
</orderedlist>
</para>
<para>
If you are intent on adding kgdb architecture specific support
for a new architecture, the architecture should define
<constant>HAVE_ARCH_KGDB</constant> in the architecture specific
Kconfig file. This will enable kgdb for the architecture, and
at that point you must create an architecture specific kgdb
implementation.
</para>
<para>
There are a few flags which must be set on every architecture in
their <asm/kgdb.h> file. These are:
<itemizedlist>
<listitem>
<para>
NUMREGBYTES: The size in bytes of all of the registers, so
that we can ensure they will all fit into a packet.
</para>
<para>
BUFMAX: The size in bytes of the buffer GDB will read into.
This must be larger than NUMREGBYTES.
</para>
<para>
CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
flush_cache_range or flush_icache_range. On some architectures,
these functions may not be safe to call on SMP since we keep other
CPUs in a holding pattern.
</para>
</listitem>
</itemizedlist>
</para>
<para>
There are also the following functions for the common backend,
found in kernel/kgdb.c, that must be supplied by the
architecture-specific backend unless marked as (optional), in
which case a default function maybe used if the architecture
does not need to provide a specific implementation.
</para>
!Iinclude/linux/kgdb.h
</sect1>
<sect1 id="kgdbocDesign">
<title>kgdboc internals</title>
<para>
The kgdboc driver is actually a very thin driver that relies on the
underlying low level to the hardware driver having "polling hooks"
which the to which the tty driver is attached. In the initial
implementation of kgdboc it the serial_core was changed to expose a
low level uart hook for doing polled mode reading and writing of a
single character while in an atomic context. When kgdb makes an I/O
request to the debugger, kgdboc invokes a call back in the serial
core which in turn uses the call back in the uart driver. It is
certainly possible to extend kgdboc to work with non-uart based
consoles in the future.
</para>
<para>
When using kgdboc with a uart, the uart driver must implement two callbacks in the <constant>struct uart_ops</constant>. Example from drivers/8250.c:<programlisting>
#ifdef CONFIG_CONSOLE_POLL
.poll_get_char = serial8250_get_poll_char,
.poll_put_char = serial8250_put_poll_char,
#endif
</programlisting>
Any implementation specifics around creating a polling driver use the
<constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
Keep in mind that polling hooks have to be implemented in such a way
that they can be called from an atomic context and have to restore
the state of the uart chip on return such that the system can return
to normal when the debugger detaches. You need to be very careful
with any kind of lock you consider, because failing here is most
going to mean pressing the reset button.
</para>
</sect1>
</chapter>
<chapter id="credits">
<title>Credits</title>
<para>
The following people have contributed to this document:
<orderedlist>
<listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
<listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
</orderedlist>
In March 2008 this document was completely rewritten by:
<itemizedlist>
<listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
</itemizedlist>
</para>
</chapter>
</book>
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