forked from luck/tmp_suning_uos_patched
d34a792da9
gdb command "set remote debug 1" is not valid, change to correct command. Signed-off-by: Frank Rowand <frank.rowand@am.sony.com> Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
460 lines
19 KiB
XML
460 lines
19 KiB
XML
<?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="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 debug remote 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>
|
|
|