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Jason Wessele3e2aaf2008-03-20 13:43:45 -05001<?xml version="1.0" encoding="UTF-8"?>
2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
4
5<book id="kgdbOnLinux">
6 <bookinfo>
7 <title>Using kgdb and the kgdb Internals</title>
8
9 <authorgroup>
10 <author>
11 <firstname>Jason</firstname>
12 <surname>Wessel</surname>
13 <affiliation>
14 <address>
15 <email>jason.wessel@windriver.com</email>
16 </address>
17 </affiliation>
18 </author>
19 </authorgroup>
20
21 <authorgroup>
22 <author>
23 <firstname>Tom</firstname>
24 <surname>Rini</surname>
25 <affiliation>
26 <address>
27 <email>trini@kernel.crashing.org</email>
28 </address>
29 </affiliation>
30 </author>
31 </authorgroup>
32
33 <authorgroup>
34 <author>
35 <firstname>Amit S.</firstname>
36 <surname>Kale</surname>
37 <affiliation>
38 <address>
39 <email>amitkale@linsyssoft.com</email>
40 </address>
41 </affiliation>
42 </author>
43 </authorgroup>
44
45 <copyright>
46 <year>2008</year>
47 <holder>Wind River Systems, Inc.</holder>
48 </copyright>
49 <copyright>
50 <year>2004-2005</year>
51 <holder>MontaVista Software, Inc.</holder>
52 </copyright>
53 <copyright>
54 <year>2004</year>
55 <holder>Amit S. Kale</holder>
56 </copyright>
57
58 <legalnotice>
59 <para>
60 This file is licensed under the terms of the GNU General Public License
61 version 2. This program is licensed "as is" without any warranty of any
62 kind, whether express or implied.
63 </para>
64
65 </legalnotice>
66 </bookinfo>
67
68<toc></toc>
69 <chapter id="Introduction">
70 <title>Introduction</title>
71 <para>
72 kgdb is a source level debugger for linux kernel. It is used along
73 with gdb to debug a linux kernel. The expectation is that gdb can
74 be used to "break in" to the kernel to inspect memory, variables
grzegorz.chwesewicz@chilan.comf9250932008-04-24 16:57:22 -050075 and look through call stack information similar to what an
Jason Wessele3e2aaf2008-03-20 13:43:45 -050076 application developer would use gdb for. It is possible to place
77 breakpoints in kernel code and perform some limited execution
78 stepping.
79 </para>
80 <para>
81 Two machines are required for using kgdb. One of these machines is a
82 development machine and the other is a test machine. The kernel
83 to be debugged runs on the test machine. The development machine
84 runs an instance of gdb against the vmlinux file which contains
85 the symbols (not boot image such as bzImage, zImage, uImage...).
86 In gdb the developer specifies the connection parameters and
Jason Wessela606b5e2008-06-24 10:52:55 -050087 connects to kgdb. The type of connection a developer makes with
88 gdb depends on the availability of kgdb I/O modules compiled as
89 builtin's or kernel modules in the test machine's kernel.
Jason Wessele3e2aaf2008-03-20 13:43:45 -050090 </para>
91 </chapter>
92 <chapter id="CompilingAKernel">
93 <title>Compiling a kernel</title>
94 <para>
grzegorz.chwesewicz@chilan.comf9250932008-04-24 16:57:22 -050095 To enable <symbol>CONFIG_KGDB</symbol> you should first turn on
96 "Prompt for development and/or incomplete code/drivers"
97 (CONFIG_EXPERIMENTAL) in "General setup", then under the
98 "Kernel debugging" select "KGDB: kernel debugging with remote gdb".
Jason Wessele3e2aaf2008-03-20 13:43:45 -050099 </para>
100 <para>
Jason Wessel5f5ddfb2008-08-01 08:39:34 -0500101 It is advised, but not required that you turn on the
102 CONFIG_FRAME_POINTER kernel option. This option inserts code to
103 into the compiled executable which saves the frame information in
104 registers or on the stack at different points which will allow a
105 debugger such as gdb to more accurately construct stack back traces
106 while debugging the kernel.
107 </para>
108 <para>
Jason Wessela9b60bf2008-08-01 08:39:34 -0500109 If the architecture that you are using supports the kernel option
110 CONFIG_DEBUG_RODATA, you should consider turning it off. This
111 option will prevent the use of software breakpoints because it
112 marks certain regions of the kernel's memory space as read-only.
113 If kgdb supports it for the architecture you are using, you can
114 use hardware breakpoints if you desire to run with the
115 CONFIG_DEBUG_RODATA option turned on, else you need to turn off
116 this option.
117 </para>
118 <para>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500119 Next you should choose one of more I/O drivers to interconnect debugging
120 host and debugged target. Early boot debugging requires a KGDB
121 I/O driver that supports early debugging and the driver must be
122 built into the kernel directly. Kgdb I/O driver configuration
123 takes place via kernel or module parameters, see following
124 chapter.
125 </para>
126 <para>
127 The kgdb test compile options are described in the kgdb test suite chapter.
128 </para>
129
130 </chapter>
131 <chapter id="EnableKGDB">
132 <title>Enable kgdb for debugging</title>
133 <para>
134 In order to use kgdb you must activate it by passing configuration
135 information to one of the kgdb I/O drivers. If you do not pass any
136 configuration information kgdb will not do anything at all. Kgdb
137 will only actively hook up to the kernel trap hooks if a kgdb I/O
138 driver is loaded and configured. If you unconfigure a kgdb I/O
139 driver, kgdb will unregister all the kernel hook points.
140 </para>
141 <para>
142 All drivers can be reconfigured at run time, if
143 <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
144 are enabled, by echo'ing a new config string to
145 <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
146 The driver can be unconfigured by passing an empty string. You cannot
147 change the configuration while the debugger is attached. Make sure
148 to detach the debugger with the <constant>detach</constant> command
149 prior to trying unconfigure a kgdb I/O driver.
150 </para>
151 <sect1 id="kgdbwait">
152 <title>Kernel parameter: kgdbwait</title>
153 <para>
154 The Kernel command line option <constant>kgdbwait</constant> makes
155 kgdb wait for a debugger connection during booting of a kernel. You
156 can only use this option you compiled a kgdb I/O driver into the
157 kernel and you specified the I/O driver configuration as a kernel
158 command line option. The kgdbwait parameter should always follow the
159 configuration parameter for the kgdb I/O driver in the kernel
160 command line else the I/O driver will not be configured prior to
161 asking the kernel to use it to wait.
162 </para>
163 <para>
164 The kernel will stop and wait as early as the I/O driver and
165 architecture will allow when you use this option. If you build the
166 kgdb I/O driver as a kernel module kgdbwait will not do anything.
167 </para>
168 </sect1>
169 <sect1 id="kgdboc">
170 <title>Kernel parameter: kgdboc</title>
171 <para>
172 The kgdboc driver was originally an abbreviation meant to stand for
173 "kgdb over console". Kgdboc is designed to work with a single
Jason Wessel225a4422008-04-01 16:55:26 -0500174 serial port. It was meant to cover the circumstance
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500175 where you wanted to use a serial console as your primary console as
Jason Wessel225a4422008-04-01 16:55:26 -0500176 well as using it to perform kernel debugging. Of course you can
177 also use kgdboc without assigning a console to the same port.
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500178 </para>
179 <sect2 id="UsingKgdboc">
180 <title>Using kgdboc</title>
181 <para>
182 You can configure kgdboc via sysfs or a module or kernel boot line
183 parameter depending on if you build with CONFIG_KGDBOC as a module
184 or built-in.
185 <orderedlist>
186 <listitem><para>From the module load or build-in</para>
187 <para><constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
188 <para>
189 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>
190 </para>
191 </listitem>
192 <listitem><para>From sysfs</para>
193 <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para>
194 </listitem>
195 </orderedlist>
196 </para>
197 <para>
198 NOTE: Kgdboc does not support interrupting the target via the
199 gdb remote protocol. You must manually send a sysrq-g unless you
200 have a proxy that splits console output to a terminal problem and
201 has a separate port for the debugger to connect to that sends the
202 sysrq-g for you.
203 </para>
204 <para>When using kgdboc with no debugger proxy, you can end up
205 connecting the debugger for one of two entry points. If an
206 exception occurs after you have loaded kgdboc a message should print
207 on the console stating it is waiting for the debugger. In case you
208 disconnect your terminal program and then connect the debugger in
209 its place. If you want to interrupt the target system and forcibly
210 enter a debug session you have to issue a Sysrq sequence and then
211 type the letter <constant>g</constant>. Then you disconnect the
212 terminal session and connect gdb. Your options if you don't like
213 this are to hack gdb to send the sysrq-g for you as well as on the
214 initial connect, or to use a debugger proxy that allows an
215 unmodified gdb to do the debugging.
216 </para>
217 </sect2>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500218 </sect1>
219 <sect1 id="kgdbcon">
220 <title>Kernel parameter: kgdbcon</title>
221 <para>
222 Kgdb supports using the gdb serial protocol to send console messages
223 to the debugger when the debugger is connected and running. There
224 are two ways to activate this feature.
225 <orderedlist>
226 <listitem><para>Activate with the kernel command line option:</para>
227 <para><constant>kgdbcon</constant></para>
228 </listitem>
229 <listitem><para>Use sysfs before configuring an io driver</para>
230 <para>
231 <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
232 </para>
233 <para>
234 NOTE: If you do this after you configure the kgdb I/O driver, the
235 setting will not take effect until the next point the I/O is
236 reconfigured.
237 </para>
238 </listitem>
239 </orderedlist>
240 </para>
241 <para>
242 IMPORTANT NOTE: Using this option with kgdb over the console
Jason Wessela606b5e2008-06-24 10:52:55 -0500243 (kgdboc) is not supported.
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500244 </para>
245 </sect1>
246 </chapter>
247 <chapter id="ConnectingGDB">
248 <title>Connecting gdb</title>
249 <para>
250 If you are using kgdboc, you need to have used kgdbwait as a boot
251 argument, issued a sysrq-g, or the system you are going to debug
252 has already taken an exception and is waiting for the debugger to
253 attach before you can connect gdb.
254 </para>
255 <para>
256 If you are not using different kgdb I/O driver other than kgdboc,
257 you should be able to connect and the target will automatically
258 respond.
259 </para>
260 <para>
261 Example (using a serial port):
262 </para>
263 <programlisting>
264 % gdb ./vmlinux
265 (gdb) set remotebaud 115200
266 (gdb) target remote /dev/ttyS0
267 </programlisting>
268 <para>
Jason Wessela606b5e2008-06-24 10:52:55 -0500269 Example (kgdb to a terminal server on tcp port 2012):
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500270 </para>
271 <programlisting>
272 % gdb ./vmlinux
Jason Wessela606b5e2008-06-24 10:52:55 -0500273 (gdb) target remote 192.168.2.2:2012
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500274 </programlisting>
275 <para>
276 Once connected, you can debug a kernel the way you would debug an
277 application program.
278 </para>
279 <para>
280 If you are having problems connecting or something is going
281 seriously wrong while debugging, it will most often be the case
282 that you want to enable gdb to be verbose about its target
283 communications. You do this prior to issuing the <constant>target
Frank Rowandd34a7922009-05-15 07:56:25 -0500284 remote</constant> command by typing in: <constant>set debug remote 1</constant>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500285 </para>
286 </chapter>
287 <chapter id="KGDBTestSuite">
288 <title>kgdb Test Suite</title>
289 <para>
290 When kgdb is enabled in the kernel config you can also elect to
291 enable the config parameter KGDB_TESTS. Turning this on will
292 enable a special kgdb I/O module which is designed to test the
293 kgdb internal functions.
294 </para>
295 <para>
296 The kgdb tests are mainly intended for developers to test the kgdb
297 internals as well as a tool for developing a new kgdb architecture
298 specific implementation. These tests are not really for end users
299 of the Linux kernel. The primary source of documentation would be
300 to look in the drivers/misc/kgdbts.c file.
301 </para>
302 <para>
303 The kgdb test suite can also be configured at compile time to run
304 the core set of tests by setting the kernel config parameter
305 KGDB_TESTS_ON_BOOT. This particular option is aimed at automated
306 regression testing and does not require modifying the kernel boot
307 config arguments. If this is turned on, the kgdb test suite can
308 be disabled by specifying "kgdbts=" as a kernel boot argument.
309 </para>
310 </chapter>
311 <chapter id="CommonBackEndReq">
Jason Wessel225a4422008-04-01 16:55:26 -0500312 <title>KGDB Internals</title>
313 <sect1 id="kgdbArchitecture">
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500314 <title>Architecture Specifics</title>
315 <para>
316 Kgdb is organized into three basic components:
317 <orderedlist>
318 <listitem><para>kgdb core</para>
319 <para>
320 The kgdb core is found in kernel/kgdb.c. It contains:
321 <itemizedlist>
322 <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
323 <listitem><para>A generic OS exception handler which includes sync'ing the processors into a stopped state on an multi cpu system.</para></listitem>
324 <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
325 <listitem><para>The API to make calls to the arch specific kgdb implementation</para></listitem>
326 <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
327 <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
328 </itemizedlist>
329 </para>
330 </listitem>
331 <listitem><para>kgdb arch specific implementation</para>
332 <para>
333 This implementation is generally found in arch/*/kernel/kgdb.c.
334 As an example, arch/x86/kernel/kgdb.c contains the specifics to
335 implement HW breakpoint as well as the initialization to
336 dynamically register and unregister for the trap handlers on
337 this architecture. The arch specific portion implements:
338 <itemizedlist>
339 <listitem><para>contains an arch specific trap catcher which
340 invokes kgdb_handle_exception() to start kgdb about doing its
341 work</para></listitem>
342 <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
343 <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
344 <listitem><para>Any special exception handling and cleanup</para></listitem>
345 <listitem><para>NMI exception handling and cleanup</para></listitem>
346 <listitem><para>(optional)HW breakpoints</para></listitem>
347 </itemizedlist>
348 </para>
349 </listitem>
350 <listitem><para>kgdb I/O driver</para>
351 <para>
Jason Wessel225a4422008-04-01 16:55:26 -0500352 Each kgdb I/O driver has to provide an implemenation for the following:
353 <itemizedlist>
354 <listitem><para>configuration via builtin or module</para></listitem>
355 <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
356 <listitem><para>read and write character interface</para></listitem>
357 <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
358 <listitem><para>(optional) Early debug methodology</para></listitem>
359 </itemizedlist>
360 Any given kgdb I/O driver has to operate very closely with the
361 hardware and must do it in such a way that does not enable
362 interrupts or change other parts of the system context without
363 completely restoring them. The kgdb core will repeatedly "poll"
364 a kgdb I/O driver for characters when it needs input. The I/O
365 driver is expected to return immediately if there is no data
366 available. Doing so allows for the future possibility to touch
367 watch dog hardware in such a way as to have a target system not
368 reset when these are enabled.
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500369 </para>
370 </listitem>
371 </orderedlist>
372 </para>
373 <para>
374 If you are intent on adding kgdb architecture specific support
375 for a new architecture, the architecture should define
376 <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
377 Kconfig file. This will enable kgdb for the architecture, and
378 at that point you must create an architecture specific kgdb
379 implementation.
380 </para>
381 <para>
382 There are a few flags which must be set on every architecture in
383 their &lt;asm/kgdb.h&gt; file. These are:
384 <itemizedlist>
385 <listitem>
386 <para>
387 NUMREGBYTES: The size in bytes of all of the registers, so
388 that we can ensure they will all fit into a packet.
389 </para>
390 <para>
391 BUFMAX: The size in bytes of the buffer GDB will read into.
392 This must be larger than NUMREGBYTES.
393 </para>
394 <para>
395 CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
396 flush_cache_range or flush_icache_range. On some architectures,
397 these functions may not be safe to call on SMP since we keep other
398 CPUs in a holding pattern.
399 </para>
400 </listitem>
401 </itemizedlist>
402 </para>
403 <para>
404 There are also the following functions for the common backend,
405 found in kernel/kgdb.c, that must be supplied by the
406 architecture-specific backend unless marked as (optional), in
407 which case a default function maybe used if the architecture
408 does not need to provide a specific implementation.
409 </para>
410!Iinclude/linux/kgdb.h
Jason Wessel225a4422008-04-01 16:55:26 -0500411 </sect1>
412 <sect1 id="kgdbocDesign">
413 <title>kgdboc internals</title>
414 <para>
415 The kgdboc driver is actually a very thin driver that relies on the
416 underlying low level to the hardware driver having "polling hooks"
417 which the to which the tty driver is attached. In the initial
418 implementation of kgdboc it the serial_core was changed to expose a
419 low level uart hook for doing polled mode reading and writing of a
420 single character while in an atomic context. When kgdb makes an I/O
421 request to the debugger, kgdboc invokes a call back in the serial
422 core which in turn uses the call back in the uart driver. It is
423 certainly possible to extend kgdboc to work with non-uart based
424 consoles in the future.
425 </para>
426 <para>
427 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>
428#ifdef CONFIG_CONSOLE_POLL
429 .poll_get_char = serial8250_get_poll_char,
430 .poll_put_char = serial8250_put_poll_char,
431#endif
432 </programlisting>
433 Any implementation specifics around creating a polling driver use the
434 <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
435 Keep in mind that polling hooks have to be implemented in such a way
436 that they can be called from an atomic context and have to restore
437 the state of the uart chip on return such that the system can return
438 to normal when the debugger detaches. You need to be very careful
439 with any kind of lock you consider, because failing here is most
440 going to mean pressing the reset button.
441 </para>
442 </sect1>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500443 </chapter>
444 <chapter id="credits">
445 <title>Credits</title>
446 <para>
447 The following people have contributed to this document:
448 <orderedlist>
449 <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
450 <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500451 </orderedlist>
Jason Wessel225a4422008-04-01 16:55:26 -0500452 In March 2008 this document was completely rewritten by:
453 <itemizedlist>
454 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
455 </itemizedlist>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500456 </para>
457 </chapter>
458</book>
459