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2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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4
5<book id="kgdbOnLinux">
6 <bookinfo>
7 <title>Using kgdb, kdb and the kernel debugger 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 <copyright>
21 <year>2008,2010</year>
22 <holder>Wind River Systems, Inc.</holder>
23 </copyright>
24 <copyright>
25 <year>2004-2005</year>
26 <holder>MontaVista Software, Inc.</holder>
27 </copyright>
28 <copyright>
29 <year>2004</year>
30 <holder>Amit S. Kale</holder>
31 </copyright>
32
33 <legalnotice>
34 <para>
35 This file is licensed under the terms of the GNU General Public License
36 version 2. This program is licensed "as is" without any warranty of any
37 kind, whether express or implied.
38 </para>
39
40 </legalnotice>
41 </bookinfo>
42
43<toc></toc>
44 <chapter id="Introduction">
45 <title>Introduction</title>
46 <para>
47 The kernel has two different debugger front ends (kdb and kgdb)
48 which interface to the debug core. It is possible to use either
49 of the debugger front ends and dynamically transition between them
50 if you configure the kernel properly at compile and runtime.
51 </para>
52 <para>
53 Kdb is simplistic shell-style interface which you can use on a
54 system console with a keyboard or serial console. You can use it
55 to inspect memory, registers, process lists, dmesg, and even set
56 breakpoints to stop in a certain location. Kdb is not a source
57 level debugger, although you can set breakpoints and execute some
58 basic kernel run control. Kdb is mainly aimed at doing some
59 analysis to aid in development or diagnosing kernel problems. You
60 can access some symbols by name in kernel built-ins or in kernel
61 modules if the code was built
62 with <symbol>CONFIG_KALLSYMS</symbol>.
63 </para>
64 <para>
65 Kgdb is intended to be used as a source level debugger for the
66 Linux kernel. It is used along with gdb to debug a Linux kernel.
67 The expectation is that gdb can be used to "break in" to the
68 kernel to inspect memory, variables and look through call stack
69 information similar to the way an application developer would use
70 gdb to debug an application. It is possible to place breakpoints
71 in kernel code and perform some limited execution stepping.
72 </para>
73 <para>
74 Two machines are required for using kgdb. One of these machines is
75 a development machine and the other is the target machine. The
76 kernel to be debugged runs on the target machine. The development
77 machine runs an instance of gdb against the vmlinux file which
78 contains the symbols (not boot image such as bzImage, zImage,
79 uImage...). In gdb the developer specifies the connection
80 parameters and connects to kgdb. The type of connection a
81 developer makes with gdb depends on the availability of kgdb I/O
82 modules compiled as built-ins or loadable kernel modules in the test
83 machine's kernel.
84 </para>
85 </chapter>
86 <chapter id="CompilingAKernel">
87 <title>Compiling a kernel</title>
88 <para>
89 <itemizedlist>
90 <listitem><para>In order to enable compilation of kdb, you must first enable kgdb.</para></listitem>
91 <listitem><para>The kgdb test compile options are described in the kgdb test suite chapter.</para></listitem>
92 </itemizedlist>
93 </para>
94 <sect1 id="CompileKGDB">
95 <title>Kernel config options for kgdb</title>
96 <para>
97 To enable <symbol>CONFIG_KGDB</symbol> you should first turn on
98 "Prompt for development and/or incomplete code/drivers"
99 (CONFIG_EXPERIMENTAL) in "General setup", then under the
100 "Kernel debugging" select "KGDB: kernel debugger".
101 </para>
102 <para>
103 While it is not a hard requirement that you have symbols in your
104 vmlinux file, gdb tends not to be very useful without the symbolic
105 data, so you will want to turn
106 on <symbol>CONFIG_DEBUG_INFO</symbol> which is called "Compile the
107 kernel with debug info" in the config menu.
108 </para>
109 <para>
110 It is advised, but not required that you turn on the
111 <symbol>CONFIG_FRAME_POINTER</symbol> kernel option which is called "Compile the
112 kernel with frame pointers" in the config menu. This option
113 inserts code to into the compiled executable which saves the frame
114 information in registers or on the stack at different points which
115 allows a debugger such as gdb to more accurately construct
116 stack back traces while debugging the kernel.
117 </para>
118 <para>
119 If the architecture that you are using supports the kernel option
120 CONFIG_DEBUG_RODATA, you should consider turning it off. This
121 option will prevent the use of software breakpoints because it
122 marks certain regions of the kernel's memory space as read-only.
123 If kgdb supports it for the architecture you are using, you can
124 use hardware breakpoints if you desire to run with the
125 CONFIG_DEBUG_RODATA option turned on, else you need to turn off
126 this option.
127 </para>
128 <para>
129 Next you should choose one of more I/O drivers to interconnect
130 debugging host and debugged target. Early boot debugging requires
131 a KGDB I/O driver that supports early debugging and the driver
132 must be built into the kernel directly. Kgdb I/O driver
133 configuration takes place via kernel or module parameters which
134 you can learn more about in the in the section that describes the
135 parameter "kgdboc".
136 </para>
137 <para>Here is an example set of .config symbols to enable or
138 disable for kgdb:
139 <itemizedlist>
140 <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
141 <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
142 <listitem><para>CONFIG_KGDB=y</para></listitem>
143 <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
144 </itemizedlist>
145 </para>
146 </sect1>
147 <sect1 id="CompileKDB">
148 <title>Kernel config options for kdb</title>
149 <para>Kdb is quite a bit more complex than the simple gdbstub
150 sitting on top of the kernel's debug core. Kdb must implement a
151 shell, and also adds some helper functions in other parts of the
152 kernel, responsible for printing out interesting data such as what
153 you would see if you ran "lsmod", or "ps". In order to build kdb
154 into the kernel you follow the same steps as you would for kgdb.
155 </para>
156 <para>The main config option for kdb
157 is <symbol>CONFIG_KGDB_KDB</symbol> which is called "KGDB_KDB:
158 include kdb frontend for kgdb" in the config menu. In theory you
159 would have already also selected an I/O driver such as the
160 CONFIG_KGDB_SERIAL_CONSOLE interface if you plan on using kdb on a
161 serial port, when you were configuring kgdb.
162 </para>
163 <para>If you want to use a PS/2-style keyboard with kdb, you would
164 select CONFIG_KDB_KEYBOARD which is called "KGDB_KDB: keyboard as
165 input device" in the config menu. The CONFIG_KDB_KEYBOARD option
166 is not used for anything in the gdb interface to kgdb. The
167 CONFIG_KDB_KEYBOARD option only works with kdb.
168 </para>
169 <para>Here is an example set of .config symbols to enable/disable kdb:
170 <itemizedlist>
171 <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
172 <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
173 <listitem><para>CONFIG_KGDB=y</para></listitem>
174 <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
175 <listitem><para>CONFIG_KGDB_KDB=y</para></listitem>
176 <listitem><para>CONFIG_KDB_KEYBOARD=y</para></listitem>
177 </itemizedlist>
178 </para>
179 </sect1>
180 </chapter>
181 <chapter id="kgdbKernelArgs">
182 <title>Kernel Debugger Boot Arguments</title>
183 <para>This section describes the various runtime kernel
184 parameters that affect the configuration of the kernel debugger.
185 The following chapter covers using kdb and kgdb as well as
186 provides some examples of the configuration parameters.</para>
187 <sect1 id="kgdboc">
188 <title>Kernel parameter: kgdboc</title>
189 <para>The kgdboc driver was originally an abbreviation meant to
190 stand for "kgdb over console". Today it is the primary mechanism
191 to configure how to communicate from gdb to kgdb as well as the
192 devices you want to use to interact with the kdb shell.
193 </para>
194 <para>For kgdb/gdb, kgdboc is designed to work with a single serial
195 port. It is intended to cover the circumstance where you want to
196 use a serial console as your primary console as well as using it to
197 perform kernel debugging. It is also possible to use kgdb on a
198 serial port which is not designated as a system console. Kgdboc
199 may be configured as a kernel built-in or a kernel loadable module.
200 You can only make use of <constant>kgdbwait</constant> and early
201 debugging if you build kgdboc into the kernel as a built-in.
202 <para>Optionally you can elect to activate kms (Kernel Mode
203 Setting) integration. When you use kms with kgdboc and you have a
204 video driver that has atomic mode setting hooks, it is possible to
205 enter the debugger on the graphics console. When the kernel
206 execution is resumed, the previous graphics mode will be restored.
207 This integration can serve as a useful tool to aid in diagnosing
208 crashes or doing analysis of memory with kdb while allowing the
209 full graphics console applications to run.
210 </para>
211 </para>
212 <sect2 id="kgdbocArgs">
213 <title>kgdboc arguments</title>
214 <para>Usage: <constant>kgdboc=[kms][[,]kbd][[,]serial_device][,baud]</constant></para>
215 <para>The order listed above must be observed if you use any of the
216 optional configurations together.
217 </para>
218 <para>Abbreviations:
219 <itemizedlist>
220 <listitem><para>kms = Kernel Mode Setting</para></listitem>
221 <listitem><para>kbd = Keyboard</para></listitem>
222 </itemizedlist>
223 </para>
224 <para>You can configure kgdboc to use the keyboard, and or a serial
225 device depending on if you are using kdb and or kgdb, in one of the
226 following scenarios. The order listed above must be observed if
227 you use any of the optional configurations together. Using kms +
228 only gdb is generally not a useful combination.</para>
229 <sect3 id="kgdbocArgs1">
230 <title>Using loadable module or built-in</title>
231 <para>
232 <orderedlist>
233 <listitem><para>As a kernel built-in:</para>
234 <para>Use the kernel boot argument: <constant>kgdboc=<tty-device>,[baud]</constant></para></listitem>
235 <listitem>
236 <para>As a kernel loadable module:</para>
237 <para>Use the command: <constant>modprobe kgdboc kgdboc=<tty-device>,[baud]</constant></para>
238 <para>Here are two examples of how you might format the kgdboc
239 string. The first is for an x86 target using the first serial port.
240 The second example is for the ARM Versatile AB using the second
241 serial port.
242 <orderedlist>
243 <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
244 <listitem><para><constant>kgdboc=ttyAMA1,115200</constant></para></listitem>
245 </orderedlist>
246 </para>
247 </listitem>
248 </orderedlist></para>
249 </sect3>
250 <sect3 id="kgdbocArgs2">
251 <title>Configure kgdboc at runtime with sysfs</title>
252 <para>At run time you can enable or disable kgdboc by echoing a
253 parameters into the sysfs. Here are two examples:</para>
254 <orderedlist>
255 <listitem><para>Enable kgdboc on ttyS0</para>
256 <para><constant>echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
257 <listitem><para>Disable kgdboc</para>
258 <para><constant>echo "" > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
259 </orderedlist>
260 <para>NOTE: You do not need to specify the baud if you are
261 configuring the console on tty which is already configured or
262 open.</para>
263 </sect3>
264 <sect3 id="kgdbocArgs3">
265 <title>More examples</title>
266 <para>You can configure kgdboc to use the keyboard, and or a serial
267 device depending on if you are using kdb and or kgdb, in one of the
268 following scenarios.</para>
269 <para>You can configure kgdboc to use the keyboard, and or a serial device
270 depending on if you are using kdb and or kgdb, in one of the
271 following scenarios.
272 <orderedlist>
273 <listitem><para>kdb and kgdb over only a serial port</para>
274 <para><constant>kgdboc=<serial_device>[,baud]</constant></para>
275 <para>Example: <constant>kgdboc=ttyS0,115200</constant></para>
276 </listitem>
277 <listitem><para>kdb and kgdb with keyboard and a serial port</para>
278 <para><constant>kgdboc=kbd,<serial_device>[,baud]</constant></para>
279 <para>Example: <constant>kgdboc=kbd,ttyS0,115200</constant></para>
280 </listitem>
281 <listitem><para>kdb with a keyboard</para>
282 <para><constant>kgdboc=kbd</constant></para>
283 </listitem>
284 <listitem><para>kdb with kernel mode setting</para>
285 <para><constant>kgdboc=kms,kbd</constant></para>
286 </listitem>
287 <listitem><para>kdb with kernel mode setting and kgdb over a serial port</para>
288 <para><constant>kgdboc=kms,kbd,ttyS0,115200</constant></para>
289 </listitem>
290 </orderedlist>
291 </para>
292 </sect3>
293 <para>NOTE: Kgdboc does not support interrupting the target via the
294 gdb remote protocol. You must manually send a sysrq-g unless you
295 have a proxy that splits console output to a terminal program.
296 A console proxy has a separate TCP port for the debugger and a separate
297 TCP port for the "human" console. The proxy can take care of sending
298 the sysrq-g for you.
299 </para>
300 <para>When using kgdboc with no debugger proxy, you can end up
301 connecting the debugger at one of two entry points. If an
302 exception occurs after you have loaded kgdboc, a message should
303 print on the console stating it is waiting for the debugger. In
304 this case you disconnect your terminal program and then connect the
305 debugger in its place. If you want to interrupt the target system
306 and forcibly enter a debug session you have to issue a Sysrq
307 sequence and then type the letter <constant>g</constant>. Then
308 you disconnect the terminal session and connect gdb. Your options
309 if you don't like this are to hack gdb to send the sysrq-g for you
310 as well as on the initial connect, or to use a debugger proxy that
311 allows an unmodified gdb to do the debugging.
312 </para>
313 </sect2>
314 </sect1>
315 <sect1 id="kgdbwait">
316 <title>Kernel parameter: kgdbwait</title>
317 <para>
318 The Kernel command line option <constant>kgdbwait</constant> makes
319 kgdb wait for a debugger connection during booting of a kernel. You
320 can only use this option you compiled a kgdb I/O driver into the
321 kernel and you specified the I/O driver configuration as a kernel
322 command line option. The kgdbwait parameter should always follow the
323 configuration parameter for the kgdb I/O driver in the kernel
324 command line else the I/O driver will not be configured prior to
325 asking the kernel to use it to wait.
326 </para>
327 <para>
328 The kernel will stop and wait as early as the I/O driver and
329 architecture allows when you use this option. If you build the
330 kgdb I/O driver as a loadable kernel module kgdbwait will not do
331 anything.
332 </para>
333 </sect1>
334 <sect1 id="kgdbcon">
335 <title>Kernel parameter: kgdbcon</title>
336 <para> The kgdbcon feature allows you to see printk() messages
337 inside gdb while gdb is connected to the kernel. Kdb does not make
338 use of the kgdbcon feature.
339 </para>
340 <para>Kgdb supports using the gdb serial protocol to send console
341 messages to the debugger when the debugger is connected and running.
342 There are two ways to activate this feature.
343 <orderedlist>
344 <listitem><para>Activate with the kernel command line option:</para>
345 <para><constant>kgdbcon</constant></para>
346 </listitem>
347 <listitem><para>Use sysfs before configuring an I/O driver</para>
348 <para>
349 <constant>echo 1 > /sys/module/kgdb/parameters/kgdb_use_con</constant>
350 </para>
351 <para>
352 NOTE: If you do this after you configure the kgdb I/O driver, the
353 setting will not take effect until the next point the I/O is
354 reconfigured.
355 </para>
356 </listitem>
357 </orderedlist>
358 <para>IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an
359 active system console. An example incorrect usage is <constant>console=ttyS0,115200 kgdboc=ttyS0 kgdbcon</constant>
360 </para>
361 <para>It is possible to use this option with kgdboc on a tty that is not a system console.
362 </para>
363 </para>
364 </sect1>
365 </chapter>
366 <chapter id="usingKDB">
367 <title>Using kdb</title>
368 <para>
369 </para>
370 <sect1 id="quickKDBserial">
371 <title>Quick start for kdb on a serial port</title>
372 <para>This is a quick example of how to use kdb.</para>
373 <para><orderedlist>
374 <listitem><para>Boot kernel with arguments:
375 <itemizedlist>
376 <listitem><para><constant>console=ttyS0,115200 kgdboc=ttyS0,115200</constant></para></listitem>
377 </itemizedlist></para>
378 <para>OR</para>
379 <para>Configure kgdboc after the kernel booted; assuming you are using a serial port console:
380 <itemizedlist>
381 <listitem><para><constant>echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
382 </itemizedlist>
383 </para>
384 </listitem>
385 <listitem><para>Enter the kernel debugger manually or by waiting for an oops or fault. There are several ways you can enter the kernel debugger manually; all involve using the sysrq-g, which means you must have enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.</para>
386 <itemizedlist>
387 <listitem><para>When logged in as root or with a super user session you can run:</para>
388 <para><constant>echo g > /proc/sysrq-trigger</constant></para></listitem>
389 <listitem><para>Example using minicom 2.2</para>
390 <para>Press: <constant>Control-a</constant></para>
391 <para>Press: <constant>f</constant></para>
392 <para>Press: <constant>g</constant></para>
393 </listitem>
394 <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
395 <para>Press: <constant>Control-]</constant></para>
396 <para>Type in:<constant>send break</constant></para>
397 <para>Press: <constant>Enter</constant></para>
398 <para>Press: <constant>g</constant></para>
399 </listitem>
400 </itemizedlist>
401 </listitem>
402 <listitem><para>From the kdb prompt you can run the "help" command to see a complete list of the commands that are available.</para>
403 <para>Some useful commands in kdb include:
404 <itemizedlist>
405 <listitem><para>lsmod -- Shows where kernel modules are loaded</para></listitem>
406 <listitem><para>ps -- Displays only the active processes</para></listitem>
407 <listitem><para>ps A -- Shows all the processes</para></listitem>
408 <listitem><para>summary -- Shows kernel version info and memory usage</para></listitem>
409 <listitem><para>bt -- Get a backtrace of the current process using dump_stack()</para></listitem>
410 <listitem><para>dmesg -- View the kernel syslog buffer</para></listitem>
411 <listitem><para>go -- Continue the system</para></listitem>
412 </itemizedlist>
413 </para>
414 </listitem>
415 <listitem>
416 <para>When you are done using kdb you need to consider rebooting the
417 system or using the "go" command to resuming normal kernel
418 execution. If you have paused the kernel for a lengthy period of
419 time, applications that rely on timely networking or anything to do
420 with real wall clock time could be adversely affected, so you
421 should take this into consideration when using the kernel
422 debugger.</para>
423 </listitem>
424 </orderedlist></para>
425 </sect1>
426 <sect1 id="quickKDBkeyboard">
427 <title>Quick start for kdb using a keyboard connected console</title>
428 <para>This is a quick example of how to use kdb with a keyboard.</para>
429 <para><orderedlist>
430 <listitem><para>Boot kernel with arguments:
431 <itemizedlist>
432 <listitem><para><constant>kgdboc=kbd</constant></para></listitem>
433 </itemizedlist></para>
434 <para>OR</para>
435 <para>Configure kgdboc after the kernel booted:
436 <itemizedlist>
437 <listitem><para><constant>echo kbd > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
438 </itemizedlist>
439 </para>
440 </listitem>
441 <listitem><para>Enter the kernel debugger manually or by waiting for an oops or fault. There are several ways you can enter the kernel debugger manually; all involve using the sysrq-g, which means you must have enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.</para>
442 <itemizedlist>
443 <listitem><para>When logged in as root or with a super user session you can run:</para>
444 <para><constant>echo g > /proc/sysrq-trigger</constant></para></listitem>
445 <listitem><para>Example using a laptop keyboard</para>
446 <para>Press and hold down: <constant>Alt</constant></para>
447 <para>Press and hold down: <constant>Fn</constant></para>
448 <para>Press and release the key with the label: <constant>SysRq</constant></para>
449 <para>Release: <constant>Fn</constant></para>
450 <para>Press and release: <constant>g</constant></para>
451 <para>Release: <constant>Alt</constant></para>
452 </listitem>
453 <listitem><para>Example using a PS/2 101-key keyboard</para>
454 <para>Press and hold down: <constant>Alt</constant></para>
455 <para>Press and release the key with the label: <constant>SysRq</constant></para>
456 <para>Press and release: <constant>g</constant></para>
457 <para>Release: <constant>Alt</constant></para>
458 </listitem>
459 </itemizedlist>
460 </listitem>
461 <listitem>
462 <para>Now type in a kdb command such as "help", "dmesg", "bt" or "go" to continue kernel execution.</para>
463 </listitem>
464 </orderedlist></para>
465 </sect1>
466 </chapter>
467 <chapter id="EnableKGDB">
468 <title>Using kgdb / gdb</title>
469 <para>In order to use kgdb you must activate it by passing
470 configuration information to one of the kgdb I/O drivers. If you
471 do not pass any configuration information kgdb will not do anything
472 at all. Kgdb will only actively hook up to the kernel trap hooks
473 if a kgdb I/O driver is loaded and configured. If you unconfigure
474 a kgdb I/O driver, kgdb will unregister all the kernel hook points.
475 </para>
476 <para> All kgdb I/O drivers can be reconfigured at run time, if
477 <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
478 are enabled, by echo'ing a new config string to
479 <constant>/sys/module/<driver>/parameter/<option></constant>.
480 The driver can be unconfigured by passing an empty string. You cannot
481 change the configuration while the debugger is attached. Make sure
482 to detach the debugger with the <constant>detach</constant> command
483 prior to trying to unconfigure a kgdb I/O driver.
484 </para>
485 <sect1 id="ConnectingGDB">
486 <title>Connecting with gdb to a serial port</title>
487 <orderedlist>
488 <listitem><para>Configure kgdboc</para>
489 <para>Boot kernel with arguments:
490 <itemizedlist>
491 <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
492 </itemizedlist></para>
493 <para>OR</para>
494 <para>Configure kgdboc after the kernel booted:
495 <itemizedlist>
496 <listitem><para><constant>echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
497 </itemizedlist></para>
498 </listitem>
499 <listitem>
500 <para>Stop kernel execution (break into the debugger)</para>
501 <para>In order to connect to gdb via kgdboc, the kernel must
502 first be stopped. There are several ways to stop the kernel which
503 include using kgdbwait as a boot argument, via a sysrq-g, or running
504 the kernel until it takes an exception where it waits for the
505 debugger to attach.
506 <itemizedlist>
507 <listitem><para>When logged in as root or with a super user session you can run:</para>
508 <para><constant>echo g > /proc/sysrq-trigger</constant></para></listitem>
509 <listitem><para>Example using minicom 2.2</para>
510 <para>Press: <constant>Control-a</constant></para>
511 <para>Press: <constant>f</constant></para>
512 <para>Press: <constant>g</constant></para>
513 </listitem>
514 <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
515 <para>Press: <constant>Control-]</constant></para>
516 <para>Type in:<constant>send break</constant></para>
517 <para>Press: <constant>Enter</constant></para>
518 <para>Press: <constant>g</constant></para>
519 </listitem>
520 </itemizedlist>
521 </para>
522 </listitem>
523 <listitem>
524 <para>Connect from from gdb</para>
525 <para>
526 Example (using a directly connected port):
527 </para>
528 <programlisting>
529 % gdb ./vmlinux
530 (gdb) set remotebaud 115200
531 (gdb) target remote /dev/ttyS0
532 </programlisting>
533 <para>
534 Example (kgdb to a terminal server on TCP port 2012):
535 </para>
536 <programlisting>
537 % gdb ./vmlinux
538 (gdb) target remote 192.168.2.2:2012
539 </programlisting>
540 <para>
541 Once connected, you can debug a kernel the way you would debug an
542 application program.
543 </para>
544 <para>
545 If you are having problems connecting or something is going
546 seriously wrong while debugging, it will most often be the case
547 that you want to enable gdb to be verbose about its target
548 communications. You do this prior to issuing the <constant>target
549 remote</constant> command by typing in: <constant>set debug remote 1</constant>
550 </para>
551 </listitem>
552 </orderedlist>
553 <para>Remember if you continue in gdb, and need to "break in" again,
554 you need to issue an other sysrq-g. It is easy to create a simple
555 entry point by putting a breakpoint at <constant>sys_sync</constant>
556 and then you can run "sync" from a shell or script to break into the
557 debugger.</para>
558 </sect1>
559 </chapter>
560 <chapter id="switchKdbKgdb">
561 <title>kgdb and kdb interoperability</title>
562 <para>It is possible to transition between kdb and kgdb dynamically.
563 The debug core will remember which you used the last time and
564 automatically start in the same mode.</para>
565 <sect1>
566 <title>Switching between kdb and kgdb</title>
567 <sect2>
568 <title>Switching from kgdb to kdb</title>
569 <para>
570 There are two ways to switch from kgdb to kdb: you can use gdb to
571 issue a maintenance packet, or you can blindly type the command $3#33.
572 Whenever kernel debugger stops in kgdb mode it will print the
573 message <constant>KGDB or $3#33 for KDB</constant>. It is important
574 to note that you have to type the sequence correctly in one pass.
575 You cannot type a backspace or delete because kgdb will interpret
576 that as part of the debug stream.
577 <orderedlist>
578 <listitem><para>Change from kgdb to kdb by blindly typing:</para>
579 <para><constant>$3#33</constant></para></listitem>
580 <listitem><para>Change from kgdb to kdb with gdb</para>
581 <para><constant>maintenance packet 3</constant></para>
582 <para>NOTE: Now you must kill gdb. Typically you press control-z and
583 issue the command: kill -9 %</para></listitem>
584 </orderedlist>
585 </para>
586 </sect2>
587 <sect2>
588 <title>Change from kdb to kgdb</title>
589 <para>There are two ways you can change from kdb to kgdb. You can
590 manually enter kgdb mode by issuing the kgdb command from the kdb
591 shell prompt, or you can connect gdb while the kdb shell prompt is
592 active. The kdb shell looks for the typical first commands that gdb
593 would issue with the gdb remote protocol and if it sees one of those
594 commands it automatically changes into kgdb mode.</para>
595 <orderedlist>
596 <listitem><para>From kdb issue the command:</para>
597 <para><constant>kgdb</constant></para>
598 <para>Now disconnect your terminal program and connect gdb in its place</para></listitem>
599 <listitem><para>At the kdb prompt, disconnect the terminal program and connect gdb in its place.</para></listitem>
600 </orderedlist>
601 </sect2>
602 </sect1>
603 <sect1>
604 <title>Running kdb commands from gdb</title>
605 <para>It is possible to run a limited set of kdb commands from gdb,
606 using the gdb monitor command. You don't want to execute any of the
607 run control or breakpoint operations, because it can disrupt the
608 state of the kernel debugger. You should be using gdb for
609 breakpoints and run control operations if you have gdb connected.
610 The more useful commands to run are things like lsmod, dmesg, ps or
611 possibly some of the memory information commands. To see all the kdb
612 commands you can run <constant>monitor help</constant>.</para>
613 <para>Example:
614 <informalexample><programlisting>
615(gdb) monitor ps
6161 idle process (state I) and
61727 sleeping system daemon (state M) processes suppressed,
618use 'ps A' to see all.
619Task Addr Pid Parent [*] cpu State Thread Command
620
6210xc78291d0 1 0 0 0 S 0xc7829404 init
6220xc7954150 942 1 0 0 S 0xc7954384 dropbear
6230xc78789c0 944 1 0 0 S 0xc7878bf4 sh
624(gdb)
625 </programlisting></informalexample>
626 </para>
627 </sect1>
628 </chapter>
629 <chapter id="KGDBTestSuite">
630 <title>kgdb Test Suite</title>
631 <para>
632 When kgdb is enabled in the kernel config you can also elect to
633 enable the config parameter KGDB_TESTS. Turning this on will
634 enable a special kgdb I/O module which is designed to test the
635 kgdb internal functions.
636 </para>
637 <para>
638 The kgdb tests are mainly intended for developers to test the kgdb
639 internals as well as a tool for developing a new kgdb architecture
640 specific implementation. These tests are not really for end users
641 of the Linux kernel. The primary source of documentation would be
642 to look in the drivers/misc/kgdbts.c file.
643 </para>
644 <para>
645 The kgdb test suite can also be configured at compile time to run
646 the core set of tests by setting the kernel config parameter
647 KGDB_TESTS_ON_BOOT. This particular option is aimed at automated
648 regression testing and does not require modifying the kernel boot
649 config arguments. If this is turned on, the kgdb test suite can
650 be disabled by specifying "kgdbts=" as a kernel boot argument.
651 </para>
652 </chapter>
653 <chapter id="CommonBackEndReq">
654 <title>Kernel Debugger Internals</title>
655 <sect1 id="kgdbArchitecture">
656 <title>Architecture Specifics</title>
657 <para>
658 The kernel debugger is organized into a number of components:
659 <orderedlist>
660 <listitem><para>The debug core</para>
661 <para>
662 The debug core is found in kernel/debugger/debug_core.c. It contains:
663 <itemizedlist>
664 <listitem><para>A generic OS exception handler which includes
665 sync'ing the processors into a stopped state on an multi-CPU
666 system.</para></listitem>
667 <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
668 <listitem><para>The API to make calls to the arch-specific kgdb implementation</para></listitem>
669 <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
670 <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
671 <listitem><para>The API to invoke either the kdb or kgdb frontend to the debug core.</para></listitem>
672 <listitem><para>The structures and callback API for atomic kernel mode setting.</para>
673 <para>NOTE: kgdboc is where the kms callbacks are invoked.</para></listitem>
674 </itemizedlist>
675 </para>
676 </listitem>
677 <listitem><para>kgdb arch-specific implementation</para>
678 <para>
679 This implementation is generally found in arch/*/kernel/kgdb.c.
680 As an example, arch/x86/kernel/kgdb.c contains the specifics to
681 implement HW breakpoint as well as the initialization to
682 dynamically register and unregister for the trap handlers on
683 this architecture. The arch-specific portion implements:
684 <itemizedlist>
685 <listitem><para>contains an arch-specific trap catcher which
686 invokes kgdb_handle_exception() to start kgdb about doing its
687 work</para></listitem>
688 <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
689 <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
690 <listitem><para>Any special exception handling and cleanup</para></listitem>
691 <listitem><para>NMI exception handling and cleanup</para></listitem>
692 <listitem><para>(optional)HW breakpoints</para></listitem>
693 </itemizedlist>
694 </para>
695 </listitem>
696 <listitem><para>gdbstub frontend (aka kgdb)</para>
697 <para>The gdbstub is located in kernel/debug/gdbstub.c. It contains:</para>
698 <itemizedlist>
699 <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
700 </itemizedlist>
701 </listitem>
702 <listitem><para>kdb frontend</para>
703 <para>The kdb debugger shell is broken down into a number of
704 components. The kdb core is located in kernel/debug/kdb. There
705 are a number of helper functions in some of the other kernel
706 components to make it possible for kdb to examine and report
707 information about the kernel without taking locks that could
708 cause a kernel deadlock. The kdb core contains implements the following functionality.</para>
709 <itemizedlist>
710 <listitem><para>A simple shell</para></listitem>
711 <listitem><para>The kdb core command set</para></listitem>
712 <listitem><para>A registration API to register additional kdb shell commands.</para>
713 <itemizedlist>
714 <listitem><para>A good example of a self-contained kdb module
715 is the "ftdump" command for dumping the ftrace buffer. See:
716 kernel/trace/trace_kdb.c</para></listitem>
717 <listitem><para>For an example of how to dynamically register
718 a new kdb command you can build the kdb_hello.ko kernel module
719 from samples/kdb/kdb_hello.c. To build this example you can
720 set CONFIG_SAMPLES=y and CONFIG_SAMPLE_KDB=m in your kernel
721 config. Later run "modprobe kdb_hello" and the next time you
722 enter the kdb shell, you can run the "hello"
723 command.</para></listitem>
724 </itemizedlist></listitem>
725 <listitem><para>The implementation for kdb_printf() which
726 emits messages directly to I/O drivers, bypassing the kernel
727 log.</para></listitem>
728 <listitem><para>SW / HW breakpoint management for the kdb shell</para></listitem>
729 </itemizedlist>
730 </listitem>
731 <listitem><para>kgdb I/O driver</para>
732 <para>
733 Each kgdb I/O driver has to provide an implementation for the following:
734 <itemizedlist>
735 <listitem><para>configuration via built-in or module</para></listitem>
736 <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
737 <listitem><para>read and write character interface</para></listitem>
738 <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
739 <listitem><para>(optional) Early debug methodology</para></listitem>
740 </itemizedlist>
741 Any given kgdb I/O driver has to operate very closely with the
742 hardware and must do it in such a way that does not enable
743 interrupts or change other parts of the system context without
744 completely restoring them. The kgdb core will repeatedly "poll"
745 a kgdb I/O driver for characters when it needs input. The I/O
746 driver is expected to return immediately if there is no data
747 available. Doing so allows for the future possibility to touch
748 watch dog hardware in such a way as to have a target system not
749 reset when these are enabled.
750 </para>
751 </listitem>
752 </orderedlist>
753 </para>
754 <para>
755 If you are intent on adding kgdb architecture specific support
756 for a new architecture, the architecture should define
757 <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
758 Kconfig file. This will enable kgdb for the architecture, and
759 at that point you must create an architecture specific kgdb
760 implementation.
761 </para>
762 <para>
763 There are a few flags which must be set on every architecture in
764 their <asm/kgdb.h> file. These are:
765 <itemizedlist>
766 <listitem>
767 <para>
768 NUMREGBYTES: The size in bytes of all of the registers, so
769 that we can ensure they will all fit into a packet.
770 </para>
771 <para>
772 BUFMAX: The size in bytes of the buffer GDB will read into.
773 This must be larger than NUMREGBYTES.
774 </para>
775 <para>
776 CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
777 flush_cache_range or flush_icache_range. On some architectures,
778 these functions may not be safe to call on SMP since we keep other
779 CPUs in a holding pattern.
780 </para>
781 </listitem>
782 </itemizedlist>
783 </para>
784 <para>
785 There are also the following functions for the common backend,
786 found in kernel/kgdb.c, that must be supplied by the
787 architecture-specific backend unless marked as (optional), in
788 which case a default function maybe used if the architecture
789 does not need to provide a specific implementation.
790 </para>
791!Iinclude/linux/kgdb.h
792 </sect1>
793 <sect1 id="kgdbocDesign">
794 <title>kgdboc internals</title>
795 <sect2>
796 <title>kgdboc and uarts</title>
797 <para>
798 The kgdboc driver is actually a very thin driver that relies on the
799 underlying low level to the hardware driver having "polling hooks"
800 which the to which the tty driver is attached. In the initial
801 implementation of kgdboc it the serial_core was changed to expose a
802 low level UART hook for doing polled mode reading and writing of a
803 single character while in an atomic context. When kgdb makes an I/O
804 request to the debugger, kgdboc invokes a callback in the serial
805 core which in turn uses the callback in the UART driver.</para>
806 <para>
807 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>
808#ifdef CONFIG_CONSOLE_POLL
809 .poll_get_char = serial8250_get_poll_char,
810 .poll_put_char = serial8250_put_poll_char,
811#endif
812 </programlisting>
813 Any implementation specifics around creating a polling driver use the
814 <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
815 Keep in mind that polling hooks have to be implemented in such a way
816 that they can be called from an atomic context and have to restore
817 the state of the UART chip on return such that the system can return
818 to normal when the debugger detaches. You need to be very careful
819 with any kind of lock you consider, because failing here is most likely
820 going to mean pressing the reset button.
821 </para>
822 </sect2>
823 <sect2 id="kgdbocKbd">
824 <title>kgdboc and keyboards</title>
825 <para>The kgdboc driver contains logic to configure communications
826 with an attached keyboard. The keyboard infrastructure is only
827 compiled into the kernel when CONFIG_KDB_KEYBOARD=y is set in the
828 kernel configuration.</para>
829 <para>The core polled keyboard driver driver for PS/2 type keyboards
830 is in drivers/char/kdb_keyboard.c. This driver is hooked into the
831 debug core when kgdboc populates the callback in the array
832 called <constant>kdb_poll_funcs[]</constant>. The
833 kdb_get_kbd_char() is the top-level function which polls hardware
834 for single character input.
835 </para>
836 </sect2>
837 <sect2 id="kgdbocKms">
838 <title>kgdboc and kms</title>
839 <para>The kgdboc driver contains logic to request the graphics
840 display to switch to a text context when you are using
841 "kgdboc=kms,kbd", provided that you have a video driver which has a
842 frame buffer console and atomic kernel mode setting support.</para>
843 <para>
844 Every time the kernel
845 debugger is entered it calls kgdboc_pre_exp_handler() which in turn
846 calls con_debug_enter() in the virtual console layer. On resuming kernel
847 execution, the kernel debugger calls kgdboc_post_exp_handler() which
848 in turn calls con_debug_leave().</para>
849 <para>Any video driver that wants to be compatible with the kernel
850 debugger and the atomic kms callbacks must implement the
851 mode_set_base_atomic, fb_debug_enter and fb_debug_leave operations.
852 For the fb_debug_enter and fb_debug_leave the option exists to use
853 the generic drm fb helper functions or implement something custom for
854 the hardware. The following example shows the initialization of the
855 .mode_set_base_atomic operation in
856 drivers/gpu/drm/i915/intel_display.c:
857 <informalexample>
858 <programlisting>
859static const struct drm_crtc_helper_funcs intel_helper_funcs = {
860[...]
861 .mode_set_base_atomic = intel_pipe_set_base_atomic,
862[...]
863};
864 </programlisting>
865 </informalexample>
866 </para>
867 <para>Here is an example of how the i915 driver initializes the fb_debug_enter and fb_debug_leave functions to use the generic drm helpers in
868 drivers/gpu/drm/i915/intel_fb.c:
869 <informalexample>
870 <programlisting>
871static struct fb_ops intelfb_ops = {
872[...]
873 .fb_debug_enter = drm_fb_helper_debug_enter,
874 .fb_debug_leave = drm_fb_helper_debug_leave,
875[...]
876};
877 </programlisting>
878 </informalexample>
879 </para>
880 </sect2>
881 </sect1>
882 </chapter>
883 <chapter id="credits">
884 <title>Credits</title>
885 <para>
886 The following people have contributed to this document:
887 <orderedlist>
888 <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
889 <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
890 </orderedlist>
891 In March 2008 this document was completely rewritten by:
892 <itemizedlist>
893 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
894 </itemizedlist>
895 In Jan 2010 this document was updated to include kdb.
896 <itemizedlist>
897 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
898 </itemizedlist>
899 </para>
900 </chapter>
901</book>
902