1 =================================================
2 Using kgdb, kdb and the kernel debugger internals
3 =================================================
4
5 :Author: Jason Wessel
6
7 Introduction
8 ============
9
10 The kernel has two different debugger front ends (kdb and kgdb) which
11 interface to the debug core. It is possible to use either of the
12 debugger front ends and dynamically transition between them if you
13 configure the kernel properly at compile and runtime.
14
15 Kdb is simplistic shell-style interface which you can use on a system
16 console with a keyboard or serial console. You can use it to inspect
17 memory, registers, process lists, dmesg, and even set breakpoints to
18 stop in a certain location. Kdb is not a source level debugger, although
19 you can set breakpoints and execute some basic kernel run control. Kdb
20 is mainly aimed at doing some analysis to aid in development or
21 diagnosing kernel problems. You can access some symbols by name in
22 kernel built-ins or in kernel modules if the code was built with
23 ``CONFIG_KALLSYMS``.
24
25 Kgdb is intended to be used as a source level debugger for the Linux
26 kernel. It is used along with gdb to debug a Linux kernel. The
27 expectation is that gdb can be used to "break in" to the kernel to
28 inspect memory, variables and look through call stack information
29 similar to the way an application developer would use gdb to debug an
30 application. It is possible to place breakpoints in kernel code and
31 perform some limited execution stepping.
32
33 Two machines are required for using kgdb. One of these machines is a
34 development machine and the other is the target machine. The kernel to
35 be debugged runs on the target machine. The development machine runs an
36 instance of gdb against the vmlinux file which contains the symbols (not
37 a boot image such as bzImage, zImage, uImage...). In gdb the developer
38 specifies the connection parameters and connects to kgdb. The type of
39 connection a developer makes with gdb depends on the availability of
40 kgdb I/O modules compiled as built-ins or loadable kernel modules in the
41 test machine's kernel.
42
43 Compiling a kernel
44 ==================
45
46 - In order to enable compilation of kdb, you must first enable kgdb.
47
48 - The kgdb test compile options are described in the kgdb test suite
49 chapter.
50
51 Kernel config options for kgdb
52 ------------------------------
53
54 To enable ``CONFIG_KGDB`` you should look under
55 :menuselection:`Kernel hacking --> Kernel debugging` and select
56 :menuselection:`KGDB: kernel debugger`.
57
58 While it is not a hard requirement that you have symbols in your vmlinux
59 file, gdb tends not to be very useful without the symbolic data, so you
60 will want to turn on ``CONFIG_DEBUG_INFO`` which is called
61 :menuselection:`Compile the kernel with debug info` in the config menu.
62
63 It is advised, but not required, that you turn on the
64 ``CONFIG_FRAME_POINTER`` kernel option which is called :menuselection:`Compile
65 the kernel with frame pointers` in the config menu. This option inserts code
66 into the compiled executable which saves the frame information in registers
67 or on the stack at different points which allows a debugger such as gdb to
68 more accurately construct stack back traces while debugging the kernel.
69
70 If the architecture that you are using supports the kernel option
71 ``CONFIG_STRICT_KERNEL_RWX``, you should consider turning it off. This
72 option will prevent the use of software breakpoints because it marks
73 certain regions of the kernel's memory space as read-only. If kgdb
74 supports it for the architecture you are using, you can use hardware
75 breakpoints if you desire to run with the ``CONFIG_STRICT_KERNEL_RWX``
76 option turned on, else you need to turn off this option.
77
78 Next you should choose one of more I/O drivers to interconnect debugging
79 host and debugged target. Early boot debugging requires a KGDB I/O
80 driver that supports early debugging and the driver must be built into
81 the kernel directly. Kgdb I/O driver configuration takes place via
82 kernel or module parameters which you can learn more about in the in the
83 section that describes the parameter kgdboc.
84
85 Here is an example set of ``.config`` symbols to enable or disable for kgdb::
86
87 # CONFIG_STRICT_KERNEL_RWX is not set
88 CONFIG_FRAME_POINTER=y
89 CONFIG_KGDB=y
90 CONFIG_KGDB_SERIAL_CONSOLE=y
91
92 Kernel config options for kdb
93 -----------------------------
94
95 Kdb is quite a bit more complex than the simple gdbstub sitting on top
96 of the kernel's debug core. Kdb must implement a shell, and also adds
97 some helper functions in other parts of the kernel, responsible for
98 printing out interesting data such as what you would see if you ran
99 ``lsmod``, or ``ps``. In order to build kdb into the kernel you follow the
100 same steps as you would for kgdb.
101
102 The main config option for kdb is ``CONFIG_KGDB_KDB`` which is called
103 :menuselection:`KGDB_KDB: include kdb frontend for kgdb` in the config menu.
104 In theory you would have already also selected an I/O driver such as the
105 ``CONFIG_KGDB_SERIAL_CONSOLE`` interface if you plan on using kdb on a
106 serial port, when you were configuring kgdb.
107
108 If you want to use a PS/2-style keyboard with kdb, you would select
109 ``CONFIG_KDB_KEYBOARD`` which is called :menuselection:`KGDB_KDB: keyboard as
110 input device` in the config menu. The ``CONFIG_KDB_KEYBOARD`` option is not
111 used for anything in the gdb interface to kgdb. The ``CONFIG_KDB_KEYBOARD``
112 option only works with kdb.
113
114 Here is an example set of ``.config`` symbols to enable/disable kdb::
115
116 # CONFIG_STRICT_KERNEL_RWX is not set
117 CONFIG_FRAME_POINTER=y
118 CONFIG_KGDB=y
119 CONFIG_KGDB_SERIAL_CONSOLE=y
120 CONFIG_KGDB_KDB=y
121 CONFIG_KDB_KEYBOARD=y
122
123 Kernel Debugger Boot Arguments
124 ==============================
125
126 This section describes the various runtime kernel parameters that affect
127 the configuration of the kernel debugger. The following chapter covers
128 using kdb and kgdb as well as providing some examples of the
129 configuration parameters.
130
131 Kernel parameter: kgdboc
132 ------------------------
133
134 The kgdboc driver was originally an abbreviation meant to stand for
135 "kgdb over console". Today it is the primary mechanism to configure how
136 to communicate from gdb to kgdb as well as the devices you want to use
137 to interact with the kdb shell.
138
139 For kgdb/gdb, kgdboc is designed to work with a single serial port. It
140 is intended to cover the circumstance where you want to use a serial
141 console as your primary console as well as using it to perform kernel
142 debugging. It is also possible to use kgdb on a serial port which is not
143 designated as a system console. Kgdboc may be configured as a kernel
144 built-in or a kernel loadable module. You can only make use of
145 ``kgdbwait`` and early debugging if you build kgdboc into the kernel as
146 a built-in.
147
148 Optionally you can elect to activate kms (Kernel Mode Setting)
149 integration. When you use kms with kgdboc and you have a video driver
150 that has atomic mode setting hooks, it is possible to enter the debugger
151 on the graphics console. When the kernel execution is resumed, the
152 previous graphics mode will be restored. This integration can serve as a
153 useful tool to aid in diagnosing crashes or doing analysis of memory
154 with kdb while allowing the full graphics console applications to run.
155
156 kgdboc arguments
157 ~~~~~~~~~~~~~~~~
158
159 Usage::
160
161 kgdboc=[kms][[,]kbd][[,]serial_device][,baud]
162
163 The order listed above must be observed if you use any of the optional
164 configurations together.
165
166 Abbreviations:
167
168 - kms = Kernel Mode Setting
169
170 - kbd = Keyboard
171
172 You can configure kgdboc to use the keyboard, and/or a serial device
173 depending on if you are using kdb and/or kgdb, in one of the following
174 scenarios. The order listed above must be observed if you use any of the
175 optional configurations together. Using kms + only gdb is generally not
176 a useful combination.
177
178 Using loadable module or built-in
179 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
180
181 1. As a kernel built-in:
182
183 Use the kernel boot argument::
184
185 kgdboc=<tty-device>,[baud]
186
187 2. As a kernel loadable module:
188
189 Use the command::
190
191 modprobe kgdboc kgdboc=<tty-device>,[baud]
192
193 Here are two examples of how you might format the kgdboc string. The
194 first is for an x86 target using the first serial port. The second
195 example is for the ARM Versatile AB using the second serial port.
196
197 1. ``kgdboc=ttyS0,115200``
198
199 2. ``kgdboc=ttyAMA1,115200``
200
201 Configure kgdboc at runtime with sysfs
202 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
203
204 At run time you can enable or disable kgdboc by echoing a parameters
205 into the sysfs. Here are two examples:
206
207 1. Enable kgdboc on ttyS0::
208
209 echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
210
211 2. Disable kgdboc::
212
213 echo "" > /sys/module/kgdboc/parameters/kgdboc
214
215 .. note::
216
217 You do not need to specify the baud if you are configuring the
218 console on tty which is already configured or open.
219
220 More examples
221 ^^^^^^^^^^^^^
222
223 You can configure kgdboc to use the keyboard, and/or a serial device
224 depending on if you are using kdb and/or kgdb, in one of the following
225 scenarios.
226
227 1. kdb and kgdb over only a serial port::
228
229 kgdboc=<serial_device>[,baud]
230
231 Example::
232
233 kgdboc=ttyS0,115200
234
235 2. kdb and kgdb with keyboard and a serial port::
236
237 kgdboc=kbd,<serial_device>[,baud]
238
239 Example::
240
241 kgdboc=kbd,ttyS0,115200
242
243 3. kdb with a keyboard::
244
245 kgdboc=kbd
246
247 4. kdb with kernel mode setting::
248
249 kgdboc=kms,kbd
250
251 5. kdb with kernel mode setting and kgdb over a serial port::
252
253 kgdboc=kms,kbd,ttyS0,115200
254
255 .. note::
256
257 Kgdboc does not support interrupting the target via the gdb remote
258 protocol. You must manually send a :kbd:`SysRq-G` unless you have a proxy
259 that splits console output to a terminal program. A console proxy has a
260 separate TCP port for the debugger and a separate TCP port for the
261 "human" console. The proxy can take care of sending the :kbd:`SysRq-G`
262 for you.
263
264 When using kgdboc with no debugger proxy, you can end up connecting the
265 debugger at one of two entry points. If an exception occurs after you
266 have loaded kgdboc, a message should print on the console stating it is
267 waiting for the debugger. In this case you disconnect your terminal
268 program and then connect the debugger in its place. If you want to
269 interrupt the target system and forcibly enter a debug session you have
270 to issue a :kbd:`Sysrq` sequence and then type the letter :kbd:`g`. Then you
271 disconnect the terminal session and connect gdb. Your options if you
272 don't like this are to hack gdb to send the :kbd:`SysRq-G` for you as well as
273 on the initial connect, or to use a debugger proxy that allows an
274 unmodified gdb to do the debugging.
275
276 Kernel parameter: ``kgdboc_earlycon``
277 -------------------------------------
278
279 If you specify the kernel parameter ``kgdboc_earlycon`` and your serial
280 driver registers a boot console that supports polling (doesn't need
281 interrupts and implements a nonblocking read() function) kgdb will attempt
282 to work using the boot console until it can transition to the regular
283 tty driver specified by the ``kgdboc`` parameter.
284
285 Normally there is only one boot console (especially that implements the
286 read() function) so just adding ``kgdboc_earlycon`` on its own is
287 sufficient to make this work. If you have more than one boot console you
288 can add the boot console's name to differentiate. Note that names that
289 are registered through the boot console layer and the tty layer are not
290 the same for the same port.
291
292 For instance, on one board to be explicit you might do::
293
294 kgdboc_earlycon=qcom_geni kgdboc=ttyMSM0
295
296 If the only boot console on the device was "qcom_geni", you could simplify::
297
298 kgdboc_earlycon kgdboc=ttyMSM0
299
300 Kernel parameter: ``kgdbwait``
301 ------------------------------
302
303 The Kernel command line option ``kgdbwait`` makes kgdb wait for a
304 debugger connection during booting of a kernel. You can only use this
305 option if you compiled a kgdb I/O driver into the kernel and you
306 specified the I/O driver configuration as a kernel command line option.
307 The kgdbwait parameter should always follow the configuration parameter
308 for the kgdb I/O driver in the kernel command line else the I/O driver
309 will not be configured prior to asking the kernel to use it to wait.
310
311 The kernel will stop and wait as early as the I/O driver and
312 architecture allows when you use this option. If you build the kgdb I/O
313 driver as a loadable kernel module kgdbwait will not do anything.
314
315 Kernel parameter: ``kgdbcon``
316 -----------------------------
317
318 The ``kgdbcon`` feature allows you to see printk() messages inside gdb
319 while gdb is connected to the kernel. Kdb does not make use of the kgdbcon
320 feature.
321
322 Kgdb supports using the gdb serial protocol to send console messages to
323 the debugger when the debugger is connected and running. There are two
324 ways to activate this feature.
325
326 1. Activate with the kernel command line option::
327
328 kgdbcon
329
330 2. Use sysfs before configuring an I/O driver::
331
332 echo 1 > /sys/module/kgdb/parameters/kgdb_use_con
333
334 .. note::
335
336 If you do this after you configure the kgdb I/O driver, the
337 setting will not take effect until the next point the I/O is
338 reconfigured.
339
340 .. important::
341
342 You cannot use kgdboc + kgdbcon on a tty that is an
343 active system console. An example of incorrect usage is::
344
345 console=ttyS0,115200 kgdboc=ttyS0 kgdbcon
346
347 It is possible to use this option with kgdboc on a tty that is not a
348 system console.
349
350 Run time parameter: ``kgdbreboot``
351 ----------------------------------
352
353 The kgdbreboot feature allows you to change how the debugger deals with
354 the reboot notification. You have 3 choices for the behavior. The
355 default behavior is always set to 0.
356
357 .. tabularcolumns:: |p{0.4cm}|p{11.5cm}|p{5.6cm}|
358
359 .. flat-table::
360 :widths: 1 10 8
361
362 * - 1
363 - ``echo -1 > /sys/module/debug_core/parameters/kgdbreboot``
364 - Ignore the reboot notification entirely.
365
366 * - 2
367 - ``echo 0 > /sys/module/debug_core/parameters/kgdbreboot``
368 - Send the detach message to any attached debugger client.
369
370 * - 3
371 - ``echo 1 > /sys/module/debug_core/parameters/kgdbreboot``
372 - Enter the debugger on reboot notify.
373
374 Kernel parameter: ``nokaslr``
375 -----------------------------
376
377 If the architecture that you are using enable KASLR by default,
378 you should consider turning it off. KASLR randomizes the
379 virtual address where the kernel image is mapped and confuse
380 gdb which resolve kernel symbol address from symbol table
381 of vmlinux.
382
383 Using kdb
384 =========
385
386 Quick start for kdb on a serial port
387 ------------------------------------
388
389 This is a quick example of how to use kdb.
390
391 1. Configure kgdboc at boot using kernel parameters::
392
393 console=ttyS0,115200 kgdboc=ttyS0,115200 nokaslr
394
395 OR
396
397 Configure kgdboc after the kernel has booted; assuming you are using
398 a serial port console::
399
400 echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
401
402 2. Enter the kernel debugger manually or by waiting for an oops or
403 fault. There are several ways you can enter the kernel debugger
404 manually; all involve using the :kbd:`SysRq-G`, which means you must have
405 enabled ``CONFIG_MAGIC_SYSRQ=y`` in your kernel config.
406
407 - When logged in as root or with a super user session you can run::
408
409 echo g > /proc/sysrq-trigger
410
411 - Example using minicom 2.2
412
413 Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`
414
415 - When you have telneted to a terminal server that supports sending
416 a remote break
417
418 Press: :kbd:`CTRL-]`
419
420 Type in: ``send break``
421
422 Press: :kbd:`Enter` :kbd:`g`
423
424 3. From the kdb prompt you can run the ``help`` command to see a complete
425 list of the commands that are available.
426
427 Some useful commands in kdb include:
428
429 =========== =================================================================
430 ``lsmod`` Shows where kernel modules are loaded
431 ``ps`` Displays only the active processes
432 ``ps A`` Shows all the processes
433 ``summary`` Shows kernel version info and memory usage
434 ``bt`` Get a backtrace of the current process using dump_stack()
435 ``dmesg`` View the kernel syslog buffer
436 ``go`` Continue the system
437 =========== =================================================================
438
439 4. When you are done using kdb you need to consider rebooting the system
440 or using the ``go`` command to resuming normal kernel execution. If you
441 have paused the kernel for a lengthy period of time, applications
442 that rely on timely networking or anything to do with real wall clock
443 time could be adversely affected, so you should take this into
444 consideration when using the kernel debugger.
445
446 Quick start for kdb using a keyboard connected console
447 ------------------------------------------------------
448
449 This is a quick example of how to use kdb with a keyboard.
450
451 1. Configure kgdboc at boot using kernel parameters::
452
453 kgdboc=kbd
454
455 OR
456
457 Configure kgdboc after the kernel has booted::
458
459 echo kbd > /sys/module/kgdboc/parameters/kgdboc
460
461 2. Enter the kernel debugger manually or by waiting for an oops or
462 fault. There are several ways you can enter the kernel debugger
463 manually; all involve using the :kbd:`SysRq-G`, which means you must have
464 enabled ``CONFIG_MAGIC_SYSRQ=y`` in your kernel config.
465
466 - When logged in as root or with a super user session you can run::
467
468 echo g > /proc/sysrq-trigger
469
470 - Example using a laptop keyboard:
471
472 Press and hold down: :kbd:`Alt`
473
474 Press and hold down: :kbd:`Fn`
475
476 Press and release the key with the label: :kbd:`SysRq`
477
478 Release: :kbd:`Fn`
479
480 Press and release: :kbd:`g`
481
482 Release: :kbd:`Alt`
483
484 - Example using a PS/2 101-key keyboard
485
486 Press and hold down: :kbd:`Alt`
487
488 Press and release the key with the label: :kbd:`SysRq`
489
490 Press and release: :kbd:`g`
491
492 Release: :kbd:`Alt`
493
494 3. Now type in a kdb command such as ``help``, ``dmesg``, ``bt`` or ``go`` to
495 continue kernel execution.
496
497 Using kgdb / gdb
498 ================
499
500 In order to use kgdb you must activate it by passing configuration
501 information to one of the kgdb I/O drivers. If you do not pass any
502 configuration information kgdb will not do anything at all. Kgdb will
503 only actively hook up to the kernel trap hooks if a kgdb I/O driver is
504 loaded and configured. If you unconfigure a kgdb I/O driver, kgdb will
505 unregister all the kernel hook points.
506
507 All kgdb I/O drivers can be reconfigured at run time, if
508 ``CONFIG_SYSFS`` and ``CONFIG_MODULES`` are enabled, by echo'ing a new
509 config string to ``/sys/module/<driver>/parameter/<option>``. The driver
510 can be unconfigured by passing an empty string. You cannot change the
511 configuration while the debugger is attached. Make sure to detach the
512 debugger with the ``detach`` command prior to trying to unconfigure a
513 kgdb I/O driver.
514
515 Connecting with gdb to a serial port
516 ------------------------------------
517
518 1. Configure kgdboc
519
520 Configure kgdboc at boot using kernel parameters::
521
522 kgdboc=ttyS0,115200
523
524 OR
525
526 Configure kgdboc after the kernel has booted::
527
528 echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
529
530 2. Stop kernel execution (break into the debugger)
531
532 In order to connect to gdb via kgdboc, the kernel must first be
533 stopped. There are several ways to stop the kernel which include
534 using kgdbwait as a boot argument, via a :kbd:`SysRq-G`, or running the
535 kernel until it takes an exception where it waits for the debugger to
536 attach.
537
538 - When logged in as root or with a super user session you can run::
539
540 echo g > /proc/sysrq-trigger
541
542 - Example using minicom 2.2
543
544 Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`
545
546 - When you have telneted to a terminal server that supports sending
547 a remote break
548
549 Press: :kbd:`CTRL-]`
550
551 Type in: ``send break``
552
553 Press: :kbd:`Enter` :kbd:`g`
554
555 3. Connect from gdb
556
557 Example (using a directly connected port)::
558
559 % gdb ./vmlinux
560 (gdb) set serial baud 115200
561 (gdb) target remote /dev/ttyS0
562
563
564 Example (kgdb to a terminal server on TCP port 2012)::
565
566 % gdb ./vmlinux
567 (gdb) target remote 192.168.2.2:2012
568
569
570 Once connected, you can debug a kernel the way you would debug an
571 application program.
572
573 If you are having problems connecting or something is going seriously
574 wrong while debugging, it will most often be the case that you want
575 to enable gdb to be verbose about its target communications. You do
576 this prior to issuing the ``target remote`` command by typing in::
577
578 set debug remote 1
579
580 Remember if you continue in gdb, and need to "break in" again, you need
581 to issue an other :kbd:`SysRq-G`. It is easy to create a simple entry point by
582 putting a breakpoint at ``sys_sync`` and then you can run ``sync`` from a
583 shell or script to break into the debugger.
584
585 kgdb and kdb interoperability
586 =============================
587
588 It is possible to transition between kdb and kgdb dynamically. The debug
589 core will remember which you used the last time and automatically start
590 in the same mode.
591
592 Switching between kdb and kgdb
593 ------------------------------
594
595 Switching from kgdb to kdb
596 ~~~~~~~~~~~~~~~~~~~~~~~~~~
597
598 There are two ways to switch from kgdb to kdb: you can use gdb to issue
599 a maintenance packet, or you can blindly type the command ``$3#33``.
600 Whenever the kernel debugger stops in kgdb mode it will print the
601 message ``KGDB or $3#33 for KDB``. It is important to note that you have
602 to type the sequence correctly in one pass. You cannot type a backspace
603 or delete because kgdb will interpret that as part of the debug stream.
604
605 1. Change from kgdb to kdb by blindly typing::
606
607 $3#33
608
609 2. Change from kgdb to kdb with gdb::
610
611 maintenance packet 3
612
613 .. note::
614
615 Now you must kill gdb. Typically you press :kbd:`CTRL-Z` and issue
616 the command::
617
618 kill -9 %
619
620 Change from kdb to kgdb
621 ~~~~~~~~~~~~~~~~~~~~~~~
622
623 There are two ways you can change from kdb to kgdb. You can manually
624 enter kgdb mode by issuing the kgdb command from the kdb shell prompt,
625 or you can connect gdb while the kdb shell prompt is active. The kdb
626 shell looks for the typical first commands that gdb would issue with the
627 gdb remote protocol and if it sees one of those commands it
628 automatically changes into kgdb mode.
629
630 1. From kdb issue the command::
631
632 kgdb
633
634 Now disconnect your terminal program and connect gdb in its place
635
636 2. At the kdb prompt, disconnect the terminal program and connect gdb in
637 its place.
638
639 Running kdb commands from gdb
640 -----------------------------
641
642 It is possible to run a limited set of kdb commands from gdb, using the
643 gdb monitor command. You don't want to execute any of the run control or
644 breakpoint operations, because it can disrupt the state of the kernel
645 debugger. You should be using gdb for breakpoints and run control
646 operations if you have gdb connected. The more useful commands to run
647 are things like lsmod, dmesg, ps or possibly some of the memory
648 information commands. To see all the kdb commands you can run
649 ``monitor help``.
650
651 Example::
652
653 (gdb) monitor ps
654 1 idle process (state I) and
655 27 sleeping system daemon (state M) processes suppressed,
656 use 'ps A' to see all.
657 Task Addr Pid Parent [*] cpu State Thread Command
658
659 0xc78291d0 1 0 0 0 S 0xc7829404 init
660 0xc7954150 942 1 0 0 S 0xc7954384 dropbear
661 0xc78789c0 944 1 0 0 S 0xc7878bf4 sh
662 (gdb)
663
664 kgdb Test Suite
665 ===============
666
667 When kgdb is enabled in the kernel config you can also elect to enable
668 the config parameter ``KGDB_TESTS``. Turning this on will enable a special
669 kgdb I/O module which is designed to test the kgdb internal functions.
670
671 The kgdb tests are mainly intended for developers to test the kgdb
672 internals as well as a tool for developing a new kgdb architecture
673 specific implementation. These tests are not really for end users of the
674 Linux kernel. The primary source of documentation would be to look in
675 the ``drivers/misc/kgdbts.c`` file.
676
677 The kgdb test suite can also be configured at compile time to run the
678 core set of tests by setting the kernel config parameter
679 ``KGDB_TESTS_ON_BOOT``. This particular option is aimed at automated
680 regression testing and does not require modifying the kernel boot config
681 arguments. If this is turned on, the kgdb test suite can be disabled by
682 specifying ``kgdbts=`` as a kernel boot argument.
683
684 Kernel Debugger Internals
685 =========================
686
687 Architecture Specifics
688 ----------------------
689
690 The kernel debugger is organized into a number of components:
691
692 1. The debug core
693
694 The debug core is found in ``kernel/debugger/debug_core.c``. It
695 contains:
696
697 - A generic OS exception handler which includes sync'ing the
698 processors into a stopped state on an multi-CPU system.
699
700 - The API to talk to the kgdb I/O drivers
701
702 - The API to make calls to the arch-specific kgdb implementation
703
704 - The logic to perform safe memory reads and writes to memory while
705 using the debugger
706
707 - A full implementation for software breakpoints unless overridden
708 by the arch
709
710 - The API to invoke either the kdb or kgdb frontend to the debug
711 core.
712
713 - The structures and callback API for atomic kernel mode setting.
714
715 .. note:: kgdboc is where the kms callbacks are invoked.
716
717 2. kgdb arch-specific implementation
718
719 This implementation is generally found in ``arch/*/kernel/kgdb.c``. As
720 an example, ``arch/x86/kernel/kgdb.c`` contains the specifics to
721 implement HW breakpoint as well as the initialization to dynamically
722 register and unregister for the trap handlers on this architecture.
723 The arch-specific portion implements:
724
725 - contains an arch-specific trap catcher which invokes
726 kgdb_handle_exception() to start kgdb about doing its work
727
728 - translation to and from gdb specific packet format to struct pt_regs
729
730 - Registration and unregistration of architecture specific trap
731 hooks
732
733 - Any special exception handling and cleanup
734
735 - NMI exception handling and cleanup
736
737 - (optional) HW breakpoints
738
739 3. gdbstub frontend (aka kgdb)
740
741 The gdbstub is located in ``kernel/debug/gdbstub.c``. It contains:
742
743 - All the logic to implement the gdb serial protocol
744
745 4. kdb frontend
746
747 The kdb debugger shell is broken down into a number of components.
748 The kdb core is located in kernel/debug/kdb. There are a number of
749 helper functions in some of the other kernel components to make it
750 possible for kdb to examine and report information about the kernel
751 without taking locks that could cause a kernel deadlock. The kdb core
752 contains implements the following functionality.
753
754 - A simple shell
755
756 - The kdb core command set
757
758 - A registration API to register additional kdb shell commands.
759
760 - A good example of a self-contained kdb module is the ``ftdump``
761 command for dumping the ftrace buffer. See:
762 ``kernel/trace/trace_kdb.c``
763
764 - For an example of how to dynamically register a new kdb command
765 you can build the kdb_hello.ko kernel module from
766 ``samples/kdb/kdb_hello.c``. To build this example you can set
767 ``CONFIG_SAMPLES=y`` and ``CONFIG_SAMPLE_KDB=m`` in your kernel
768 config. Later run ``modprobe kdb_hello`` and the next time you
769 enter the kdb shell, you can run the ``hello`` command.
770
771 - The implementation for kdb_printf() which emits messages directly
772 to I/O drivers, bypassing the kernel log.
773
774 - SW / HW breakpoint management for the kdb shell
775
776 5. kgdb I/O driver
777
778 Each kgdb I/O driver has to provide an implementation for the
779 following:
780
781 - configuration via built-in or module
782
783 - dynamic configuration and kgdb hook registration calls
784
785 - read and write character interface
786
787 - A cleanup handler for unconfiguring from the kgdb core
788
789 - (optional) Early debug methodology
790
791 Any given kgdb I/O driver has to operate very closely with the
792 hardware and must do it in such a way that does not enable interrupts
793 or change other parts of the system context without completely
794 restoring them. The kgdb core will repeatedly "poll" a kgdb I/O
795 driver for characters when it needs input. The I/O driver is expected
796 to return immediately if there is no data available. Doing so allows
797 for the future possibility to touch watchdog hardware in such a way
798 as to have a target system not reset when these are enabled.
799
800 If you are intent on adding kgdb architecture specific support for a new
801 architecture, the architecture should define ``HAVE_ARCH_KGDB`` in the
802 architecture specific Kconfig file. This will enable kgdb for the
803 architecture, and at that point you must create an architecture specific
804 kgdb implementation.
805
806 There are a few flags which must be set on every architecture in their
807 ``asm/kgdb.h`` file. These are:
808
809 - ``NUMREGBYTES``:
810 The size in bytes of all of the registers, so that we
811 can ensure they will all fit into a packet.
812
813 - ``BUFMAX``:
814 The size in bytes of the buffer GDB will read into. This must
815 be larger than NUMREGBYTES.
816
817 - ``CACHE_FLUSH_IS_SAFE``:
818 Set to 1 if it is always safe to call
819 flush_cache_range or flush_icache_range. On some architectures,
820 these functions may not be safe to call on SMP since we keep other
821 CPUs in a holding pattern.
822
823 There are also the following functions for the common backend, found in
824 ``kernel/kgdb.c``, that must be supplied by the architecture-specific
825 backend unless marked as (optional), in which case a default function
826 maybe used if the architecture does not need to provide a specific
827 implementation.
828
829 .. kernel-doc:: include/linux/kgdb.h
830 :internal:
831
832 kgdboc internals
833 ----------------
834
835 kgdboc and uarts
836 ~~~~~~~~~~~~~~~~
837
838 The kgdboc driver is actually a very thin driver that relies on the
839 underlying low level to the hardware driver having "polling hooks" to
840 which the tty driver is attached. In the initial implementation of
841 kgdboc the serial_core was changed to expose a low level UART hook for
842 doing polled mode reading and writing of a single character while in an
843 atomic context. When kgdb makes an I/O request to the debugger, kgdboc
844 invokes a callback in the serial core which in turn uses the callback in
845 the UART driver.
846
847 When using kgdboc with a UART, the UART driver must implement two
848 callbacks in the struct uart_ops.
849 Example from ``drivers/8250.c``::
850
851
852 #ifdef CONFIG_CONSOLE_POLL
853 .poll_get_char = serial8250_get_poll_char,
854 .poll_put_char = serial8250_put_poll_char,
855 #endif
856
857
858 Any implementation specifics around creating a polling driver use the
859 ``#ifdef CONFIG_CONSOLE_POLL``, as shown above. Keep in mind that
860 polling hooks have to be implemented in such a way that they can be
861 called from an atomic context and have to restore the state of the UART
862 chip on return such that the system can return to normal when the
863 debugger detaches. You need to be very careful with any kind of lock you
864 consider, because failing here is most likely going to mean pressing the
865 reset button.
866
867 kgdboc and keyboards
868 ~~~~~~~~~~~~~~~~~~~~~~~~
869
870 The kgdboc driver contains logic to configure communications with an
871 attached keyboard. The keyboard infrastructure is only compiled into the
872 kernel when ``CONFIG_KDB_KEYBOARD=y`` is set in the kernel configuration.
873
874 The core polled keyboard driver for PS/2 type keyboards is in
875 ``drivers/char/kdb_keyboard.c``. This driver is hooked into the debug core
876 when kgdboc populates the callback in the array called
877 :c:expr:`kdb_poll_funcs[]`. The kdb_get_kbd_char() is the top-level
878 function which polls hardware for single character input.
879
880 kgdboc and kms
881 ~~~~~~~~~~~~~~~~~~
882
883 The kgdboc driver contains logic to request the graphics display to
884 switch to a text context when you are using ``kgdboc=kms,kbd``, provided
885 that you have a video driver which has a frame buffer console and atomic
886 kernel mode setting support.
887
888 Every time the kernel debugger is entered it calls
889 kgdboc_pre_exp_handler() which in turn calls con_debug_enter()
890 in the virtual console layer. On resuming kernel execution, the kernel
891 debugger calls kgdboc_post_exp_handler() which in turn calls
892 con_debug_leave().
893
894 Any video driver that wants to be compatible with the kernel debugger
895 and the atomic kms callbacks must implement the ``mode_set_base_atomic``,
896 ``fb_debug_enter`` and ``fb_debug_leave operations``. For the
897 ``fb_debug_enter`` and ``fb_debug_leave`` the option exists to use the
898 generic drm fb helper functions or implement something custom for the
899 hardware. The following example shows the initialization of the
900 .mode_set_base_atomic operation in
901 drivers/gpu/drm/i915/intel_display.c::
902
903
904 static const struct drm_crtc_helper_funcs intel_helper_funcs = {
905 [...]
906 .mode_set_base_atomic = intel_pipe_set_base_atomic,
907 [...]
908 };
909
910
911 Here is an example of how the i915 driver initializes the
912 fb_debug_enter and fb_debug_leave functions to use the generic drm
913 helpers in ``drivers/gpu/drm/i915/intel_fb.c``::
914
915
916 static struct fb_ops intelfb_ops = {
917 [...]
918 .fb_debug_enter = drm_fb_helper_debug_enter,
919 .fb_debug_leave = drm_fb_helper_debug_leave,
920 [...]
921 };
922
923
924 Credits
925 =======
926
927 The following people have contributed to this document:
928
929 1. Amit Kale <amitkale@linsyssoft.com>
930
931 2. Tom Rini <trini@kernel.crashing.org>
932
933 In March 2008 this document was completely rewritten by:
934
935 - Jason Wessel <jason.wessel@windriver.com>
936
937 In Jan 2010 this document was updated to include kdb.
938
939 - Jason Wessel <jason.wessel@windriver.com>
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