Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 EST.
1 <?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, 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 a 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 look under 98 "Kernel hacking" / "Kernel debugging" and select "KGDB: kernel debugger". 99 </para> 100 <para> 101 While it is not a hard requirement that you have symbols in your 102 vmlinux file, gdb tends not to be very useful without the symbolic 103 data, so you will want to turn 104 on <symbol>CONFIG_DEBUG_INFO</symbol> which is called "Compile the 105 kernel with debug info" in the config menu. 106 </para> 107 <para> 108 It is advised, but not required, that you turn on the 109 <symbol>CONFIG_FRAME_POINTER</symbol> kernel option which is called "Compile the 110 kernel with frame pointers" in the config menu. This option 111 inserts code to into the compiled executable which saves the frame 112 information in registers or on the stack at different points which 113 allows a debugger such as gdb to more accurately construct 114 stack back traces while debugging the kernel. 115 </para> 116 <para> 117 If the architecture that you are using supports the kernel option 118 CONFIG_DEBUG_RODATA, you should consider turning it off. This 119 option will prevent the use of software breakpoints because it 120 marks certain regions of the kernel's memory space as read-only. 121 If kgdb supports it for the architecture you are using, you can 122 use hardware breakpoints if you desire to run with the 123 CONFIG_DEBUG_RODATA option turned on, else you need to turn off 124 this option. 125 </para> 126 <para> 127 Next you should choose one of more I/O drivers to interconnect 128 debugging host and debugged target. Early boot debugging requires 129 a KGDB I/O driver that supports early debugging and the driver 130 must be built into the kernel directly. Kgdb I/O driver 131 configuration takes place via kernel or module parameters which 132 you can learn more about in the in the section that describes the 133 parameter "kgdboc". 134 </para> 135 <para>Here is an example set of .config symbols to enable or 136 disable for kgdb: 137 <itemizedlist> 138 <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem> 139 <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem> 140 <listitem><para>CONFIG_KGDB=y</para></listitem> 141 <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem> 142 </itemizedlist> 143 </para> 144 </sect1> 145 <sect1 id="CompileKDB"> 146 <title>Kernel config options for kdb</title> 147 <para>Kdb is quite a bit more complex than the simple gdbstub 148 sitting on top of the kernel's debug core. Kdb must implement a 149 shell, and also adds some helper functions in other parts of the 150 kernel, responsible for printing out interesting data such as what 151 you would see if you ran "lsmod", or "ps". In order to build kdb 152 into the kernel you follow the same steps as you would for kgdb. 153 </para> 154 <para>The main config option for kdb 155 is <symbol>CONFIG_KGDB_KDB</symbol> which is called "KGDB_KDB: 156 include kdb frontend for kgdb" in the config menu. In theory you 157 would have already also selected an I/O driver such as the 158 CONFIG_KGDB_SERIAL_CONSOLE interface if you plan on using kdb on a 159 serial port, when you were configuring kgdb. 160 </para> 161 <para>If you want to use a PS/2-style keyboard with kdb, you would 162 select CONFIG_KDB_KEYBOARD which is called "KGDB_KDB: keyboard as 163 input device" in the config menu. The CONFIG_KDB_KEYBOARD option 164 is not used for anything in the gdb interface to kgdb. The 165 CONFIG_KDB_KEYBOARD option only works with kdb. 166 </para> 167 <para>Here is an example set of .config symbols to enable/disable kdb: 168 <itemizedlist> 169 <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem> 170 <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem> 171 <listitem><para>CONFIG_KGDB=y</para></listitem> 172 <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem> 173 <listitem><para>CONFIG_KGDB_KDB=y</para></listitem> 174 <listitem><para>CONFIG_KDB_KEYBOARD=y</para></listitem> 175 </itemizedlist> 176 </para> 177 </sect1> 178 </chapter> 179 <chapter id="kgdbKernelArgs"> 180 <title>Kernel Debugger Boot Arguments</title> 181 <para>This section describes the various runtime kernel 182 parameters that affect the configuration of the kernel debugger. 183 The following chapter covers using kdb and kgdb as well as 184 providing some examples of the configuration parameters.</para> 185 <sect1 id="kgdboc"> 186 <title>Kernel parameter: kgdboc</title> 187 <para>The kgdboc driver was originally an abbreviation meant to 188 stand for "kgdb over console". Today it is the primary mechanism 189 to configure how to communicate from gdb to kgdb as well as the 190 devices you want to use to interact with the kdb shell. 191 </para> 192 <para>For kgdb/gdb, kgdboc is designed to work with a single serial 193 port. It is intended to cover the circumstance where you want to 194 use a serial console as your primary console as well as using it to 195 perform kernel debugging. It is also possible to use kgdb on a 196 serial port which is not designated as a system console. Kgdboc 197 may be configured as a kernel built-in or a kernel loadable module. 198 You can only make use of <constant>kgdbwait</constant> and early 199 debugging if you build kgdboc into the kernel as a built-in. 200 </para> 201 <para>Optionally you can elect to activate kms (Kernel Mode 202 Setting) integration. When you use kms with kgdboc and you have a 203 video driver that has atomic mode setting hooks, it is possible to 204 enter the debugger on the graphics console. When the kernel 205 execution is resumed, the previous graphics mode will be restored. 206 This integration can serve as a useful tool to aid in diagnosing 207 crashes or doing analysis of memory with kdb while allowing the 208 full graphics console applications to run. 209 </para> 210 <sect2 id="kgdbocArgs"> 211 <title>kgdboc arguments</title> 212 <para>Usage: <constant>kgdboc=[kms][[,]kbd][[,]serial_device][,baud]</constant></para> 213 <para>The order listed above must be observed if you use any of the 214 optional configurations together. 215 </para> 216 <para>Abbreviations: 217 <itemizedlist> 218 <listitem><para>kms = Kernel Mode Setting</para></listitem> 219 <listitem><para>kbd = Keyboard</para></listitem> 220 </itemizedlist> 221 </para> 222 <para>You can configure kgdboc to use the keyboard, and/or a serial 223 device depending on if you are using kdb and/or kgdb, in one of the 224 following scenarios. The order listed above must be observed if 225 you use any of the optional configurations together. Using kms + 226 only gdb is generally not a useful combination.</para> 227 <sect3 id="kgdbocArgs1"> 228 <title>Using loadable module or built-in</title> 229 <para> 230 <orderedlist> 231 <listitem><para>As a kernel built-in:</para> 232 <para>Use the kernel boot argument: <constant>kgdboc=<tty-device>,[baud]</constant></para></listitem> 233 <listitem> 234 <para>As a kernel loadable module:</para> 235 <para>Use the command: <constant>modprobe kgdboc kgdboc=<tty-device>,[baud]</constant></para> 236 <para>Here are two examples of how you might format the kgdboc 237 string. The first is for an x86 target using the first serial port. 238 The second example is for the ARM Versatile AB using the second 239 serial port. 240 <orderedlist> 241 <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem> 242 <listitem><para><constant>kgdboc=ttyAMA1,115200</constant></para></listitem> 243 </orderedlist> 244 </para> 245 </listitem> 246 </orderedlist></para> 247 </sect3> 248 <sect3 id="kgdbocArgs2"> 249 <title>Configure kgdboc at runtime with sysfs</title> 250 <para>At run time you can enable or disable kgdboc by echoing a 251 parameters into the sysfs. Here are two examples:</para> 252 <orderedlist> 253 <listitem><para>Enable kgdboc on ttyS0</para> 254 <para><constant>echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem> 255 <listitem><para>Disable kgdboc</para> 256 <para><constant>echo "" > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem> 257 </orderedlist> 258 <para>NOTE: You do not need to specify the baud if you are 259 configuring the console on tty which is already configured or 260 open.</para> 261 </sect3> 262 <sect3 id="kgdbocArgs3"> 263 <title>More examples</title> 264 <para>You can configure kgdboc to use the keyboard, and/or a serial device 265 depending on if you are using kdb and/or kgdb, in one of the 266 following scenarios. 267 <orderedlist> 268 <listitem><para>kdb and kgdb over only a serial port</para> 269 <para><constant>kgdboc=<serial_device>[,baud]</constant></para> 270 <para>Example: <constant>kgdboc=ttyS0,115200</constant></para> 271 </listitem> 272 <listitem><para>kdb and kgdb with keyboard and a serial port</para> 273 <para><constant>kgdboc=kbd,<serial_device>[,baud]</constant></para> 274 <para>Example: <constant>kgdboc=kbd,ttyS0,115200</constant></para> 275 </listitem> 276 <listitem><para>kdb with a keyboard</para> 277 <para><constant>kgdboc=kbd</constant></para> 278 </listitem> 279 <listitem><para>kdb with kernel mode setting</para> 280 <para><constant>kgdboc=kms,kbd</constant></para> 281 </listitem> 282 <listitem><para>kdb with kernel mode setting and kgdb over a serial port</para> 283 <para><constant>kgdboc=kms,kbd,ttyS0,115200</constant></para> 284 </listitem> 285 </orderedlist> 286 </para> 287 <para>NOTE: Kgdboc does not support interrupting the target via the 288 gdb remote protocol. You must manually send a sysrq-g unless you 289 have a proxy that splits console output to a terminal program. 290 A console proxy has a separate TCP port for the debugger and a separate 291 TCP port for the "human" console. The proxy can take care of sending 292 the sysrq-g for you. 293 </para> 294 <para>When using kgdboc with no debugger proxy, you can end up 295 connecting the debugger at one of two entry points. If an 296 exception occurs after you have loaded kgdboc, a message should 297 print on the console stating it is waiting for the debugger. In 298 this case you disconnect your terminal program and then connect the 299 debugger in its place. If you want to interrupt the target system 300 and forcibly enter a debug session you have to issue a Sysrq 301 sequence and then type the letter <constant>g</constant>. Then 302 you disconnect the terminal session and connect gdb. Your options 303 if you don't like this are to hack gdb to send the sysrq-g for you 304 as well as on the initial connect, or to use a debugger proxy that 305 allows an unmodified gdb to do the debugging. 306 </para> 307 </sect3> 308 </sect2> 309 </sect1> 310 <sect1 id="kgdbwait"> 311 <title>Kernel parameter: kgdbwait</title> 312 <para> 313 The Kernel command line option <constant>kgdbwait</constant> makes 314 kgdb wait for a debugger connection during booting of a kernel. You 315 can only use this option if you compiled a kgdb I/O driver into the 316 kernel and you specified the I/O driver configuration as a kernel 317 command line option. The kgdbwait parameter should always follow the 318 configuration parameter for the kgdb I/O driver in the kernel 319 command line else the I/O driver will not be configured prior to 320 asking the kernel to use it to wait. 321 </para> 322 <para> 323 The kernel will stop and wait as early as the I/O driver and 324 architecture allows when you use this option. If you build the 325 kgdb I/O driver as a loadable kernel module kgdbwait will not do 326 anything. 327 </para> 328 </sect1> 329 <sect1 id="kgdbcon"> 330 <title>Kernel parameter: kgdbcon</title> 331 <para> The kgdbcon feature allows you to see printk() messages 332 inside gdb while gdb is connected to the kernel. Kdb does not make 333 use of the kgdbcon feature. 334 </para> 335 <para>Kgdb supports using the gdb serial protocol to send console 336 messages to the debugger when the debugger is connected and running. 337 There are two ways to activate this feature. 338 <orderedlist> 339 <listitem><para>Activate with the kernel command line option:</para> 340 <para><constant>kgdbcon</constant></para> 341 </listitem> 342 <listitem><para>Use sysfs before configuring an I/O driver</para> 343 <para> 344 <constant>echo 1 > /sys/module/kgdb/parameters/kgdb_use_con</constant> 345 </para> 346 <para> 347 NOTE: If you do this after you configure the kgdb I/O driver, the 348 setting will not take effect until the next point the I/O is 349 reconfigured. 350 </para> 351 </listitem> 352 </orderedlist> 353 </para> 354 <para>IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an 355 active system console. An example of incorrect usage is <constant>console=ttyS0,115200 kgdboc=ttyS0 kgdbcon</constant> 356 </para> 357 <para>It is possible to use this option with kgdboc on a tty that is not a system console. 358 </para> 359 </sect1> 360 <sect1 id="kgdbreboot"> 361 <title>Run time parameter: kgdbreboot</title> 362 <para> The kgdbreboot feature allows you to change how the debugger 363 deals with the reboot notification. You have 3 choices for the 364 behavior. The default behavior is always set to 0.</para> 365 <orderedlist> 366 <listitem><para>echo -1 > /sys/module/debug_core/parameters/kgdbreboot</para> 367 <para>Ignore the reboot notification entirely.</para> 368 </listitem> 369 <listitem><para>echo 0 > /sys/module/debug_core/parameters/kgdbreboot</para> 370 <para>Send the detach message to any attached debugger client.</para> 371 </listitem> 372 <listitem><para>echo 1 > /sys/module/debug_core/parameters/kgdbreboot</para> 373 <para>Enter the debugger on reboot notify.</para> 374 </listitem> 375 </orderedlist> 376 </sect1> 377 </chapter> 378 <chapter id="usingKDB"> 379 <title>Using kdb</title> 380 <para> 381 </para> 382 <sect1 id="quickKDBserial"> 383 <title>Quick start for kdb on a serial port</title> 384 <para>This is a quick example of how to use kdb.</para> 385 <para><orderedlist> 386 <listitem><para>Configure kgdboc at boot using kernel parameters: 387 <itemizedlist> 388 <listitem><para><constant>console=ttyS0,115200 kgdboc=ttyS0,115200</constant></para></listitem> 389 </itemizedlist></para> 390 <para>OR</para> 391 <para>Configure kgdboc after the kernel has booted; assuming you are using a serial port console: 392 <itemizedlist> 393 <listitem><para><constant>echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem> 394 </itemizedlist> 395 </para> 396 </listitem> 397 <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> 398 <itemizedlist> 399 <listitem><para>When logged in as root or with a super user session you can run:</para> 400 <para><constant>echo g > /proc/sysrq-trigger</constant></para></listitem> 401 <listitem><para>Example using minicom 2.2</para> 402 <para>Press: <constant>Control-a</constant></para> 403 <para>Press: <constant>f</constant></para> 404 <para>Press: <constant>g</constant></para> 405 </listitem> 406 <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para> 407 <para>Press: <constant>Control-]</constant></para> 408 <para>Type in:<constant>send break</constant></para> 409 <para>Press: <constant>Enter</constant></para> 410 <para>Press: <constant>g</constant></para> 411 </listitem> 412 </itemizedlist> 413 </listitem> 414 <listitem><para>From the kdb prompt you can run the "help" command to see a complete list of the commands that are available.</para> 415 <para>Some useful commands in kdb include: 416 <itemizedlist> 417 <listitem><para>lsmod -- Shows where kernel modules are loaded</para></listitem> 418 <listitem><para>ps -- Displays only the active processes</para></listitem> 419 <listitem><para>ps A -- Shows all the processes</para></listitem> 420 <listitem><para>summary -- Shows kernel version info and memory usage</para></listitem> 421 <listitem><para>bt -- Get a backtrace of the current process using dump_stack()</para></listitem> 422 <listitem><para>dmesg -- View the kernel syslog buffer</para></listitem> 423 <listitem><para>go -- Continue the system</para></listitem> 424 </itemizedlist> 425 </para> 426 </listitem> 427 <listitem> 428 <para>When you are done using kdb you need to consider rebooting the 429 system or using the "go" command to resuming normal kernel 430 execution. If you have paused the kernel for a lengthy period of 431 time, applications that rely on timely networking or anything to do 432 with real wall clock time could be adversely affected, so you 433 should take this into consideration when using the kernel 434 debugger.</para> 435 </listitem> 436 </orderedlist></para> 437 </sect1> 438 <sect1 id="quickKDBkeyboard"> 439 <title>Quick start for kdb using a keyboard connected console</title> 440 <para>This is a quick example of how to use kdb with a keyboard.</para> 441 <para><orderedlist> 442 <listitem><para>Configure kgdboc at boot using kernel parameters: 443 <itemizedlist> 444 <listitem><para><constant>kgdboc=kbd</constant></para></listitem> 445 </itemizedlist></para> 446 <para>OR</para> 447 <para>Configure kgdboc after the kernel has booted: 448 <itemizedlist> 449 <listitem><para><constant>echo kbd > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem> 450 </itemizedlist> 451 </para> 452 </listitem> 453 <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> 454 <itemizedlist> 455 <listitem><para>When logged in as root or with a super user session you can run:</para> 456 <para><constant>echo g > /proc/sysrq-trigger</constant></para></listitem> 457 <listitem><para>Example using a laptop keyboard</para> 458 <para>Press and hold down: <constant>Alt</constant></para> 459 <para>Press and hold down: <constant>Fn</constant></para> 460 <para>Press and release the key with the label: <constant>SysRq</constant></para> 461 <para>Release: <constant>Fn</constant></para> 462 <para>Press and release: <constant>g</constant></para> 463 <para>Release: <constant>Alt</constant></para> 464 </listitem> 465 <listitem><para>Example using a PS/2 101-key keyboard</para> 466 <para>Press and hold down: <constant>Alt</constant></para> 467 <para>Press and release the key with the label: <constant>SysRq</constant></para> 468 <para>Press and release: <constant>g</constant></para> 469 <para>Release: <constant>Alt</constant></para> 470 </listitem> 471 </itemizedlist> 472 </listitem> 473 <listitem> 474 <para>Now type in a kdb command such as "help", "dmesg", "bt" or "go" to continue kernel execution.</para> 475 </listitem> 476 </orderedlist></para> 477 </sect1> 478 </chapter> 479 <chapter id="EnableKGDB"> 480 <title>Using kgdb / gdb</title> 481 <para>In order to use kgdb you must activate it by passing 482 configuration information to one of the kgdb I/O drivers. If you 483 do not pass any configuration information kgdb will not do anything 484 at all. Kgdb will only actively hook up to the kernel trap hooks 485 if a kgdb I/O driver is loaded and configured. If you unconfigure 486 a kgdb I/O driver, kgdb will unregister all the kernel hook points. 487 </para> 488 <para> All kgdb I/O drivers can be reconfigured at run time, if 489 <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol> 490 are enabled, by echo'ing a new config string to 491 <constant>/sys/module/<driver>/parameter/<option></constant>. 492 The driver can be unconfigured by passing an empty string. You cannot 493 change the configuration while the debugger is attached. Make sure 494 to detach the debugger with the <constant>detach</constant> command 495 prior to trying to unconfigure a kgdb I/O driver. 496 </para> 497 <sect1 id="ConnectingGDB"> 498 <title>Connecting with gdb to a serial port</title> 499 <orderedlist> 500 <listitem><para>Configure kgdboc</para> 501 <para>Configure kgdboc at boot using kernel parameters: 502 <itemizedlist> 503 <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem> 504 </itemizedlist></para> 505 <para>OR</para> 506 <para>Configure kgdboc after the kernel has booted: 507 <itemizedlist> 508 <listitem><para><constant>echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem> 509 </itemizedlist></para> 510 </listitem> 511 <listitem> 512 <para>Stop kernel execution (break into the debugger)</para> 513 <para>In order to connect to gdb via kgdboc, the kernel must 514 first be stopped. There are several ways to stop the kernel which 515 include using kgdbwait as a boot argument, via a sysrq-g, or running 516 the kernel until it takes an exception where it waits for the 517 debugger to attach. 518 <itemizedlist> 519 <listitem><para>When logged in as root or with a super user session you can run:</para> 520 <para><constant>echo g > /proc/sysrq-trigger</constant></para></listitem> 521 <listitem><para>Example using minicom 2.2</para> 522 <para>Press: <constant>Control-a</constant></para> 523 <para>Press: <constant>f</constant></para> 524 <para>Press: <constant>g</constant></para> 525 </listitem> 526 <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para> 527 <para>Press: <constant>Control-]</constant></para> 528 <para>Type in:<constant>send break</constant></para> 529 <para>Press: <constant>Enter</constant></para> 530 <para>Press: <constant>g</constant></para> 531 </listitem> 532 </itemizedlist> 533 </para> 534 </listitem> 535 <listitem> 536 <para>Connect from gdb</para> 537 <para> 538 Example (using a directly connected port): 539 </para> 540 <programlisting> 541 % gdb ./vmlinux 542 (gdb) set remotebaud 115200 543 (gdb) target remote /dev/ttyS0 544 </programlisting> 545 <para> 546 Example (kgdb to a terminal server on TCP port 2012): 547 </para> 548 <programlisting> 549 % gdb ./vmlinux 550 (gdb) target remote 192.168.2.2:2012 551 </programlisting> 552 <para> 553 Once connected, you can debug a kernel the way you would debug an 554 application program. 555 </para> 556 <para> 557 If you are having problems connecting or something is going 558 seriously wrong while debugging, it will most often be the case 559 that you want to enable gdb to be verbose about its target 560 communications. You do this prior to issuing the <constant>target 561 remote</constant> command by typing in: <constant>set debug remote 1</constant> 562 </para> 563 </listitem> 564 </orderedlist> 565 <para>Remember if you continue in gdb, and need to "break in" again, 566 you need to issue an other sysrq-g. It is easy to create a simple 567 entry point by putting a breakpoint at <constant>sys_sync</constant> 568 and then you can run "sync" from a shell or script to break into the 569 debugger.</para> 570 </sect1> 571 </chapter> 572 <chapter id="switchKdbKgdb"> 573 <title>kgdb and kdb interoperability</title> 574 <para>It is possible to transition between kdb and kgdb dynamically. 575 The debug core will remember which you used the last time and 576 automatically start in the same mode.</para> 577 <sect1> 578 <title>Switching between kdb and kgdb</title> 579 <sect2> 580 <title>Switching from kgdb to kdb</title> 581 <para> 582 There are two ways to switch from kgdb to kdb: you can use gdb to 583 issue a maintenance packet, or you can blindly type the command $3#33. 584 Whenever the kernel debugger stops in kgdb mode it will print the 585 message <constant>KGDB or $3#33 for KDB</constant>. It is important 586 to note that you have to type the sequence correctly in one pass. 587 You cannot type a backspace or delete because kgdb will interpret 588 that as part of the debug stream. 589 <orderedlist> 590 <listitem><para>Change from kgdb to kdb by blindly typing:</para> 591 <para><constant>$3#33</constant></para></listitem> 592 <listitem><para>Change from kgdb to kdb with gdb</para> 593 <para><constant>maintenance packet 3</constant></para> 594 <para>NOTE: Now you must kill gdb. Typically you press control-z and 595 issue the command: kill -9 %</para></listitem> 596 </orderedlist> 597 </para> 598 </sect2> 599 <sect2> 600 <title>Change from kdb to kgdb</title> 601 <para>There are two ways you can change from kdb to kgdb. You can 602 manually enter kgdb mode by issuing the kgdb command from the kdb 603 shell prompt, or you can connect gdb while the kdb shell prompt is 604 active. The kdb shell looks for the typical first commands that gdb 605 would issue with the gdb remote protocol and if it sees one of those 606 commands it automatically changes into kgdb mode.</para> 607 <orderedlist> 608 <listitem><para>From kdb issue the command:</para> 609 <para><constant>kgdb</constant></para> 610 <para>Now disconnect your terminal program and connect gdb in its place</para></listitem> 611 <listitem><para>At the kdb prompt, disconnect the terminal program and connect gdb in its place.</para></listitem> 612 </orderedlist> 613 </sect2> 614 </sect1> 615 <sect1> 616 <title>Running kdb commands from gdb</title> 617 <para>It is possible to run a limited set of kdb commands from gdb, 618 using the gdb monitor command. You don't want to execute any of the 619 run control or breakpoint operations, because it can disrupt the 620 state of the kernel debugger. You should be using gdb for 621 breakpoints and run control operations if you have gdb connected. 622 The more useful commands to run are things like lsmod, dmesg, ps or 623 possibly some of the memory information commands. To see all the kdb 624 commands you can run <constant>monitor help</constant>.</para> 625 <para>Example: 626 <informalexample><programlisting> 627 (gdb) monitor ps 628 1 idle process (state I) and 629 27 sleeping system daemon (state M) processes suppressed, 630 use 'ps A' to see all. 631 Task Addr Pid Parent [*] cpu State Thread Command 632 633 0xc78291d0 1 0 0 0 S 0xc7829404 init 634 0xc7954150 942 1 0 0 S 0xc7954384 dropbear 635 0xc78789c0 944 1 0 0 S 0xc7878bf4 sh 636 (gdb) 637 </programlisting></informalexample> 638 </para> 639 </sect1> 640 </chapter> 641 <chapter id="KGDBTestSuite"> 642 <title>kgdb Test Suite</title> 643 <para> 644 When kgdb is enabled in the kernel config you can also elect to 645 enable the config parameter KGDB_TESTS. Turning this on will 646 enable a special kgdb I/O module which is designed to test the 647 kgdb internal functions. 648 </para> 649 <para> 650 The kgdb tests are mainly intended for developers to test the kgdb 651 internals as well as a tool for developing a new kgdb architecture 652 specific implementation. These tests are not really for end users 653 of the Linux kernel. The primary source of documentation would be 654 to look in the drivers/misc/kgdbts.c file. 655 </para> 656 <para> 657 The kgdb test suite can also be configured at compile time to run 658 the core set of tests by setting the kernel config parameter 659 KGDB_TESTS_ON_BOOT. This particular option is aimed at automated 660 regression testing and does not require modifying the kernel boot 661 config arguments. If this is turned on, the kgdb test suite can 662 be disabled by specifying "kgdbts=" as a kernel boot argument. 663 </para> 664 </chapter> 665 <chapter id="CommonBackEndReq"> 666 <title>Kernel Debugger Internals</title> 667 <sect1 id="kgdbArchitecture"> 668 <title>Architecture Specifics</title> 669 <para> 670 The kernel debugger is organized into a number of components: 671 <orderedlist> 672 <listitem><para>The debug core</para> 673 <para> 674 The debug core is found in kernel/debugger/debug_core.c. It contains: 675 <itemizedlist> 676 <listitem><para>A generic OS exception handler which includes 677 sync'ing the processors into a stopped state on an multi-CPU 678 system.</para></listitem> 679 <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem> 680 <listitem><para>The API to make calls to the arch-specific kgdb implementation</para></listitem> 681 <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem> 682 <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem> 683 <listitem><para>The API to invoke either the kdb or kgdb frontend to the debug core.</para></listitem> 684 <listitem><para>The structures and callback API for atomic kernel mode setting.</para> 685 <para>NOTE: kgdboc is where the kms callbacks are invoked.</para></listitem> 686 </itemizedlist> 687 </para> 688 </listitem> 689 <listitem><para>kgdb arch-specific implementation</para> 690 <para> 691 This implementation is generally found in arch/*/kernel/kgdb.c. 692 As an example, arch/x86/kernel/kgdb.c contains the specifics to 693 implement HW breakpoint as well as the initialization to 694 dynamically register and unregister for the trap handlers on 695 this architecture. The arch-specific portion implements: 696 <itemizedlist> 697 <listitem><para>contains an arch-specific trap catcher which 698 invokes kgdb_handle_exception() to start kgdb about doing its 699 work</para></listitem> 700 <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem> 701 <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem> 702 <listitem><para>Any special exception handling and cleanup</para></listitem> 703 <listitem><para>NMI exception handling and cleanup</para></listitem> 704 <listitem><para>(optional) HW breakpoints</para></listitem> 705 </itemizedlist> 706 </para> 707 </listitem> 708 <listitem><para>gdbstub frontend (aka kgdb)</para> 709 <para>The gdbstub is located in kernel/debug/gdbstub.c. It contains:</para> 710 <itemizedlist> 711 <listitem><para>All the logic to implement the gdb serial protocol</para></listitem> 712 </itemizedlist> 713 </listitem> 714 <listitem><para>kdb frontend</para> 715 <para>The kdb debugger shell is broken down into a number of 716 components. The kdb core is located in kernel/debug/kdb. There 717 are a number of helper functions in some of the other kernel 718 components to make it possible for kdb to examine and report 719 information about the kernel without taking locks that could 720 cause a kernel deadlock. The kdb core contains implements the following functionality.</para> 721 <itemizedlist> 722 <listitem><para>A simple shell</para></listitem> 723 <listitem><para>The kdb core command set</para></listitem> 724 <listitem><para>A registration API to register additional kdb shell commands.</para> 725 <itemizedlist> 726 <listitem><para>A good example of a self-contained kdb module 727 is the "ftdump" command for dumping the ftrace buffer. See: 728 kernel/trace/trace_kdb.c</para></listitem> 729 <listitem><para>For an example of how to dynamically register 730 a new kdb command you can build the kdb_hello.ko kernel module 731 from samples/kdb/kdb_hello.c. To build this example you can 732 set CONFIG_SAMPLES=y and CONFIG_SAMPLE_KDB=m in your kernel 733 config. Later run "modprobe kdb_hello" and the next time you 734 enter the kdb shell, you can run the "hello" 735 command.</para></listitem> 736 </itemizedlist></listitem> 737 <listitem><para>The implementation for kdb_printf() which 738 emits messages directly to I/O drivers, bypassing the kernel 739 log.</para></listitem> 740 <listitem><para>SW / HW breakpoint management for the kdb shell</para></listitem> 741 </itemizedlist> 742 </listitem> 743 <listitem><para>kgdb I/O driver</para> 744 <para> 745 Each kgdb I/O driver has to provide an implementation for the following: 746 <itemizedlist> 747 <listitem><para>configuration via built-in or module</para></listitem> 748 <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem> 749 <listitem><para>read and write character interface</para></listitem> 750 <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem> 751 <listitem><para>(optional) Early debug methodology</para></listitem> 752 </itemizedlist> 753 Any given kgdb I/O driver has to operate very closely with the 754 hardware and must do it in such a way that does not enable 755 interrupts or change other parts of the system context without 756 completely restoring them. The kgdb core will repeatedly "poll" 757 a kgdb I/O driver for characters when it needs input. The I/O 758 driver is expected to return immediately if there is no data 759 available. Doing so allows for the future possibility to touch 760 watchdog hardware in such a way as to have a target system not 761 reset when these are enabled. 762 </para> 763 </listitem> 764 </orderedlist> 765 </para> 766 <para> 767 If you are intent on adding kgdb architecture specific support 768 for a new architecture, the architecture should define 769 <constant>HAVE_ARCH_KGDB</constant> in the architecture specific 770 Kconfig file. This will enable kgdb for the architecture, and 771 at that point you must create an architecture specific kgdb 772 implementation. 773 </para> 774 <para> 775 There are a few flags which must be set on every architecture in 776 their <asm/kgdb.h> file. These are: 777 <itemizedlist> 778 <listitem> 779 <para> 780 NUMREGBYTES: The size in bytes of all of the registers, so 781 that we can ensure they will all fit into a packet. 782 </para> 783 </listitem> 784 <listitem> 785 <para> 786 BUFMAX: The size in bytes of the buffer GDB will read into. 787 This must be larger than NUMREGBYTES. 788 </para> 789 </listitem> 790 <listitem> 791 <para> 792 CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call 793 flush_cache_range or flush_icache_range. On some architectures, 794 these functions may not be safe to call on SMP since we keep other 795 CPUs in a holding pattern. 796 </para> 797 </listitem> 798 </itemizedlist> 799 </para> 800 <para> 801 There are also the following functions for the common backend, 802 found in kernel/kgdb.c, that must be supplied by the 803 architecture-specific backend unless marked as (optional), in 804 which case a default function maybe used if the architecture 805 does not need to provide a specific implementation. 806 </para> 807 !Iinclude/linux/kgdb.h 808 </sect1> 809 <sect1 id="kgdbocDesign"> 810 <title>kgdboc internals</title> 811 <sect2> 812 <title>kgdboc and uarts</title> 813 <para> 814 The kgdboc driver is actually a very thin driver that relies on the 815 underlying low level to the hardware driver having "polling hooks" 816 to which the tty driver is attached. In the initial 817 implementation of kgdboc the serial_core was changed to expose a 818 low level UART hook for doing polled mode reading and writing of a 819 single character while in an atomic context. When kgdb makes an I/O 820 request to the debugger, kgdboc invokes a callback in the serial 821 core which in turn uses the callback in the UART driver.</para> 822 <para> 823 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> 824 #ifdef CONFIG_CONSOLE_POLL 825 .poll_get_char = serial8250_get_poll_char, 826 .poll_put_char = serial8250_put_poll_char, 827 #endif 828 </programlisting> 829 Any implementation specifics around creating a polling driver use the 830 <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above. 831 Keep in mind that polling hooks have to be implemented in such a way 832 that they can be called from an atomic context and have to restore 833 the state of the UART chip on return such that the system can return 834 to normal when the debugger detaches. You need to be very careful 835 with any kind of lock you consider, because failing here is most likely 836 going to mean pressing the reset button. 837 </para> 838 </sect2> 839 <sect2 id="kgdbocKbd"> 840 <title>kgdboc and keyboards</title> 841 <para>The kgdboc driver contains logic to configure communications 842 with an attached keyboard. The keyboard infrastructure is only 843 compiled into the kernel when CONFIG_KDB_KEYBOARD=y is set in the 844 kernel configuration.</para> 845 <para>The core polled keyboard driver driver for PS/2 type keyboards 846 is in drivers/char/kdb_keyboard.c. This driver is hooked into the 847 debug core when kgdboc populates the callback in the array 848 called <constant>kdb_poll_funcs[]</constant>. The 849 kdb_get_kbd_char() is the top-level function which polls hardware 850 for single character input. 851 </para> 852 </sect2> 853 <sect2 id="kgdbocKms"> 854 <title>kgdboc and kms</title> 855 <para>The kgdboc driver contains logic to request the graphics 856 display to switch to a text context when you are using 857 "kgdboc=kms,kbd", provided that you have a video driver which has a 858 frame buffer console and atomic kernel mode setting support.</para> 859 <para> 860 Every time the kernel 861 debugger is entered it calls kgdboc_pre_exp_handler() which in turn 862 calls con_debug_enter() in the virtual console layer. On resuming kernel 863 execution, the kernel debugger calls kgdboc_post_exp_handler() which 864 in turn calls con_debug_leave().</para> 865 <para>Any video driver that wants to be compatible with the kernel 866 debugger and the atomic kms callbacks must implement the 867 mode_set_base_atomic, fb_debug_enter and fb_debug_leave operations. 868 For the fb_debug_enter and fb_debug_leave the option exists to use 869 the generic drm fb helper functions or implement something custom for 870 the hardware. The following example shows the initialization of the 871 .mode_set_base_atomic operation in 872 drivers/gpu/drm/i915/intel_display.c: 873 <informalexample> 874 <programlisting> 875 static const struct drm_crtc_helper_funcs intel_helper_funcs = { 876 [...] 877 .mode_set_base_atomic = intel_pipe_set_base_atomic, 878 [...] 879 }; 880 </programlisting> 881 </informalexample> 882 </para> 883 <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 884 drivers/gpu/drm/i915/intel_fb.c: 885 <informalexample> 886 <programlisting> 887 static struct fb_ops intelfb_ops = { 888 [...] 889 .fb_debug_enter = drm_fb_helper_debug_enter, 890 .fb_debug_leave = drm_fb_helper_debug_leave, 891 [...] 892 }; 893 </programlisting> 894 </informalexample> 895 </para> 896 </sect2> 897 </sect1> 898 </chapter> 899 <chapter id="credits"> 900 <title>Credits</title> 901 <para> 902 The following people have contributed to this document: 903 <orderedlist> 904 <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem> 905 <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem> 906 </orderedlist> 907 In March 2008 this document was completely rewritten by: 908 <itemizedlist> 909 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem> 910 </itemizedlist> 911 In Jan 2010 this document was updated to include kdb. 912 <itemizedlist> 913 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem> 914 </itemizedlist> 915 </para> 916 </chapter> 917 </book>