Documentation/DocBook/kgdb.tmpl

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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 and the kgdb Internals</title>
   8
   9   <authorgroup>
  10    <author>
  11     <firstname>Jason</firstname>
  12     <surname>Wessel</surname>
  13     <affiliation>
  14      <address>
  15       <email>jason.wessel@windriver.com</email>
  16      </address>
  17     </affiliation>
  18    </author>
  19   </authorgroup>
  20
  21   <authorgroup>
  22    <author>
  23     <firstname>Tom</firstname>
  24     <surname>Rini</surname>
  25     <affiliation>
  26      <address>
  27       <email>trini@kernel.crashing.org</email>
  28      </address>
  29     </affiliation>
  30    </author>
  31   </authorgroup>
  32
  33   <authorgroup>
  34    <author>
  35     <firstname>Amit S.</firstname>
  36     <surname>Kale</surname>
  37     <affiliation>
  38      <address>
  39       <email>amitkale@linsyssoft.com</email>
  40      </address>
  41     </affiliation>
  42    </author>
  43   </authorgroup>
  44
  45   <copyright>
  46    <year>2008</year>
  47    <holder>Wind River Systems, Inc.</holder>
  48   </copyright>
  49   <copyright>
  50    <year>2004-2005</year>
  51    <holder>MontaVista Software, Inc.</holder>
  52   </copyright>
  53   <copyright>
  54    <year>2004</year>
  55    <holder>Amit S. Kale</holder>
  56   </copyright>
  57
  58   <legalnotice>
  59    <para>
  60    This file is licensed under the terms of the GNU General Public License
  61    version 2. This program is licensed "as is" without any warranty of any
  62    kind, whether express or implied.
  63    </para>
  64
  65   </legalnotice>
  66  </bookinfo>
  67
  68 <toc></toc>
  69   <chapter id="Introduction">
  70     <title>Introduction</title>
  71     <para>
  72     kgdb is a source level debugger for linux kernel. It is used along
  73     with gdb to debug a linux kernel.  The expectation is that gdb can
  74     be used to "break in" to the kernel to inspect memory, variables
  75     and look through call stack information similar to what an
  76     application developer would use gdb for.  It is possible to place
  77     breakpoints in kernel code and perform some limited execution
  78     stepping.
  79     </para>
  80     <para>
  81     Two machines are required for using kgdb. One of these machines is a
  82     development machine and the other is a test machine.  The kernel
  83     to be debugged runs on the test machine. The development machine
  84     runs an instance of gdb against the vmlinux file which contains
  85     the symbols (not boot image such as bzImage, zImage, uImage...).
  86     In gdb the developer specifies the connection parameters and
  87     connects to kgdb.  The type of connection a developer makes with
  88     gdb depends on the availability of kgdb I/O modules compiled as
  89     builtin's or kernel modules in the test machine's kernel.
  90     </para>
  91   </chapter>
  92   <chapter id="CompilingAKernel">
  93     <title>Compiling a kernel</title>
  94     <para>
  95     To enable <symbol>CONFIG_KGDB</symbol> you should first turn on
  96     "Prompt for development and/or incomplete code/drivers"
  97     (CONFIG_EXPERIMENTAL) in  "General setup", then under the
  98     "Kernel debugging" select "KGDB: kernel debugging with remote gdb".
  99     </para>
 100     <para>
 101     It is advised, but not required that you turn on the
 102     CONFIG_FRAME_POINTER kernel option.  This option inserts code to
 103     into the compiled executable which saves the frame information in
 104     registers or on the stack at different points which will allow a
 105     debugger such as gdb to more accurately construct stack back traces
 106     while debugging the kernel.
 107     </para>
 108     <para>
 109     If the architecture that you are using supports the kernel option
 110     CONFIG_DEBUG_RODATA, you should consider turning it off.  This
 111     option will prevent the use of software breakpoints because it
 112     marks certain regions of the kernel's memory space as read-only.
 113     If kgdb supports it for the architecture you are using, you can
 114     use hardware breakpoints if you desire to run with the
 115     CONFIG_DEBUG_RODATA option turned on, else you need to turn off
 116     this option.
 117     </para>
 118     <para>
 119     Next you should choose one of more I/O drivers to interconnect debugging
 120     host and debugged target.  Early boot debugging requires a KGDB
 121     I/O driver that supports early debugging and the driver must be
 122     built into the kernel directly. Kgdb I/O driver configuration
 123     takes place via kernel or module parameters, see following
 124     chapter.
 125     </para>
 126     <para>
 127     The kgdb test compile options are described in the kgdb test suite chapter.
 128     </para>
 129
 130   </chapter>
 131   <chapter id="EnableKGDB">
 132    <title>Enable kgdb for debugging</title>
 133    <para>
 134    In order to use kgdb you must activate it by passing configuration
 135    information to one of the kgdb I/O drivers.  If you do not pass any
 136    configuration information kgdb will not do anything at all.  Kgdb
 137    will only actively hook up to the kernel trap hooks if a kgdb I/O
 138    driver is loaded and configured.  If you unconfigure a kgdb I/O
 139    driver, kgdb will unregister all the kernel hook points.
 140    </para>
 141    <para>
 142    All drivers can be reconfigured at run time, if
 143    <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
 144    are enabled, by echo'ing a new config string to
 145    <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
 146    The driver can be unconfigured by passing an empty string.  You cannot
 147    change the configuration while the debugger is attached.  Make sure
 148    to detach the debugger with the <constant>detach</constant> command
 149    prior to trying unconfigure a kgdb I/O driver.
 150    </para>
 151    <sect1 id="kgdbwait">
 152    <title>Kernel parameter: kgdbwait</title>
 153    <para>
 154    The Kernel command line option <constant>kgdbwait</constant> makes
 155    kgdb wait for a debugger connection during booting of a kernel.  You
 156    can only use this option you compiled a kgdb I/O driver into the
 157    kernel and you specified the I/O driver configuration as a kernel
 158    command line option.  The kgdbwait parameter should always follow the
 159    configuration parameter for the kgdb I/O driver in the kernel
 160    command line else the I/O driver will not be configured prior to
 161    asking the kernel to use it to wait.
 162    </para>
 163    <para>
 164    The kernel will stop and wait as early as the I/O driver and
 165    architecture will allow when you use this option.  If you build the
 166    kgdb I/O driver as a kernel module kgdbwait will not do anything.
 167    </para>
 168    </sect1>
 169   <sect1 id="kgdboc">
 170   <title>Kernel parameter: kgdboc</title>
 171   <para>
 172   The kgdboc driver was originally an abbreviation meant to stand for
 173   "kgdb over console".  Kgdboc is designed to work with a single
 174   serial port. It was meant to cover the circumstance
 175   where you wanted to use a serial console as your primary console as
 176   well as using it to perform kernel debugging.  Of course you can
 177   also use kgdboc without assigning a console to the same port.
 178   </para>
 179   <sect2 id="UsingKgdboc">
 180   <title>Using kgdboc</title>
 181   <para>
 182   You can configure kgdboc via sysfs or a module or kernel boot line
 183   parameter depending on if you build with CONFIG_KGDBOC as a module
 184   or built-in.
 185   <orderedlist>
 186   <listitem><para>From the module load or build-in</para>
 187   <para><constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
 188   <para>
 189   The example here would be if your console port was typically ttyS0, you would use something like <constant>kgdboc=ttyS0,115200</constant> or on the ARM Versatile AB you would likely use <constant>kgdboc=ttyAMA0,115200</constant>
 190   </para>
 191   </listitem>
 192   <listitem><para>From sysfs</para>
 193   <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para>
 194   </listitem>
 195   </orderedlist>
 196   </para>
 197   <para>
 198   NOTE: Kgdboc does not support interrupting the target via the
 199   gdb remote protocol.  You must manually send a sysrq-g unless you
 200   have a proxy that splits console output to a terminal problem and
 201   has a separate port for the debugger to connect to that sends the
 202   sysrq-g for you.
 203   </para>
 204   <para>When using kgdboc with no debugger proxy, you can end up
 205   connecting the debugger for one of two entry points.  If an
 206   exception occurs after you have loaded kgdboc a message should print
 207   on the console stating it is waiting for the debugger.  In case you
 208   disconnect your terminal program and then connect the debugger in
 209   its place.  If you want to interrupt the target system and forcibly
 210   enter a debug session you have to issue a Sysrq sequence and then
 211   type the letter <constant>g</constant>.  Then you disconnect the
 212   terminal session and connect gdb.  Your options if you don't like
 213   this are to hack gdb to send the sysrq-g for you as well as on the
 214   initial connect, or to use a debugger proxy that allows an
 215   unmodified gdb to do the debugging.
 216   </para>
 217   </sect2>
 218   </sect1>
 219   <sect1 id="kgdbcon">
 220   <title>Kernel parameter: kgdbcon</title>
 221   <para>
 222   Kgdb supports using the gdb serial protocol to send console messages
 223   to the debugger when the debugger is connected and running.  There
 224   are two ways to activate this feature.
 225   <orderedlist>
 226   <listitem><para>Activate with the kernel command line option:</para>
 227   <para><constant>kgdbcon</constant></para>
 228   </listitem>
 229   <listitem><para>Use sysfs before configuring an io driver</para>
 230   <para>
 231   <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
 232   </para>
 233   <para>
 234   NOTE: If you do this after you configure the kgdb I/O driver, the
 235   setting will not take effect until the next point the I/O is
 236   reconfigured.
 237   </para>
 238   </listitem>
 239   </orderedlist>
 240   </para>
 241   <para>
 242   IMPORTANT NOTE: Using this option with kgdb over the console
 243   (kgdboc) is not supported.
 244   </para>
 245   </sect1>
 246   </chapter>
 247   <chapter id="ConnectingGDB">
 248   <title>Connecting gdb</title>
 249     <para>
 250     If you are using kgdboc, you need to have used kgdbwait as a boot
 251     argument, issued a sysrq-g, or the system you are going to debug
 252     has already taken an exception and is waiting for the debugger to
 253     attach before you can connect gdb.
 254     </para>
 255     <para>
 256     If you are not using different kgdb I/O driver other than kgdboc,
 257     you should be able to connect and the target will automatically
 258     respond.
 259     </para>
 260     <para>
 261     Example (using a serial port):
 262     </para>
 263     <programlisting>
 264     % gdb ./vmlinux
 265     (gdb) set remotebaud 115200
 266     (gdb) target remote /dev/ttyS0
 267     </programlisting>
 268     <para>
 269     Example (kgdb to a terminal server on tcp port 2012):
 270     </para>
 271     <programlisting>
 272     % gdb ./vmlinux
 273     (gdb) target remote 192.168.2.2:2012
 274     </programlisting>
 275     <para>
 276     Once connected, you can debug a kernel the way you would debug an
 277     application program.
 278     </para>
 279     <para>
 280     If you are having problems connecting or something is going
 281     seriously wrong while debugging, it will most often be the case
 282     that you want to enable gdb to be verbose about its target
 283     communications.  You do this prior to issuing the <constant>target
 284     remote</constant> command by typing in: <constant>set debug remote 1</constant>
 285     </para>
 286   </chapter>
 287   <chapter id="KGDBTestSuite">
 288     <title>kgdb Test Suite</title>
 289     <para>
 290     When kgdb is enabled in the kernel config you can also elect to
 291     enable the config parameter KGDB_TESTS.  Turning this on will
 292     enable a special kgdb I/O module which is designed to test the
 293     kgdb internal functions.
 294     </para>
 295     <para>
 296     The kgdb tests are mainly intended for developers to test the kgdb
 297     internals as well as a tool for developing a new kgdb architecture
 298     specific implementation.  These tests are not really for end users
 299     of the Linux kernel.  The primary source of documentation would be
 300     to look in the drivers/misc/kgdbts.c file.
 301     </para>
 302     <para>
 303     The kgdb test suite can also be configured at compile time to run
 304     the core set of tests by setting the kernel config parameter
 305     KGDB_TESTS_ON_BOOT.  This particular option is aimed at automated
 306     regression testing and does not require modifying the kernel boot
 307     config arguments.  If this is turned on, the kgdb test suite can
 308     be disabled by specifying "kgdbts=" as a kernel boot argument.
 309     </para>
 310   </chapter>
 311   <chapter id="CommonBackEndReq">
 312   <title>KGDB Internals</title>
 313   <sect1 id="kgdbArchitecture">
 314     <title>Architecture Specifics</title>
 315       <para>
 316       Kgdb is organized into three basic components:
 317       <orderedlist>
 318       <listitem><para>kgdb core</para>
 319       <para>
 320       The kgdb core is found in kernel/kgdb.c.  It contains:
 321       <itemizedlist>
 322       <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
 323       <listitem><para>A generic OS exception handler which includes sync'ing the processors into a stopped state on an multi cpu system.</para></listitem>
 324       <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
 325       <listitem><para>The API to make calls to the arch specific kgdb implementation</para></listitem>
 326       <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
 327       <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
 328       </itemizedlist>
 329       </para>
 330       </listitem>
 331       <listitem><para>kgdb arch specific implementation</para>
 332       <para>
 333       This implementation is generally found in arch/*/kernel/kgdb.c.
 334       As an example, arch/x86/kernel/kgdb.c contains the specifics to
 335       implement HW breakpoint as well as the initialization to
 336       dynamically register and unregister for the trap handlers on
 337       this architecture.  The arch specific portion implements:
 338       <itemizedlist>
 339       <listitem><para>contains an arch specific trap catcher which
 340       invokes kgdb_handle_exception() to start kgdb about doing its
 341       work</para></listitem>
 342       <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
 343       <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
 344       <listitem><para>Any special exception handling and cleanup</para></listitem>
 345       <listitem><para>NMI exception handling and cleanup</para></listitem>
 346       <listitem><para>(optional)HW breakpoints</para></listitem>
 347       </itemizedlist>
 348       </para>
 349       </listitem>
 350       <listitem><para>kgdb I/O driver</para>
 351       <para>
 352       Each kgdb I/O driver has to provide an implemenation for the following:
 353       <itemizedlist>
 354       <listitem><para>configuration via builtin or module</para></listitem>
 355       <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
 356       <listitem><para>read and write character interface</para></listitem>
 357       <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
 358       <listitem><para>(optional) Early debug methodology</para></listitem>
 359       </itemizedlist>
 360       Any given kgdb I/O driver has to operate very closely with the
 361       hardware and must do it in such a way that does not enable
 362       interrupts or change other parts of the system context without
 363       completely restoring them. The kgdb core will repeatedly "poll"
 364       a kgdb I/O driver for characters when it needs input.  The I/O
 365       driver is expected to return immediately if there is no data
 366       available.  Doing so allows for the future possibility to touch
 367       watch dog hardware in such a way as to have a target system not
 368       reset when these are enabled.
 369       </para>
 370       </listitem>
 371       </orderedlist>
 372       </para>
 373       <para>
 374       If you are intent on adding kgdb architecture specific support
 375       for a new architecture, the architecture should define
 376       <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
 377       Kconfig file.  This will enable kgdb for the architecture, and
 378       at that point you must create an architecture specific kgdb
 379       implementation.
 380       </para>
 381       <para>
 382       There are a few flags which must be set on every architecture in
 383       their &lt;asm/kgdb.h&gt; file.  These are:
 384       <itemizedlist>
 385         <listitem>
 386           <para>
 387           NUMREGBYTES: The size in bytes of all of the registers, so
 388           that we can ensure they will all fit into a packet.
 389           </para>
 390           <para>
 391           BUFMAX: The size in bytes of the buffer GDB will read into.
 392           This must be larger than NUMREGBYTES.
 393           </para>
 394           <para>
 395           CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
 396           flush_cache_range or flush_icache_range.  On some architectures,
 397           these functions may not be safe to call on SMP since we keep other
 398           CPUs in a holding pattern.
 399           </para>
 400         </listitem>
 401       </itemizedlist>
 402       </para>
 403       <para>
 404       There are also the following functions for the common backend,
 405       found in kernel/kgdb.c, that must be supplied by the
 406       architecture-specific backend unless marked as (optional), in
 407       which case a default function maybe used if the architecture
 408       does not need to provide a specific implementation.
 409       </para>
 410 !Iinclude/linux/kgdb.h
 411   </sect1>
 412   <sect1 id="kgdbocDesign">
 413   <title>kgdboc internals</title>
 414   <para>
 415   The kgdboc driver is actually a very thin driver that relies on the
 416   underlying low level to the hardware driver having "polling hooks"
 417   which the to which the tty driver is attached.  In the initial
 418   implementation of kgdboc it the serial_core was changed to expose a
 419   low level uart hook for doing polled mode reading and writing of a
 420   single character while in an atomic context.  When kgdb makes an I/O
 421   request to the debugger, kgdboc invokes a call back in the serial
 422   core which in turn uses the call back in the uart driver.  It is
 423   certainly possible to extend kgdboc to work with non-uart based
 424   consoles in the future.
 425   </para>
 426   <para>
 427   When using kgdboc with a uart, the uart driver must implement two callbacks in the <constant>struct uart_ops</constant>. Example from drivers/8250.c:<programlisting>
 428 #ifdef CONFIG_CONSOLE_POLL
 429         .poll_get_char = serial8250_get_poll_char,
 430         .poll_put_char = serial8250_put_poll_char,
 431 #endif
 432   </programlisting>
 433   Any implementation specifics around creating a polling driver use the
 434   <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
 435   Keep in mind that polling hooks have to be implemented in such a way
 436   that they can be called from an atomic context and have to restore
 437   the state of the uart chip on return such that the system can return
 438   to normal when the debugger detaches.  You need to be very careful
 439   with any kind of lock you consider, because failing here is most
 440   going to mean pressing the reset button.
 441   </para>
 442   </sect1>
 443   </chapter>
 444   <chapter id="credits">
 445      <title>Credits</title>
 446         <para>
 447                 The following people have contributed to this document:
 448                 <orderedlist>
 449                         <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
 450                         <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
 451                 </orderedlist>
 452                 In March 2008 this document was completely rewritten by:
 453                 <itemizedlist>
 454                 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
 455                 </itemizedlist>
 456         </para>
 457   </chapter>
 458 </book>
 459