=================================================Using kgdb, kdb and the kernel debugger internals================================================= :Author: Jason Wessel Introduction============ The kernel has two different debugger front ends (kdb and kgdb) whichinterface to the debug core. It is possible to use either of thedebugger front ends and dynamically transition between them if youconfigure the kernel properly at compile and runtime. Kdb is simplistic shell-style interface which you can use on a systemconsole with a keyboard or serial console. You can use it to inspectmemory, registers, process lists, dmesg, and even set breakpoints tostop in a certain location. Kdb is not a source level debugger, althoughyou can set breakpoints and execute some basic kernel run control. Kdbis mainly aimed at doing some analysis to aid in development ordiagnosing kernel problems. You can access some symbols by name inkernel built-ins or in kernel modules if the code was built with``CONFIG_KALLSYMS``. Kgdb is intended to be used as a source level debugger for the Linuxkernel. It is used along with gdb to debug a Linux kernel. Theexpectation is that gdb can be used to "break in" to the kernel toinspect memory, variables and look through call stack informationsimilar to the way an application developer would use gdb to debug anapplication. It is possible to place breakpoints in kernel code andperform some limited execution stepping. Two machines are required for using kgdb. One of these machines is adevelopment machine and the other is the target machine. The kernel tobe debugged runs on the target machine. The development machine runs aninstance of gdb against the vmlinux file which contains the symbols (nota boot image such as bzImage, zImage, uImage...). In gdb the developerspecifies the connection parameters and connects to kgdb. The type ofconnection a developer makes with gdb depends on the availability ofkgdb I/O modules compiled as built-ins or loadable kernel modules in thetest machine's kernel. Compiling a kernel================== -  In order to enable compilation of kdb, you must first enable kgdb. -  The kgdb test compile options are described in the kgdb test suite   chapter. Kernel config options for kgdb------------------------------ To enable ``CONFIG_KGDB`` you should look under:menuselection:`Kernel hacking --> Kernel debugging` and select:menuselection:`KGDB: kernel debugger`. While it is not a hard requirement that you have symbols in your vmlinuxfile, gdb tends not to be very useful without the symbolic data, so youwill want to turn on ``CONFIG_DEBUG_INFO`` which is called:menuselection:`Compile the kernel with debug info` in the config menu. It is advised, but not required, that you turn on the``CONFIG_FRAME_POINTER`` kernel option which is called :menuselection:`Compilethe kernel with frame pointers` in the config menu. This option inserts codeinto the compiled executable which saves the frame information in registersor on the stack at different points which allows a debugger such as gdb tomore accurately construct stack back traces while debugging the kernel. If the architecture that you are using supports the kernel option``CONFIG_STRICT_KERNEL_RWX``, you should consider turning it off. Thisoption will prevent the use of software breakpoints because it markscertain regions of the kernel's memory space as read-only. If kgdbsupports it for the architecture you are using, you can use hardwarebreakpoints if you desire to run with the ``CONFIG_STRICT_KERNEL_RWX``option turned on, else you need to turn off this option. Next you should choose one or more I/O drivers to interconnect the debugginghost and debugged target. Early boot debugging requires a KGDB I/Odriver that supports early debugging and the driver must be built intothe kernel directly. Kgdb I/O driver configuration takes place viakernel or module parameters which you can learn more about in thesection that describes the parameter kgdboc. Here is an example set of ``.config`` symbols to enable or disable for kgdb::   # CONFIG_STRICT_KERNEL_RWX is not set  CONFIG_FRAME_POINTER=y  CONFIG_KGDB=y  CONFIG_KGDB_SERIAL_CONSOLE=y Kernel config options for kdb----------------------------- Kdb is quite a bit more complex than the simple gdbstub sitting on topof the kernel's debug core. Kdb must implement a shell, and also addssome helper functions in other parts of the kernel, responsible forprinting out interesting data such as what you would see if you ran``lsmod``, or ``ps``. In order to build kdb into the kernel you follow thesame steps as you would for kgdb. The main config option for kdb is ``CONFIG_KGDB_KDB`` which is called:menuselection:`KGDB_KDB: include kdb frontend for kgdb` in the config menu.In theory you would have already also selected an I/O driver such as the``CONFIG_KGDB_SERIAL_CONSOLE`` interface if you plan on using kdb on aserial port, when you were configuring kgdb. If you want to use a PS/2-style keyboard with kdb, you would select``CONFIG_KDB_KEYBOARD`` which is called :menuselection:`KGDB_KDB: keyboard asinput device` in the config menu. The ``CONFIG_KDB_KEYBOARD`` option is notused for anything in the gdb interface to kgdb. The ``CONFIG_KDB_KEYBOARD``option only works with kdb. Here is an example set of ``.config`` symbols to enable/disable kdb::   # CONFIG_STRICT_KERNEL_RWX is not set  CONFIG_FRAME_POINTER=y  CONFIG_KGDB=y  CONFIG_KGDB_SERIAL_CONSOLE=y  CONFIG_KGDB_KDB=y  CONFIG_KDB_KEYBOARD=y Kernel Debugger Boot Arguments============================== This section describes the various runtime kernel parameters that affectthe configuration of the kernel debugger. The following chapter coversusing kdb and kgdb as well as providing some examples of theconfiguration parameters. Kernel parameter: kgdboc------------------------ The kgdboc driver was originally an abbreviation meant to stand for"kgdb over console". Today it is the primary mechanism to configure howto communicate from gdb to kgdb as well as the devices you want to useto interact with the kdb shell. For kgdb/gdb, kgdboc is designed to work with a single serial port. Itis intended to cover the circumstance where you want to use a serialconsole as your primary console as well as using it to perform kerneldebugging. It is also possible to use kgdb on a serial port which is notdesignated as a system console. Kgdboc may be configured as a kernelbuilt-in or a kernel loadable module. You can only make use of``kgdbwait`` and early debugging if you build kgdboc into the kernel asa built-in. Optionally you can elect to activate kms (Kernel Mode Setting)integration. When you use kms with kgdboc and you have a video driverthat has atomic mode setting hooks, it is possible to enter the debuggeron the graphics console. When the kernel execution is resumed, theprevious graphics mode will be restored. This integration can serve as auseful tool to aid in diagnosing crashes or doing analysis of memorywith kdb while allowing the full graphics console applications to run. kgdboc arguments~~~~~~~~~~~~~~~~ Usage:: 	kgdboc=[kms][[,]kbd][[,]serial_device][,baud] The order listed above must be observed if you use any of the optionalconfigurations together. Abbreviations: -  kms = Kernel Mode Setting -  kbd = Keyboard You can configure kgdboc to use the keyboard, and/or a serial devicedepending on if you are using kdb and/or kgdb, in one of the followingscenarios. The order listed above must be observed if you use any of theoptional configurations together. Using kms + only gdb is generally nota useful combination. Using loadable module or built-in^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1. As a kernel built-in:    Use the kernel boot argument:: 	kgdboc=<tty-device>,[baud] 2. As a kernel loadable module:    Use the command:: 	modprobe kgdboc kgdboc=<tty-device>,[baud]    Here are two examples of how you might format the kgdboc string. The   first is for an x86 target using the first serial port. The second   example is for the ARM Versatile AB using the second serial port.    1. ``kgdboc=ttyS0,115200``    2. ``kgdboc=ttyAMA1,115200`` Configure kgdboc at runtime with sysfs^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ At run time you can enable or disable kgdboc by writing parametersinto sysfs. Here are two examples: 1. Enable kgdboc on ttyS0:: 	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc 2. Disable kgdboc:: 	echo "" > /sys/module/kgdboc/parameters/kgdboc .. note::    You do not need to specify the baud if you are configuring the   console on tty which is already configured or open. More examples^^^^^^^^^^^^^ You can configure kgdboc to use the keyboard, and/or a serial devicedepending on if you are using kdb and/or kgdb, in one of the followingscenarios. 1. kdb and kgdb over only a serial port:: 	kgdboc=<serial_device>[,baud]    Example:: 	kgdboc=ttyS0,115200 2. kdb and kgdb with keyboard and a serial port:: 	kgdboc=kbd,<serial_device>[,baud]    Example:: 	kgdboc=kbd,ttyS0,115200 3. kdb with a keyboard:: 	kgdboc=kbd 4. kdb with kernel mode setting:: 	kgdboc=kms,kbd 5. kdb with kernel mode setting and kgdb over a serial port:: 	kgdboc=kms,kbd,ttyS0,115200 .. note::    Kgdboc does not support interrupting the target via the gdb remote   protocol. You must manually send a `SysRq-G` unless you have a proxy   that splits console output to a terminal program. A console proxy has a   separate TCP port for the debugger and a separate TCP port for the   "human" console. The proxy can take care of sending the `SysRq-G`   for you. When using kgdboc with no debugger proxy, you can end up connecting thedebugger at one of two entry points. If an exception occurs after youhave loaded kgdboc, a message should print on the console stating it iswaiting for the debugger. In this case you disconnect your terminalprogram and then connect the debugger in its place. If you want tointerrupt the target system and forcibly enter a debug session you haveto issue a `Sysrq` sequence and then type the letter `g`. Then youdisconnect the terminal session and connect gdb. Your options if youdon't like this are to hack gdb to send the `SysRq-G` for you as well ason the initial connect, or to use a debugger proxy that allows anunmodified gdb to do the debugging. Kernel parameter: ``kgdboc_earlycon``------------------------------------- If you specify the kernel parameter ``kgdboc_earlycon`` and your serialdriver registers a boot console that supports polling (doesn't needinterrupts and implements a nonblocking read() function) kgdb will attemptto work using the boot console until it can transition to the regulartty driver specified by the ``kgdboc`` parameter. Normally there is only one boot console (especially that implements theread() function) so just adding ``kgdboc_earlycon`` on its own issufficient to make this work. If you have more than one boot console youcan add the boot console's name to differentiate. Note that names thatare registered through the boot console layer and the tty layer are notthe same for the same port. For instance, on one board to be explicit you might do::    kgdboc_earlycon=qcom_geni kgdboc=ttyMSM0 If the only boot console on the device was "qcom_geni", you could simplify::    kgdboc_earlycon kgdboc=ttyMSM0 Kernel parameter: ``kgdbwait``------------------------------ The Kernel command line option ``kgdbwait`` makes kgdb wait for adebugger connection during booting of a kernel. You can only use thisoption if you compiled a kgdb I/O driver into the kernel and youspecified the I/O driver configuration as a kernel command line option.The kgdbwait parameter should always follow the configuration parameterfor the kgdb I/O driver in the kernel command line else the I/O driverwill not be configured prior to asking the kernel to use it to wait. The kernel will stop and wait as early as the I/O driver andarchitecture allows when you use this option. If you build the kgdb I/Odriver as a loadable kernel module kgdbwait will not do anything. Kernel parameter: ``kgdbcon``----------------------------- The ``kgdbcon`` feature allows you to see printk() messages inside gdbwhile gdb is connected to the kernel. Kdb does not make use of the kgdbconfeature. Kgdb supports using the gdb serial protocol to send console messages tothe debugger when the debugger is connected and running. There are twoways to activate this feature. 1. Activate with the kernel command line option:: 	kgdbcon 2. Use sysfs before configuring an I/O driver:: 	echo 1 > /sys/module/debug_core/parameters/kgdb_use_con .. note::    If you do this after you configure the kgdb I/O driver, the   setting will not take effect until the next point the I/O is   reconfigured. .. important::    You cannot use kgdboc + kgdbcon on a tty that is an   active system console. An example of incorrect usage is:: 	console=ttyS0,115200 kgdboc=ttyS0 kgdbcon It is possible to use this option with kgdboc on a tty that is not asystem console. Run time parameter: ``kgdbreboot``---------------------------------- The kgdbreboot feature allows you to change how the debugger deals withthe reboot notification. You have 3 choices for the behavior. Thedefault behavior is always set to 0. .. tabularcolumns:: |p{0.4cm}|p{11.5cm}|p{5.6cm}| .. flat-table::  :widths: 1 10 8   * - 1    - ``echo -1 > /sys/module/debug_core/parameters/kgdbreboot``    - Ignore the reboot notification entirely.   * - 2    - ``echo 0 > /sys/module/debug_core/parameters/kgdbreboot``    - Send the detach message to any attached debugger client.   * - 3    - ``echo 1 > /sys/module/debug_core/parameters/kgdbreboot``    - Enter the debugger on reboot notify. Kernel parameter: ``nokaslr``----------------------------- If the architecture that you are using enables KASLR by default,you should consider turning it off.  KASLR randomizes thevirtual address where the kernel image is mapped and confusesgdb which resolves addresses of kernel symbols from the symbol tableof vmlinux. Kernel parameter: ``rodata``---------------------------- ``CONFIG_STRICT_KERNEL_RWX`` is turned on by default and is notvisible to menuconfig on some architectures (arm64 for example),you can pass ``rodata=off`` to the kernel in this case. Using kdb========= Quick start for kdb on a serial port------------------------------------ This is a quick example of how to use kdb. 1. Configure kgdboc at boot using kernel parameters:: 	console=ttyS0,115200 kgdboc=ttyS0,115200 nokaslr    OR    Configure kgdboc after the kernel has booted; assuming you are using   a serial port console:: 	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc 2. 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.    -  When logged in as root or with a super user session you can run:: 	echo g > /proc/sysrq-trigger    -  Example using minicom 2.2       Press: `CTRL-A` `f` `g`    -  When you have telneted to a terminal server that supports sending      a remote break       Press: `CTRL-]`       Type in: ``send break``       Press: `Enter` `g` 3. From the kdb prompt you can run the ``help`` command to see a complete   list of the commands that are available.    Some useful commands in kdb include:    =========== =================================================================   ``lsmod``   Shows where kernel modules are loaded   ``ps``      Displays only the active processes   ``ps A``    Shows all the processes   ``summary`` Shows kernel version info and memory usage   ``bt``      Get a backtrace of the current process using dump_stack()   ``dmesg``   View the kernel syslog buffer   ``go``      Continue the system   =========== ================================================================= 4. When you are done using kdb you need to consider rebooting the system   or using the ``go`` command to resuming normal kernel execution. If you   have paused the kernel for a lengthy period of time, applications   that rely on timely networking or anything to do with real wall clock   time could be adversely affected, so you should take this into   consideration when using the kernel debugger. Quick start for kdb using a keyboard connected console------------------------------------------------------ This is a quick example of how to use kdb with a keyboard. 1. Configure kgdboc at boot using kernel parameters:: 	kgdboc=kbd    OR    Configure kgdboc after the kernel has booted:: 	echo kbd > /sys/module/kgdboc/parameters/kgdboc 2. 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.    -  When logged in as root or with a super user session you can run:: 	echo g > /proc/sysrq-trigger    -  Example using a laptop keyboard:       Press and hold down: `Alt`       Press and hold down: `Fn`       Press and release the key with the label: `SysRq`       Release: `Fn`       Press and release: `g`       Release: `Alt`    -  Example using a PS/2 101-key keyboard       Press and hold down: `Alt`       Press and release the key with the label: `SysRq`       Press and release: `g`       Release: `Alt` 

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