Chapter 5. Using kgdb / gdb

Table of Contents

Connecting with gdb to a serial port

In order to use kgdb you must activate it by passing configuration information to one of the kgdb I/O drivers. If you do not pass any configuration information kgdb will not do anything at all. Kgdb will only actively hook up to the kernel trap hooks if a kgdb I/O driver is loaded and configured. If you unconfigure a kgdb I/O driver, kgdb will unregister all the kernel hook points.

All kgdb I/O drivers can be reconfigured at run time, if CONFIG_SYSFS and CONFIG_MODULES are enabled, by echo'ing a new config string to /sys/module/<driver>/parameter/<option>. The driver can be unconfigured by passing an empty string. You cannot change the configuration while the debugger is attached. Make sure to detach the debugger with the detach command prior to trying to unconfigure a kgdb I/O driver.

Connecting with gdb to a serial port

  1. Configure kgdboc

    Boot kernel with arguments:

    • kgdboc=ttyS0,115200


    Configure kgdboc after the kernel booted:

    • echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc

  2. Stop kernel execution (break into the debugger)

    In order to connect to gdb via kgdboc, the kernel must first be stopped. There are several ways to stop the kernel which include using kgdbwait as a boot argument, via a sysrq-g, or running the kernel until it takes an exception where it waits for the debugger to attach.

    • 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: Control-a

      Press: f

      Press: g

    • When you have telneted to a terminal server that supports sending a remote break

      Press: Control-]

      Type in:send break

      Press: Enter

      Press: g

  3. Connect from from gdb

    Example (using a directly connected port):

        % gdb ./vmlinux
        (gdb) set remotebaud 115200
        (gdb) target remote /dev/ttyS0

    Example (kgdb to a terminal server on TCP port 2012):

        % gdb ./vmlinux
        (gdb) target remote

    Once connected, you can debug a kernel the way you would debug an application program.

    If you are having problems connecting or something is going seriously wrong while debugging, it will most often be the case that you want to enable gdb to be verbose about its target communications. You do this prior to issuing the target remote command by typing in: set debug remote 1

Remember if you continue in gdb, and need to "break in" again, you need to issue an other sysrq-g. It is easy to create a simple entry point by putting a breakpoint at sys_sync and then you can run "sync" from a shell or script to break into the debugger.