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KDB

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Introduction and basic resources

Here is some information about KDB - the in-kernel debugger for the Linux kernel.

The KDB and KGDB official wiki: https://kgdb.wiki.kernel.org/ (this only has 2 pages?)

Jason Wessel is the current KDB maintainer. Here is a presentation from him at LinuxCon 2010 (August 2010): http://kernel.org/pub/linux/kernel/people/jwessel/dbg_webinar/State_Of_kernel_debugging_LinuxCon2010.pdf

Here are some videos showing use of KDB and KGDB:

Documentation, up-to-date as of 2010, for KDB and KGDB is at: http://kernel.org/pub/linux/kernel/people/jwessel/kdb/

Older Information

See http://www.ibm.com/developerworks/linux/library/l-kdbug/ for a tutorial for the 2.4.20 kernel (from June 2003)

Here's an article from 2002 on KDB vs. KGDB: http://kerneltrap.org/node/112 It has a good discussion excerpt between Andrew Morton and Keith Owens about the relative merits of KDB versus KGDB.

Questions

Here are some questions to answer:

  • What kernel versions are supported?
    • 2.6.35 and on
  • what kernel configs are required to be set?
    • see below
  • how to invoke the debugger?
    • sysrq trigger ('echo g >/proc/sysrq-trigger')
    • sysrq serial console combo (in minicom, ctrl-a f g (quickly))
    • panic
    • breakpoint

Kernel configuration

The following descriptions are for a 2.6.35 kernel, using KDB over a serial line between host and target:

All these options on are the "Kernel Hacking" menu.

In order to support KDB, "KGDB" support must be turned on first (even if you aren't using kgdb/gdb)

  • CONFIG_DEBUG_KERNEL=y - includes debug information in the kernel compilation - required for basic kernel debugging support
  • CONFIG_KGDB=Y - turn on basic kernel debug agent support
  • CONFIG_KGDB_SERIAL_CONSOLE=Y - to share a serial console with kgdb.
    • Sysrq-g must be used to break in initially.
    • Selecting this will automatically set:
      • CONFIG_CONSOLE_POLL=N
      • CONFIG_MAGIC_SYSRQ=Y - turn on MAGIC-SYSRQ key support
  • CONFIG_KGDB_KDB=Y - actually turn on the KDB debugger feature

Optional other configuration settings:

  • CONFIG_FRAME_POINTER=Y - this allows for better backtrace support in KDB
  • CONFIG_DEBUG_RODATA=N - disable this in order to support hardware breakpoints on data accesses
  • CONFIG_KALLSYMS=Y - this adds symbolic information to the kernel, useful to see symbols instead of addresses
  • CONFIG_KDB_KEYBOARD - use KDB with an attached keyboard (not for use with serial console)
  • CONFIG_KGDB_TESTS - used to turn on kgdb internal self-tests - see the config help for this for more information

Using gdb to see the kernel source listing

You can use the addresses printed out in kdb, with a host-side gdb session, to see the source code or assembly instructions around a particular address.

The target address can come from a backtrace or register dump (e.g. instruction pointer).

To load the source for a kernel, start gdb (or the appropriate arch-specific gdb) with the vmlinux that matches the image running on target. The kernel should have been compiled with debug symbols (CONFIG_DEBUG_KERNEL=y). gdb will start, and load the symbol information for the kernel.

Use the following commands to see various bits of information:

  • source file and line number for an instruction address
    • info line *0x<target_addr>
  • source lines around an instruction address
    • list *0x<target_addr>
  • assembly instructions at an address
    • disas 0x<target_addr>, or
    • x/20i 0x<target_addr>

KDB environment variables

  • LINES - set the number of lines for paging output from KDB