KGDB Tutorial

Building a Kernel that supports KGDB

• Upstrteam documentation:  Using kgdb and the kgdb Internals
• Set CONFIG_KGDB=y in your kernel .config (linux-2.6/lib/Kconfig.kgdb):

menuconfig KGDB

        bool "KGDB: kernel debugging with remote gdb"

        depends on HAVE_ARCH_KGDB

        depends on DEBUG_KERNEL && EXPERIMENTAL

        help

          If you say Y here, it will be possible to remotely debug the

          kernel using gdb.  

• Set CONFIG_KGDB_CONSOLE=y to serve as the KGDB I/O driver (linux-2.6/lib/Kconfig.kgdb, linux-2.6/drivers/serial/kgdboc.c):

config KGDB_SERIAL_CONSOLE

        tristate "KGDB: use kgdb over the serial console"

        select CONSOLE_POLL

        select MAGIC_SYSRQ

        default y

        help

          Share a serial console with kgdb. Sysrq-g must be used

          to break in initially.

KGDB Boot and Module Options

1. Boot with kgdboc=<tty-device>,[baud] (ex: kgdboc=ttyAMA1 for qemu-system-arm)
2. (alternatively) From sysfs
   echo TTY_DEVICE > /sys/module/kgdboc/parameters/kgdboc

Connecting GDB to the Kernel through QEMU Emulated Serial Port

• [arm-cross: linux-2.6]$ qemu-system-arm -nographic -s -M integratorcp -kernel
  ./zImage-2.6.32-integratorcp-v5 -serial tcp:localhost:2345,server -net
  nic,vlan=0 -net tap,vlan=0,ifname=tap0,script=./scripts/qemu-ifup
  -append "console=ttyAMA0 root=/dev/nfs
  nfsroot=172.20.0.1:/nfsroot/box,nfsvers=3 rw
  ip=172.20.0.2::172.20.0.1:255.255.255.0 kgdboc=ttyAMA0 kgdbwait"
  
• [arm-cross: linux-2.6]$ telnet localhost:2345
• Wait till the telnet session show "kgdb: Waiting for connection from remote gdb..." then terminate telnet with CTRL-] then CTRL-D
  
• [arm-cross: linux-2.6]$ gdb ./vmlinux
• (gdb) target remote localhost:2345
• Trouble Shooting: (gdb) set debug remote 1

Connecting GDB to the Kernel through a Physical Serial Port

• [scottt@2530p linux-2.6]$ gdb ./vmlinux

  (gdb) set remotebaud 115200

  (gdb) target remote /dev/ttyS0