[linuxkernelnewbies] My Tech notes: Kernel (for 2.6) debugging- kgdb

Kernel (for 2.6) debugging- kgdb &

kgdb is a kernel patch that, once applied, allows for the use of the familiar gdb interface for source-level debugging of a running Linux kernel.

The kgdb patch supplies the kernel with a debugging stub. This stub uses gdb remote serial/Ethernet protocol to communicate with gdb (host machine). This patch is applied to the kernel on the machine that will run the gdb session (host machine).

The gdbmod for module debugging, gdbmod version of gdb is required to debug modules that are loaded dynamically on to the testing machine.

Configuring kgdb

Download kgdb and gdbmod patch for your kernel version from http://kgdb.linsyssoft.com/downloads.htm

Copy the patch to the kernel source directory on the development machine, and apply,

Unzip the kgdb patch
mkdir ${BASE_DIR}/patch-kgdb
cd ${BASE_DIR}/patch-kgdb
tar -jxvf linux-2.6.7-kgdb-2.2.tar.bz2
patch -p1 < ${BASE_DIR}/patch-kgdb/core-lite.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/i386-lite.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/8250.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/eth.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/i386.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/core.patch

This kernel will be for target board, the patch applied here are for i386, for ARM patch download latest patch from for kernel 2.6.15.5 http://kgdb.linsyssoft.com/downloads/kgdb-2/linux-2.6.15.5-kgdb-2.4.tar.bz2

Apply the patch for arm board as

patch -p1 < ${BASE_DIR}/patch-kgdb/core-lite.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/arm-lite.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/8250.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/eth.patch
patch -p1 < ${BASE_DIR}/patch-kgdb/core.patch

The patched kernel must now be recompiled.

        $ make menuconfig

From the main menu navigate to and select kernel hacking. You should now see an option for Remote (serial) debugging with gdb. Make sure this option is selected and then exit.

        $ make clean

        $ make bzImage

The newly compiled kernel image (e.g., bzImage) is copied to the target machine for booting up.

Compiling the kernel on the development machine

In the ${BASE_DIR}/linux-2.6.7/Makefile, set the EXTRAVERSION = -kgdb
make mrproper
make menuconfig
Select the options appropriate for the target machine Hardware.
Select the options pertaining to kgdb under "Kernel hacking" .
Remote (serial) debugging with gdb
make clean
make bzImage
Transfer the built kernel to the Target machine from the Development machine.

Copy the Kernel image from ${BASE_DIR}/linux-2.6.7/arch/i386/boot/bzImage to the target machine as /boot/vmlinuz-2.6.7-kgdb
Copy the Map file from ${BASE_DIR}/linux-2.6.7/System.map to the target machine as /boot/System.map-2.6.7-kgdb
Also create links,
ln -s /boot/vmlinuz-2.6.7-kgdb /boot/vmlinuz
ln -s /boot/System.map-2.6.7-kgdb /boot/System.map

Edit the /boot/grub/grub.conf file in the target machine to have the kgdb enabled kernel entry.
Sample is shown below :-

# Sample grub.conf which will by default boot the kgdb enabled kernel
default=1
timeout=10
splashimage=(hd0,0)/boot/grub/splash.xpm.gz

title RH
root (hd0,0)
kernel /boot/vmlinuz ro root=/dev/sda1
initrd /boot/initrd

title Linux-2.6.7-kgdb
root (hd0,0)
kernel /boot/vmlinuz-2.6.7-kgdb ro root=/dev/sda1 rootfstype=ext3 kgdbwait kgdb8250=0,115200
Using kgdb ethernet interface

Add following line in the grub entry :

[EMAIL PROTECTED]/,@192.168.2.117/ (that's [EMAIL PROTECTED]/,@REMOTE-IP/)

# Sample grub.conf which will by default boot the kgdb enabled kernel

title Linux-2.6.15.5-kgdb(eth)

root (hd0,0)

kernel /boot/vmlinuz-2.6.15.5-kgdb ro root=/dev/hda1 [EMAIL PROTECTED]/,@192.168.2.117/

console=ttyS0,115200

Then for starting the debug session give following command on gdb.

(gdb) ./vmlinux

(gdb) target remote udp:HOSTNAME:6443

Starting the debug session

After booting the target machine will wait for the host development machine to connect, by displaying the message:-

Waiting for connection from remote gdb...
For version 2.6 this message will not appear, it will just wait after printing message "booting linux, uncompressing linux", can connect to this form host machine,
cd ${BASE_DIR}/linux-2.6.7
For setting a debug session with baud rate of 115200 on /dev/ttyS0 , run as "root" user:-

gdb ./vmlinux
GNU gdb Red Hat Linux (6.0post-0.20040223.17rh)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb) shell echo -e "\003" > /dev/ttyS0
(gdb) set remotebaud 115200
(gdb) target remote /dev/ttyS0
Remote debugging using /dev/ttyS0
breakpoint () at kernel/kgdb.c:1212
1212 atomic_set(&kgdb_setting_breakpoint, 0);
warning: shared library handler failed to enable breakpoint
(gdb)
For further commands refer http://kgdb.linsyssoft.com/tockdebug.htm

Reference

Gdb quick ref guide.

http://users.ece.utexas.edu/~adnan/gdb-refcard.pdf

Simple how to

http://www.shakthimaan.com/downloads/glv/kgdb-howto/kgdb-howto.html

Remote debugging

http://www.linuxjournal.com/article/4525