NOTE: kgdb is not merged upstream yet
Required configuration to use KGDB/KDB for kernel debugging
Make sure your .config contains:
If you want to be able to use KDB (a KGDB frontend running on the console):
To enable kdb/kgdb, you need to pass the device you want to use to kgdb, either via sysfs file or palo. My palo configuration looks like this:
0/vmlinuz HOME=/ root=/dev/sda5 panic_timeout=60 panic=10 console=ttyS0,115200 kgdboc=ttyS0,115200 0: 0/vmlinuz 1: HOME=/ 2: root=/dev/sda5 3: panic_timeout=60 4: panic=10 5: console=ttyS0,115200 6: kgdboc=ttyS0,115200
<#> edit the numbered field 'b' boot with this command line 'r' restore command line 'l' list dir 'x' reset and reboot machine ? 0
If you want to enable kgdb via sysfs, do
echo ttyS0,115200 >/sys/module/kgdboc/parameters/kgdboc
Most of the PA-RISC machines have at least two serial ports, so you could also use ttyS1. In my examples i'm using ttyS0, which is also the console port.
There are several ways to enter kgdb:
- via 'echo g >/proc/sysrq-trigger
- sending a break g on the serial console, with minicom that is Ctrl-A f g
- It will also show up on a kernel crash, as long as there's no panic command line option used
When entered, kdb will look like this:
Entering kdb (current=0xefcccdb0, pid 0) on processor 1 due to Keyboard Entry kdb>
The  before the kdb prompt is the processor number kdb is currently on.
help shows you the command list:
kdb> help Command Usage Description ---------------------------------------------------------- md <vaddr> Display Memory Contents, also mdWcN, e.g. md8c1 mdr <vaddr> <bytes> Display Raw Memory mdp <paddr> <bytes> Display Physical Memory mds <vaddr> Display Memory Symbolically mm <vaddr> <contents> Modify Memory Contents go [<vaddr>] Continue Execution rd Display Registers rm <reg> <contents> Modify Registers ef <vaddr> Display exception frame bt [<vaddr>] Stack traceback btp <pid> Display stack for process <pid> bta [D|R|S|T|C|Z|E|U|I|M|A] Backtrace all processes matching state flag btc Backtrace current process on each cpu btt <vaddr> Backtrace process given its struct task address env Show environment variables set Set environment variables help Display Help Message ? Display Help Message cpu <cpunum> Switch to new cpu kgdb Enter kgdb mode ps [<flags>|A] Display active task list pid <pidnum> Switch to another task reboot Reboot the machine immediately lsmod List loaded kernel modules sr <key> Magic SysRq key dmesg [lines] Display syslog buffer defcmd name "usage" "help" Define a set of commands, down to endefcmd kill <-signal> <pid> Send a signal to a process summary Summarize the system per_cpu <sym> [<bytes>] [<cpu>] Display per_cpu variables grephelp Display help on | grep bp [<vaddr>] Set/Display breakpoints bl [<vaddr>] Display breakpoints bc <bpnum> Clear Breakpoint be <bpnum> Enable Breakpoint bd <bpnum> Disable Breakpoint ss Single Step dumpcommon Common kdb debugging dumpall First line debugging dumpcpu Same as dumpall but only tasks on cpus ftdump [skip_#lines] [cpu] Dump ftrace log
To exit kdb, just type go:
The system will resume execution where it was stopped by kdb.
To set a breakpoint, use the bp command:
# enter kdb when kernel calls do_sys_open, which is the open(2) syscall kgdb> bp do_sys_open Instruction(i) BP #0 at 0x103478b8 (do_sys_open) is enabled addr at 00000000103478b8, hardtype=0 installed=0 kgdb> go