POSTED BY: Patroklos Argyroudis / 19.01.2009

FreeBSD kernel debugging

The FreeBSD kernel can be debugged with the ddb(4) interactive kernel debugger. Although the latest production release of FreeBSD (7.1 at the time of this writing) adds some very useful features, ddb is still lacking the flexibility of gdb.

The FreeBSD developer’s handbook has a section on kernel debugging using remote gdb, but it is not directly applicable to VMware-based installations. The solution is to use VMware’s feature of creating virtual serial ports as named pipes to emulate a serial connection between two FreeBSD virtual machines.

The first FreeBSD virtual machine, let’s call it the target host to follow the handbook’s terminology, is the machine that the experimental kernel code runs on. The second, the debugging host, is the one that will run gdb and connect over the virtual serial connection to the target host. The target host’s kernel needs to be compiled with the following options:

makeoptions DEBUG=-g
options GDB
options DDB
options KDB

Furthermore, the serial port needs to be defined in the device flags in the /boot/device.hints file of the target host by setting the 0x80 bit, and the 0x10 bit for specifying that the kernel gdb backend is to be accessed via remote debugging over this port:


Also, edit the target host’s /etc/sysctl.conf file to include the following self-explanatory kernel parameters:


After the compilation and installation of the new kernel on the target host, the /usr/obj/usr/src/sys/TARGET_HOST directory (assuming you have named the new kernel TARGET_HOST) needs to be copied to the debugging host (for example with scp -r).

For the following steps both virtual machines need to be turned off. In VMware go to the tab of the target host, click Edit virtual machine settings->Add->Serial Port->Output to named pipe. Enter /tmp/com_1 (or whatever you want) as the named pipe, select This end is the server and The other end is a virtual machine. Then perform the same steps on the debugging host’s virtual machine, enter the same named pipe, but select This end is the client in this case. The /tmp/com_1 named pipe on the machine that runs VMware (Linux in our case) will be used as a virtual serial connection between the two FreeBSD guests.

Now power on the target host normally, cause a kernel panic or start the kernel debugger manually, and type gdb and then s:

[root@target_host]# sysctl debug.kdb.enter=1
KDB: enter: sysctl debug.kdb.enter
[thread pid 578 tid 100063 ]
Stopped at kdb_enter+0x2b: nop
db> gdb
Step to enter the remote GDB backend.
db> s

On the debugging host you need to find the device that corresponds to the virtual serial port you defined in VMware. On our setup it is /dev/cuad0. Then start a kgdb remote debugging session in the /usr/obj/usr/src/sys/TARGET_HOST directory, passing as arguments the serial port device and the kernel to be debugged:

[root@debugging_host ~]# cd /usr/obj/usr/src/sys/TARGET_HOST
[root@debugging_host /usr/obj/usr/src/sys/TARGET_HOST]# kgdb -r /dev/cuad0 \
GNU gdb 6.1.1 [FreeBSD]
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-marcel-freebsd".
Switching to remote protocol
kdb_enter (msg=0x23 <Address 0x23 out of bounds>) at
270 }

Unread portion of the kernel message buffer:
KDB: enter: sysctl debug.kdb.enter

#0  kdb_enter (msg=0x23 <Address 0x23 out of bounds>) at
270 }
(kgdb) bt
#0  kdb_enter (msg=0x23 <Address 0x23 out of bounds>) at
#1  0xc0657710 in kdb_sysctl_enter (oidp=0xc08d3fa0, arg1=0x0, arg2=0,
 req=0xcca54c04) at /usr/src/sys/kern/subr_kdb.c:175
#2  0xc0646f2b in sysctl_root (oidp=0x0, arg1=0x0, arg2=0, req=0xcca54c04) at
#3  0xc0647128 in userland_sysctl (td=0x23, name=0xcca54c74, namelen=3,
 old=0xcca54c04, oldlenp=0x0, inkernel=0, new=0xbfbfe428, newlen=35,
 retval=0xcca54c70, flags=35) at /usr/src/sys/kern/kern_sysctl.c:1347
#4  0xc0646fcb in __sysctl (td=0xc164d180, uap=0xcca54d04) at
#5  0xc0811dcf in syscall (frame=
 {tf_fs = 59, tf_es = 59, tf_ds = 59, tf_edi = 3, tf_esi = 0,
 tf_ebp = -1077943368, tf_isp = -861581980, tf_ebx = 672367844,
 tf_edx = 0, tf_ecx = -1077941184, tf_eax = 202, tf_trapno = 12,
 tf_err = 2, tf_eip = 671840819, tf_cs = 51, tf_eflags = 658,
 tf_esp = -1077943428, tf_ss = 59}) at /usr/src/sys/i386/i386/trap.c:976
#6  0xc08007bf in Xint0x80_syscall () at /usr/src/sys/i386/i386/exception.s:200
#7  0x00000033 in ?? ()
Previous frame inner to this frame (corrupt stack?)

That’s it. Now let’s do some debugging.