title debug ring0 code with bochs/gdb
user omin0us
ip 24.119.76.153
vol 1
lock ********

This small tutorial will describe the process for debugging kernel code using bochs x86 emulator. if you have any questions contact me at
anthony dot lineberry at gmail dot com

Requirments: As far as i know, in my example, you must be booting with GRUB
using an elf kernel. gdb must obviously be installed as well.

first download the bochs source code from http://sourceforge.net/project/showfiles.php?group_id=12580 (current release at the time of writing this is 2.3,
but I will be using 2.2.6 in this example.

unpack the sourcecode and configure it then compile and install like this:
/ /tt ./configure --enable-gdb-stub
/ /tt make
/ /tt make install

At this point you will have bochs installed. Now you will need to edit your
/ bochsrc and add this line:
/ /tt gdbstub: enabled=1, port=1234, text_base=0, data_base=0, bss_base=0

at this point when you start up bochs, you will see this:
/ /tt Waiting for gdb connection on localhost:1234

open another terminal and open your kernel with gdb. My kernel's filename is
kernel32 in this example.

/tt anthony@rupert:~/code/os/ominos$ gdb -q kernel32
/ /tt Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
/ /tt (gdb)

at the gdb prompt enter the command: target remote localhost:1234

/tt (gdb) target remote localhost:1234
/ /tt Remote debugging using localhost:1234
/ /tt 0x0000fff0 in ?? ()
/ /tt warning: shared library handler failed to enable breakpoint
/ /tt Reply contains invalid hex digit 78
/ /tt (gdb)

I always get that warning. but it has never affected anything. at this point
you can set break points. I will set a breakpoint on vfsMount.

/tt (gdb) break vfsMount
/ /tt Breakpoint 1 at 0xc0208078: file vfs.c, line 324.
/ /tt (gdb)

Then run the continue command at the gdb prompt to start bochs.

/tt (gdb) continue
/ /tt Continuing.

/ /tt Breakpoint 1, vfsMount (device=0x0, mountpoint=0x2c9c0 "�a", fstype=0x0)
/ /tt     at vfs.c:324
/ /tt 324     {
/ /tt (gdb)

I will run through a few lines of code printing variables and whatnot to show
that you can debug your kernel this way

/tt (gdb) continue
/ /tt Continuing.

/tt Breakpoint 1, vfsMount (device=0x0, mountpoint=0x2c9c0 "�a", fstype=0x0)
/ /tt     at vfs.c:324
/ /tt 324     {
/ /tt (gdb) next
/ /tt 330             kprintf("%s: mounting '%s' on '%s', fstype='%s'\n", __FUNCTION__, mountpoint, device?device:"NULL", fstype);
/ /tt (gdb) next
/ /tt 331             mount = kmalloc(sizeof(mount_t));
/ /tt (gdb) print fstype
/ /tt $1 = 0xc020c434 "rootfs"
/ /tt (gdb) next
/ /tt 332             if(mount == NULL)
/ /tt (gdb) print/x mount
/ /tt $2 = 0xd00020fa
/ /tt (gdb) next
/ /tt 335             memset(mount, 0, sizeof(mount_t));
/ /tt (gdb) next
/ /tt 337             mount->fs = getFileSystem(fstype);
/ /tt (gdb) next
/ /tt 338             if(mount->fs == NULL)
/ /tt (gdb) print *mount->fs
/ /tt $3 = {name = 0xd00020ac "rootfs", blocksize = 0, blockCount = 0,
/ /tt   fscalls = 0xc0209a14, fsInfo = 0x0, next = 0xd00020c2}
/ /tt (gdb) next
/ /tt 345             if(device != NULL)
/ /tt (gdb) next
/ /tt 354             mount->mountpoint = kstrdup(mountpoint);
/ /tt (gdb) step
/ /tt kstrdup (s=0xc020c43b "/") at kstrdup.c:6
/ /tt 6               char *ret = (char *)kmalloc(strlen(s)+1);
/ /tt (gdb) next
/ /tt 7               if(ret == NULL)
/ /tt (gdb) next
/ /tt 9               memset(ret, 0, strlen(s)+1);
/ /tt (gdb) next
/ /tt 10              strncpy(ret, s, strlen(s));
/ /tt (gdb) next
/ /tt 11              return ret;
/ /tt (gdb) next
/ /tt 12      }
/ /tt (gdb) next
/ /tt vfsMount (device=0x0, mountpoint=0xc020c43b "/", fstype=0xc020c434 "rootfs")
/ /tt     at vfs.c:355
/ /tt 355             if(mount->mountpoint == NULL)
/ /tt (gdb) print mount->mountpoint
/ /tt $4 = 0xd0002122 "/"
/ /tt (gdb) print *mount
/ /tt $5 = {device = 0, fs = 0xd0002088, mountpoint = 0xd0002122 "/",
/ /tt   rootVnode = 0x0, mountCover = 0x0, vTable = 0x0, next = 0x0}
/ /tt (gdb)

There you go. That is all there is to debugging with gdb and bochs. Look on
the internet for more indepth tutorials on actually using gdb and all its
available co