Debugging TORO with BOCHS and GDB

In the following lines I'll tray to explain how debug Toro using bochs+gdb. I often used qemu instance of bochs but the latest days I figured out some rare behaviors so I replaced qemu for bochs with good results. 

Before to start we have to compile bochs for such purpose, after download the source run the next command in order to compile it:

> ./configure --enable-cpu-level=6 --enable-all-optimizations --enable-x86-64 --enable-pci --enable-vmx --enable-disasm --enable-debugger-gui --enable-logging --enable-fpu --enable-3dnow --enable-sb16=dummy --enable-cdrom --enable-x86-debugger --enable-iodebug --disable-plugins --disable-docbook --enable-gdb-stub     

> make install

It is not possible to enable the smp support and gdb stub at the same time. If the compilation was right we'll be able to run bochs with gdb support.

Now It is needed to compile the toro application within the debug symbols, if there're any old .o or .ppu file they must be deleted because they don't have symbols information. We should execute in the toro source directory the next commands:

> fpc ToroHello.pas -g -oToroHello -Fu../rtl/ -Fu../rtl/drivers

> ./build 4 ToroHello boot.o ToroHello.img

This procedure is for ToroHello.pas but it's the same for other toro's app.

So far, we've the bochs and the Toro's image, now we have to build the .bochsrc file and launch bochs.  The following lines may be useful:

megs: 32

ata0-master: type=disk, path="ToroHello.img"

boot: c

log: bochsout.txt

gdbstub: enabled=1

Check that we've indeed to enable the gdbstub in the bochs' source file.

The next step is to run bochs and then GDB:

> bochs

> gdb ToroHello

If we run gdb in the toro/test directory .gdbinit will be used, otherwise we have to connect to bochs manually as follow:

> (gdb) target remote localhost:1234 

If everything goes well we are able to set breakpoints and uses all the tools of gdb. For instance we could do:

> (gdb) b KERNELSTART

> (gdb) c

In the first line a breakpoint is set at KERNELSTART and then the virtual machine continues until comes back when the breakpoint is reached.

Many commands could be usefull in this point like n, for running line by line, step for stepping into, info registers and soon on.

There're a lot of information that we can get at this point but that's for another tutorial ;)

Matias E. Vara

www.torokernel.org

Toro debug with ECLIPSE

Hi everyone! I figured out that the server where I had the video about "TORO debug with ECLIPSE" is down, so I've uploaded it again. It shows the TORO Builder running on Windows 2003 x64. It is interesting how we can do "step by step" debugging and "set a breakpoint". Qemu is emulating a x86_64 arch.

Enjoy!   

Matias E. Vara
www.torokernel.org

Patching GDB 7.3 for QEMU remote kernel debug

This time I will try to explain how patch GDB 7.3 in order to debug a kernel using QEMU through remote debuging. If we try to debug remotely, we'll find a error message like:

Remote packet too long: 000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 ...

I am not sure about problem but I suppose it's about register size. When the virtual machine jumps from real mode to long/protect mode, the register size changes but GDB doesn't know that. Thus, when GDB receives a bigger packet than it expects, it fails. Therefore, The patch just increments the buffer in those cases.

The first step is to download GDB 7.3 from http://www.gnu.org/s/gdb/download/, I've implemented the patch on 7.3 version but I think it works in oldest too.

Once downloaded and uncompressed, edit the file gdb-7.3/gdb/remote.c and go to 5693 line. That's the process_g_packet procedure. Now, look for and replace the original source with the following lines:

/* Further sanity checks, with knowledge of the architecture. */

//if (buf_len > 2 * rsa->sizeof_g_packet)

// error (_("Remote 'g' packet reply is too long: %s"), rs->buf);

if (buf_len > 2 * rsa->sizeof_g_packet)

{

rsa->sizeof_g_packet = buf_len;

for (i = 0; i < gdbarch_num_regs (gdbarch); i++)

{

if (rsa->regs[i].pnum == -1)

continue;

if (rsa->regs[i].offset >= rsa->sizeof_g_packet)

rsa->regs[i].in_g_packet = 0;

else

rsa->regs[i].in_g_packet = 1;

}

}

Finally, it just remains to execute:

$ ./configure

$ make


In some systems may be necessary to install termcap library, simply execute:

$ sudo apt-get install libncurses5-dev

After compilation, the binary could be found in gdb-7.3/gdb/gdb, It must be enough to run GDB correctly.

Matias E. Vara

www.torokernel.org