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