A dedicated unikernel for microservices

Thursday, August 25, 2011

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)
if (rsa->regs[i].offset >= rsa->sizeof_g_packet)
rsa->regs[i].in_g_packet = 0;
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

Tuesday, August 23, 2011

Toro in Microelectronic Conference (UNLP)

Toro will be shown in the Microelectronic Conference at University of La Plata, Argentina. In the work that I've done I will show the kernel capabilities and a few tests comparing Toro with a general purpose operative system. The conference will be the 8th of September in "Sala A" at 16.20hs.

Matias E. Vara