gdbstub

This is a simple GDB stub that can be easily dropped in to your project to allow you to debug a target platform using GDB (or another application which supports remote GDB targets). It has no library dependencies (such as libc) and requires just standard tools to build.

Protocol

Communication between the stub and the debugger takes place via the GDB Remote Serial Protocol.

Porting

This was developed for x86 systems, but it's fairly modular. With a little effort, it can be easily ported to other platforms. You will need to modify gdbstub_sys.h and gdbstub_sys.c to fit your platform's needs accordingly.

Building

Running make produces ELF binary gdbstub.elf with an entry point (dbg_start) that will simply hook the current IDT (to support debug interrupts) and break.

Additionally, a simple flat binary gdbstub.bin is created from the ELF binary. The intent for this flat binary is to be trivially loaded and jumped to.

Demo

In demo/demo.c there is a simple function that's used for demonstration and testing. To test the GDB stub out, you can launch an instance of the full-system emulator QEMU as follows:

qemu-system-i386 -serial tcp:127.0.0.1:1234,server -display none -kernel gdbstub.elf

This will launch QEMU, create a virtual machine with a virtual serial port that can be connected to through local TCP port 1234, then load and run the stub executable inside the virtual machine.

You can then launch your local GDB client with the demo.gdbinit script to get your GDB client to connect to the virtual serial port and begin debugging the demo application:

gdb --command=demo/demo.gdbinit

For example, step a couple of times and print out the value of x:

(gdb) s 2
(gdb) p/x x
$1 = 0xdeadbeef

License

This software is published under the terms of the MIT License. See LICENSE.txt for full license.

Matt Borgerson, 2016-2019