gdbstub

This is a simple, single-file¹ GDB stub library 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.

Usage

In some .c file in your project:

/* Define to enable bare metal x86 support, or define your own architecture */
#define GDBSTUB_ARCH_X86

/* Enable function definitions */
#define GDBSTUB_IMPLEMENTATION

#include "gdbstub.h"

void _start(void) {
	dbg_sys_init(); /* Setup, wait for debugger on serial port  */
}

See gdbstub.c for example usage.

Architecture Support

• GDBSTUB_ARCH_MOCK: A mock architecture for testing
• GDBSTUB_ARCH_X86: Bare-metal x86 (32-bit). You'll also need interrupt handlers (so not .

Porting

This was originally developed for embedded x86 systems, but it's fairly modular. With a little effort, it can be easily ported to other platforms.

I recommend copying the mock architecture implementation and adjusting it to fit your platform's needs accordingly.

PR's for other platforms are welcome!

Building

By default, running make produces a gdbstub program. This is simply a stub for a mock architecture (just a handful of registers and some memory) running inside a normal program that communicates over stdio.

A stub intended for bare metal x86 machines can be built with make ARCH=x86. This produces an ELF binary gdbstub.elf that will 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 easily loaded into memory and jumped to.

x86 Demo

In gdbstub.c there is a simple function that's used for demonstration and testing. To use it, build the stub with:

$ make ARCH=x86 INCLUDE_DEMO=1

Then, to test the GDB stub out, 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 to get your GDB client to connect to the virtual serial port and begin debugging the demo application:

$ gdb
(gdb) symbol-file gdbstub.elf
(gdb) target remote 127.0.0.1:1234
(gdb) b simple_loop
(gdb) layout split
(gdb) c

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

(gdb) s 3
(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, 2016-2022