gdbstub

An ergonomic and easy-to-integrate implementation of the GDB Remote Serial Protocol in Rust.

gdbstub is entirely #![no_std] compatible, and can be used on platforms without a global allocator. In embedded contexts, gdbstub can be configured to use pre-allocated buffers and communicate over any available serial I/O connection (e.g: UART).

gdbstub is particularly well suited for emulation, making it easy to add powerful, non-intrusive debugging support to an emulated system. Just provide an implementation of gdbstub::Target for your target platform, and you're ready to start debugging!

• Documentation

Debugging Features

Features marked as (optional) aren't required to be implemented, but can be implemented to enhance the debugging experience.

• Core GDB Protocol
  • Step + Continue
  • Add + Remove Software Breakpoints
  • Read/Write memory
  • Read/Write registers
  • (optional) Add + Remove Hardware Breakpoints
  • (optional) Read/Write/Access Watchpoints (i.e: value breakpoints)
  • (optional) Multithreading support
• Extended GDB Protocol
  • (optional) Handle custom debug commands (sent via GDB's monitor command)
  • (optional) Automatic architecture detection

The GDB Remote Serial Protocol is surprisingly complex, supporting advanced features such as remote file I/O, spawning new processes, "rewinding" program execution, and much, much more. Thankfully, most of these features are completely optional, and getting a basic debugging session up-and-running only requires a small subset of commands to be implemented.

If gdbstub is missing a feature you'd like to use, please file an issue / open a PR!

Feature flags

The std feature is enabled by default. In #![no_std] contexts, use default-features = false.

• alloc
  • Implements Connection for Box<dyn Connection>.
  • Adds output buffering to ConsoleOutput.
• std (implies alloc)
  • Implements Connection for TcpStream and UnixStream.
  • Implements std::error::Error for gdbstub::Error
  • Log outgoing packets via log::trace! (uses a heap-allocated output buffer)

Examples

armv4t

The armv4t example shows how gdbstub can be used to add gdb debugging support to an (incredibly simple) ARMv4T-based emulator. See examples/armv4t/README.md for details.

armv4t_multicore

A dual-core variation of the armv4t example. Implements gdbstub's multithread extensions to enable per-core debugging. See examples/armv4t_multicore/README.md for details.

Real-World Examples

Several projects are already using gdbstub.

• clicky - An emulator for classic clickwheel iPods (dual-core ARMv4T SoC)
• rustyboyadvance-ng - Nintendo GameBoy Advance emulator and debugger
• microcorruption-emu - msp430 emulator for the microcorruption.com ctf
• ts7200 - An emulator for the TS-7200, a somewhat bespoke embedded ARMv4t platform

If you end up using gdbstub in your project, feel free to open a PR and add it to this list!

Using gdbstub on bare-metal hardware

Since gdbstub is #![no_std] compatible, it should be possible to implement a gdbstub::Target which uses low-level trap instructions + context switching to debug bare-metal code.

If you happen to stumble across this crate and end up using it to debug some bare-metal code, please let me know! I'd love to link to your project!

unsafe in gdbstub

gdbstub "core" only has 2 lines of unsafe code:

• A call to NonZeroUsize::new_unchecked(1) when defining the SINGLE_THREAD_TID constant.
• A call to str::from_utf8_unchecked() when working with incoming GDB packets (the underlying &[u8] buffer is checked with is_ascii() prior to the call).

With the std feature enabled, there is one additional line of unsafe code:

• gdbstub includes an implementation of UnixStream::peek which uses libc::recv. This will be removed once rust-lang/rust#73761 is merged.

Future Plans

• Improve multiprocess / multi-thread / multi-core support
  • Support thread-specific breakpoints
  • Support non-stop mode?
  • Support disabling multiprocess extensions on older GDB clients
• Support addresses larger than 64-bits?
  • This would require plumbing-through the architecture's pointer size as a generic parameter into all the packet parsing code, which probably isn't too difficult, just time consuming.