Pyda

Pyda lets you write simple dynamic binary analysis tools using Python.

Pyda combines Dynamorio-based instrumentation with a CPython interpreter, allowing you to write hooks in Python that directly manipulate registers/memory in the target, without going through GDB or ptrace.

Features:

• Hooks: Inspect and modify registers and memory at any instruction.
• Redirect execution: Hooks can directly modify the program counter; for example, to cause a function to return early or to skip over a tricky branch.
• Syscall interception: Syscall pre/post hooks can capture and modify syscall arguments, and optionally skip the syscall altogether.
• Package support: Install and use your favorite packages like normal using pip (e.g. pwntools).
• Graceful multithreading: Writing tools for multithreaded programs is easy: program threads share a Python interpreter*, so you can use globals to track and aggregate state over several threads (see: p.tid).

Pyda is a...

• In-process, scriptable debugger: Pyda hooks can be used as GDB-style breakpoints to inspect/modify registers and memory. Several packages (e.g. pwntools) can be used to look up symbols or parse DWARF info.
• Reverse engineering tool: Answer questions like "Where do all these indirect jumps go?" in just a few lines of Python.

Quickstart

docker run -it ghcr.io/ndrewh/pyda pyda examples/ltrace.py -- ls -al

Example

Warning

This API is not stable and may change. Please provide feedback on the API by filing an issue.

from pyda import *
from pwnlib.elf.elf import ELF
from pwnlib.util.packing import u64

# Get a handle to the current process
p = process()

# You can use pwnlib to get information about the target ELF
e = ELF(p.exe_path)
e.address = p.maps[p.exe_path].base

# Define a hook/breakpoint -- this can be at any instruction
def main_hook(p):
    print(f"at main, rsp={hex(p.regs.rsp)}")
    return_addr = p.read(p.regs.rsp, 8)
    print(f"return address: {hex(u64(return_addr))}")

# Register the hook
p.hook(e.symbols["main"], main_hook)

# Tell Pyda we are ready to go!
p.run()

$ pyda examples/simple.py -- ./challenge 
You are running Pyda v0.0.1
at main, rsp=0x7fff1303f078
return address: 0x7f3c50420d90

More examples

See examples/ for additional examples.

• ltrace.py: Hook all calls to library functions, and print out their arguments
• strace.py: Hook all syscalls and print out their arguments
• resolve_indirect_calls.py: dump a list of indirect calls with objdump, and then print out the targets during execution

Limitations

• Currently Linux only (please contribute Windows support!)
• Currently X86_64 only (please contribute ARM64 support!)
• All of the limitations of Dynamorio apply. The program must be reasonably well behaved.
• Some state may be shared with the target process; while Dynamorio attempts to isolate our libc (and other libraries) from the target, OS structures (e.g. fds) are shared.

Getting started

Install

Suggested use is via Docker.

Pull the latest release:

docker pull ghcr.io/ndrewh/pyda

Or build it yourself:

docker build -t pyda .

(The Pyda image is currently based on ubuntu:22.04 and we leave the default entrypoint as /bin/bash)

Usage

pyda <script_path> [script_args] -- <bin_path> [bin_args]

"Hello World" example: ltrace over /bin/ls

pyda examples/ltrace.py -- ls

API

You can view all of the available APIs in process.py, but in summary:

# Read memory
p.read(0x100000, 8) # 8 bytes (bytes)
p.mem[0x100000] # 1 byte (int)
p.mem[0x100000:0x100008] # 8 bytes (bytes)

# Write memory
p.write(0x100000, b"\x00" * 8)
p.mem[0x100000:0x100008] = b"\x00" * 8

# Read registers
p.regs.rax # (int)

# Write registers
p.regs.rax = 0x1337133713371337

# Get process base
p.maps["libc.so.6"].base # (int)

# Get current thread id (valid in hooks and thread entrypoint)
p.tid # (int), starts from 1

# Hooks (functions called before executing the instruction at the specified PC)
p.hook(0x100000, lambda p: print(f"rsp={hex(p.regs.rsp)}"))

# New thread events: called when a new thread starts (just before entrypoint)
p.set_thread_entry(lambda p: print(f"tid {p.tid} started")) # Called when a new thread is spawned

# Syscall hooks: called for a specific syscall (specified by the first arg)
# as a pre (before syscall) or post (after syscall) hook.
#
# Pre-syscall hooks can optionally return False to skip the syscall.
# In this case, you are responsible for setting the return value
# (e.g. with p.regs.rax = 0). Returning any value other than False (or not
# returning anything at all) will still run the syscall.
p.syscall_pre(1, lambda p, syscall_num: print(f"write about to be called with {p.regs.rdx} bytes"))
p.syscall_post(1, lambda p, syscall_num: print(f"write called with {p.regs.rdx} bytes"))

FAQ

Why should I use this over GDB or other ptrace-based debuggers?

GDB breakpoints incur substantial overhead due to signal handling in the kernel and in the tracer process. This introduces an insurmountable performance challenge for high-frequency breakpoints. For multithreaded programs, this problem is compounded by "group-stop", which will stop all threads when any thread receives a stop signal.

Unlike GDB, Pyda hooks are executed directly in the target process itself (without the overhead of signals or a kernel context switch). As a result, Pyda hooks typically run faster.

Why should I use this over Frida or other dynamic instrumentation tools?

These tools are quite similar to Pyda, with mostly ergonomic differences: Pyda tools are written in Python using a relatively minimal set of APIs. Pyda relies on the existing Python ecosystem for many features (e.g. ELF parsing). As a result, Pyda tools are typically shorter and easier to write than equivalent Frida scripts.

However, Pyda is not (currently) an adequate replacement for full-fledged dynamic instrumentation frameworks. We do not provide a fine-grained instrumentation API (e.g. you cannot insert specific instructions), relying instead on hooks as the primary unit of instrumentation.

Can I use LD_LIBRARY_PATH on the target?

Generally, yes. Just run pyda with LD_LIBRARY_PATH -- the target uses a normal loader.

Can I run this tool on itself?

Probably not. But you can run the Python interpreter under it.

$ pyda <script> -- python3
Python 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 

Can my scripts parse arguments?

Yes. Script arguments can be passed before the -- when running pyda. For example:

pyda script.py --option1 --option2 -- ls

Your script can parse these options like normal with the argparse module.

How it works

Pyda runs as a Dynamorio tool: pyda is just a drrun wrapper that runs the application under dynamorio with our tool loaded. Technically, Pyda is "just" a shared object that links against libPython---both of which get loaded into the target process by Dynamorio. However, Dynamorio is designed to support targets which load the same libraries as required by tools (i.e., by including it's own private loader for tools). As you might imagine, it gets a bit messy to run CPython under a nonstandard loader, and we had to include nontrivial patches for both Dynamorio and CPython to make it all work. There were also issues with the "Client Transparency" aspects of Dynamorio: in particular, our tool's threads reside in a different process group than the target itself (despite residing in the same memory space). This causes problems with certain concurrency primatives (e.g. sem_init/sem_wait) that rely on threads being in the same process group.

Dynamorio handles all the nasty low-level details: inserting instrumentation, machine state trasitions to/from hooks, etc. Pyda provides a CPython extension for registering hooks and proxies machine state modifications to Dynamorio. Pyda itself ends up handling a lot of edge cases (think: hooks which throw exceptions, hooks which remove themselves, redirect execution, etc.) and nasty error states, especially surrounding thread creation and cleanup.

Multithreading

Unlike GDB/ptrace, which suspends all threads when any thread reaches a breakpoint, Pyda hooks do not interrupt other threads.

Programs running under Pyda get a single CPython interpreter, so all threads share the same state (globals). When a hook is called by a thread, the thread will first acquire the GIL. We maintain a thread-specific data structure using Dynamorio's TLS mechanism, which allows us to track thread creation/destruction and report thread-specific information (e.g. p.tid) in hooks.

Contributing

Issues and pull requests are welcome. If reporting an issue with a particular target, please attach the binary.