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osandov / drgn

Programmable debugger
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drgn

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.. start-introduction

drgn (pronounced "dragon") is a debugger with an emphasis on programmability. drgn exposes the types and variables in a program for easy, expressive scripting in Python. For example, you can debug the Linux kernel:

.. code-block:: pycon

>>> from drgn.helpers.linux import list_for_each_entry
>>> for mod in list_for_each_entry('struct module',
...                                prog['modules'].address_of_(),
...                                'list'):
...    if mod.refcnt.counter > 10:
...        print(mod.name)
...
(char [56])"snd"
(char [56])"evdev"
(char [56])"i915"

Although other debuggers like GDB <https://www.gnu.org/software/gdb/>_ have scripting support, drgn aims to make scripting as natural as possible so that debugging feels like coding. This makes it well-suited for introspecting the complex, inter-connected state in large programs.

Additionally, drgn is designed as a library that can be used to build debugging and introspection tools; see the official tools <https://github.com/osandov/drgn/tree/main/tools>_.

drgn was developed at Meta <https://opensource.fb.com/> for debugging the Linux kernel (as an alternative to the crash <https://crash-utility.github.io/> utility), but it can also debug userspace programs written in C. C++ support is in progress.

.. end-introduction

Documentation can be found at drgn.readthedocs.io <https://drgn.readthedocs.io>_.

.. start-installation

Installation

Package Manager ^^^^^^^^^^^^^^^

drgn can be installed using the package manager on some Linux distributions.

.. image:: https://repology.org/badge/vertical-allrepos/drgn.svg :target: https://repology.org/project/drgn/versions :alt: Packaging Status

pip ^^^

If your Linux distribution doesn't package the latest release of drgn, you can install it with pip <https://pip.pypa.io/>_.

First, install pip <https://packaging.python.org/guides/installing-using-linux-tools/#installing-pip-setuptools-wheel-with-linux-package-managers>_. Then, run:

.. code-block:: console

$ sudo pip3 install drgn

This will install a binary wheel by default. If you get a build error, then pip wasn't able to use the binary wheel. Install the dependencies listed below <#from-source>_ and try again.

Note that RHEL/CentOS 6, Debian Stretch, Ubuntu Trusty, and Ubuntu Xenial (and older) ship Python versions which are too old. Python 3.6 or newer must be installed.

From Source ^^^^^^^^^^^

To get the development version of drgn, you will need to build it from source. First, install dependencies:

Then, run:

.. code-block:: console

$ git clone https://github.com/osandov/drgn.git
$ cd drgn
$ python3 setup.py build
$ sudo python3 setup.py install

.. end-installation

See the installation documentation <https://drgn.readthedocs.io/en/latest/installation.html>_ for more options.

Quick Start

.. start-quick-start

drgn debugs the running kernel by default; run sudo drgn. To debug a running program, run sudo drgn -p $PID. To debug a core dump (either a kernel vmcore or a userspace core dump), run drgn -c $PATH. Make sure to install debugging symbols <https://drgn.readthedocs.io/en/latest/getting_debugging_symbols.html>_ for whatever you are debugging.

Then, you can access variables in the program with prog['name'] and access structure members with .:

.. code-block:: pycon

$ sudo drgn
>>> prog['init_task'].comm
(char [16])"swapper/0"

You can use various predefined helpers:

.. code-block:: pycon

>>> len(list(bpf_prog_for_each()))
11
>>> task = find_task(115)
>>> cmdline(task)
[b'findmnt', b'-p']

You can get stack traces with stack_trace() and access parameters or local variables with trace['name']:

.. code-block:: pycon

>>> trace = stack_trace(task)
>>> trace[5]
#5 at 0xffffffff8a5a32d0 (do_sys_poll+0x400/0x578) in do_poll at ./fs/select.c:961:8 (inlined)
>>> poll_list = trace[5]['list']
>>> file = fget(task, poll_list.entries[0].fd)
>>> d_path(file.f_path.address_of_())
b'/proc/115/mountinfo'

.. end-quick-start

See the user guide <https://drgn.readthedocs.io/en/latest/user_guide.html>_ for more details and features.

.. start-for-index

Getting Help

License

Copyright (c) Meta Platforms, Inc. and affiliates.

drgn is licensed under the LGPLv2.1 <https://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html>_ or later.

.. end-for-index