pylddwrap

pylddwrap

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Pylddwrap wraps ldd *nix utility to determine shared libraries required by a program.

We need to dynamically package subset of our system at deployment time. Consequently, we have to determine the dependencies on shared libraries of our binaries programmatically.

The output of ldd Linux command, while informative, is not structured enough to be easily integrated into a program. At the time of this writing, we only found two alternative ldd wrappers on Internet python-ldd <https://github.com/relip/python-ldd>_ and ldd.py <https://gist.github.com/masami256/1588876>_, but their output was either too basic for our use case or the project was still incipient.

Pylddwrap, in contrast, returns a well-structured list of the dependencies. The command-line tool outputs the dependencies either as a table (for visual inspection) or as a JSON-formatted string (for use with other tools). The included Python module lddwrap returns a Python object with type annotations so that it can be used readily by the deployment scripts and other modules.

For more information on the ldd tool, please see ldd manual <http://man7.org/linux/man-pages/man1/ldd.1.html>_.

Usage

Command-Line Tool pylddwrap

• Assume we need the dependencies of the /bin/ls. The following command gives them as a table:

.. code-block:: bash

pylddwrap /bin/ls

• The output of the command looks like this:

.. code-block:: text

soname          | path                                  | found | mem_address        | unused
----------------+---------------------------------------+-------+--------------------+-------
linux-vdso.so.1 | None                                  | True  | 0x00007ffd8750f000 | False
libselinux.so.1 | /lib/x86_64-linux-gnu/libselinux.so.1 | True  | 0x00007f4e73dc3000 | True
libc.so.6       | /lib/x86_64-linux-gnu/libc.so.6       | True  | 0x00007f4e739f9000 | False
libpcre.so.3    | /lib/x86_64-linux-gnu/libpcre.so.3    | True  | 0x00007f4e73789000 | False
libdl.so.2      | /lib/x86_64-linux-gnu/libdl.so.2      | True  | 0x00007f4e73585000 | False
None            | /lib64/ld-linux-x86-64.so.2           | True  | 0x00007f4e73fe5000 | False
libpthread.so.0 | /lib/x86_64-linux-gnu/libpthread.so.0 | True  | 0x00007f4e73368000 | False

• To obtain the dependencies as JSON, invoke:

.. code-block:: bash

pylddwrap --format json /bin/ls

• The JSON output is structured like this:

.. code-block:: text

[ { "soname": "linux-vdso.so.1", "path": "None", "found": true, "mem_address": "0x00007ffed857f000", "unused": false }, ... ]

• You can also sort the table with --sorted which will sort by soname:

.. code-block:: bash

pylddwrap /bin/pwd --sorted

• Pylddwrap gives the table sorted by soname:

.. code-block:: text

soname          | path                            | found | mem_address        | unused
----------------+---------------------------------+-------+--------------------+-------
None            | /lib64/ld-linux-x86-64.so.2     | True  | 0x00007fd54894d000 | False
libc.so.6       | /lib/x86_64-linux-gnu/libc.so.6 | True  | 0x00007fd548353000 | False
linux-vdso.so.1 | None                            | True  | 0x00007ffe0953f000 | False

Alternatively, you can sort by any other column. For example, to sort by path:

.. code-block:: bash

pylddwrap /bin/pwd --sorted path

• The output will be:

.. code-block:: text

soname          | path                            | found | mem_address        | unused
----------------+---------------------------------+-------+--------------------+-------
linux-vdso.so.1 | None                            | True  | 0x00007ffe0953f000 | False
libc.so.6       | /lib/x86_64-linux-gnu/libc.so.6 | True  | 0x00007fd548353000 | False
None            | /lib64/ld-linux-x86-64.so.2     | True  | 0x00007fd54894d000 | False

ldwrap Python Module

We provide lddwrap Python module which you can integrate into your deployment scripts and other modules.

• The following example shows how to list the dependencies of /bin/ls:

.. code-block:: python

import pathlib
import lddwrap

path = pathlib.Path("/bin/ls")
deps = lddwrap.list_dependencies(path=path)
for dep in deps:
    print(dep)

"""
soname: linux-vdso.so.1, path: None, found: True, mem_address: (0x00007ffe8e2fb000), unused: None
soname: libselinux.so.1, path: /lib/x86_64-linux-gnu/libselinux.so.1, found: True, mem_address: (0x00007f7759ccc000), unused: None
soname: libc.so.6, path: /lib/x86_64-linux-gnu/libc.so.6, found: True, mem_address: (0x00007f7759902000), unused: None
...
"""

• List all dependencies of the /bin/ls utility and check if the direct dependencies are used. If unused for list_dependencies is set to False then the unused variable of the dependencies will not be determined and are therefore unknown and set to None. Otherwise information about direct usage will be retrieved and added to the dependencies.

.. code-block:: python

import pathlib
import lddwrap

path = pathlib.Path("/bin/ls")
deps = lddwrap.list_dependencies(path=path, unused=True)
print(deps[1])
# soname: libselinux.so.1,
# path: /lib/x86_64-linux-gnu/libselinux.so.1,
# found: True,
# mem_address: (0x00007f5a6064a000),
# unused: True

• Lddwrap operates normally with the environment variables of the caller. In cases where your dependencies are determined differently than the current environment, you pass a separate environment (in form of a dictionary) as an argument:

.. code-block:: python

import os
import pathlib
import lddwrap

env = os.environ.copy()
env['LD_LIBRARY_PATH'] = "some/important/path"
path = pathlib.Path("/bin/ls")
deps = lddwrap.list_dependencies(path=path, env=env)

Installation

• Install pylddwrap with pip:

.. code-block:: bash

pip3 install pylddwrap

Development

• Check out the repository.

• In the repository root, create the virtual environment:

.. code-block:: bash

python3 -m venv venv3

• Activate the virtual environment:

.. code-block:: bash

source venv3/bin/activate

• Install the development dependencies:

.. code-block:: bash

pip3 install -e .[dev]

• Tests can be run directly using unittest:

.. code-block:: bash

python3 -m unittest discover tests/

Pre-commit Checks

We provide a set of pre-commit checks that lint and check code for formatting.

Namely, we use:

• yapf <https://github.com/google/yapf>_ to check the formatting.
• The style of the docstrings is checked with pydocstyle <https://github.com/PyCQA/pydocstyle>_.
• Static type analysis is performed with mypy <http://mypy-lang.org/>_.
• Various linter checks are done with pylint <https://www.pylint.org/>_.

Apply the automatic formatting by running the format environment:

.. code-block:: bash

tox -e format

Run the pre-commit checks and tests using tox:

.. code-block:: bash

tox

Versioning

We follow Semantic Versioning <http://semver.org/spec/v1.0.0.html>_. The version X.Y.Z indicates:

• X is the major version (backward-incompatible),
• Y is the minor version (backward-compatible), and
• Z is the patch version (backward-compatible bug fix).