Advanced Usage
==============

.. highlight:: pycon

The :doc:`user_guide` covers basic usage of drgn, but drgn also supports more
advanced use cases which are covered here.

Loading Debugging Symbols
-------------------------

drgn will automatically load debugging information based on the debugged
program (e.g., from loaded kernel modules or loaded shared libraries).
:meth:`drgn.Program.load_debug_info()` can be used to load additional debugging
information::

    >>> prog.load_debug_info(['./libfoo.so', '/usr/lib/libbar.so'])

Library
-------

In addition to the CLI, drgn is also available as a library.
:func:`drgn.program_from_core_dump()`, :func:`drgn.program_from_kernel()`, and
:func:`drgn.program_from_pid()` correspond to the ``-c``, ``-k``, and ``-p``
command line options, respectively; they return a :class:`drgn.Program` that
can be used just like the one initialized by the CLI::

    >>> import drgn
    >>> prog = drgn.program_from_kernel()

C Library
---------

The core functionality of drgn is implemented in C and is available as a C
library, ``libdrgn``. See |drgn.h|_.

.. |drgn.h| replace:: ``drgn.h``
.. _drgn.h: https://github.com/osandov/drgn/blob/main/libdrgn/drgn.h.in

Full documentation can be generated by running ``doxygen`` in the ``libdrgn``
directory of the source code. Note that the API and ABI are not yet stable.

Custom Programs
---------------

The main components of a :class:`drgn.Program` are the program memory, types,
and symbols. The CLI and equivalent library interfaces automatically determine
these. However, it is also possible to create a "blank" ``Program`` and plug in
the main components.

:meth:`drgn.Program.add_memory_segment()` defines a range of memory and how to
read that memory. The following example uses a Btrfs filesystem image as the
program "memory":

.. code-block:: python3

    import drgn
    import os
    import sys


    def btrfs_debugger(dev):
        file = open(dev, 'rb')
        size = file.seek(0, 2)

        def read_file(address, count, physical, offset):
            file.seek(offset)
            return file.read(count)

        platform = drgn.Platform(drgn.Architecture.UNKNOWN,
                                 drgn.PlatformFlags.IS_LITTLE_ENDIAN)
        prog = drgn.Program(platform)
        prog.add_memory_segment(0, size, read_file)
        prog.load_debug_info([f'/lib/modules/{os.uname().release}/kernel/fs/btrfs/btrfs.ko'])
        return prog


    prog = btrfs_debugger(sys.argv[1] if len(sys.argv) >= 2 else '/dev/sda')
    print(drgn.Object(prog, 'struct btrfs_super_block', address=65536))

:meth:`drgn.Program.add_type_finder()` and
:meth:`drgn.Program.add_symbol_finder()` are the equivalent methods for
plugging in types and symbols.

Environment Variables
---------------------

Some of drgn's behavior can be modified through environment variables:

``DRGN_MAX_DEBUG_INFO_ERRORS``
    The maximum number of individual errors to report in a
    :exc:`drgn.MissingDebugInfoError`. Any additional errors are truncated. The
    default is 5; -1 is unlimited.

``DRGN_PREFER_ORC_UNWINDER```
    Whether to prefer using `ORC
    <https://www.kernel.org/doc/html/latest/x86/orc-unwinder.html>`_ over DWARF
    for stack unwinding (0 or 1). The default is 0. Note that drgn will always
    fall back to ORC for functions lacking DWARF call frame information and
    vice versa. This environment variable is mainly intended for testing and
    may be ignored in the future.

``DRGN_USE_LIBDWFL_REPORT``
    Whether drgn should use libdwfl to find debugging information for core
    dumps instead of its own implementation (0 or 1). The default is 0. This
    environment variable is mainly intended as an escape hatch in case of bugs
    in drgn's implementation and will be ignored in the future.

``DRGN_USE_LIBKDUMPFILE_FOR_ELF``
    Whether drgn should use libkdumpfile for ELF vmcores (0 or 1). The default
    is 0. This functionality will be removed in the future.

``DRGN_USE_PROC_AND_SYS_MODULES``
    Whether drgn should use ``/proc/modules`` and ``/sys/module`` to find
    loaded kernel modules for the running kernel instead of getting them from
    the core dump (0 or 1). The default is 1. This environment variable is
    mainly intended for testing and may be ignored in the future.