User Guide ========== Quick Start ----------- .. include:: ../README.rst :start-after: start-quick-start :end-before: end-quick-start Core Concepts ------------- .. highlight:: pycon The most important interfaces in drgn are *programs*, *objects*, and *helpers*. Programs ^^^^^^^^ A program being debugged is represented by an instance of the :class:`drgn.Program` class. The drgn CLI is initialized with a ``Program`` named ``prog``; unless you are using the drgn library directly, this is usually the only ``Program`` you will need. A ``Program`` is used to look up type definitions, access variables, and read arbitrary memory:: >>> prog.type('unsigned long') prog.int_type(name='unsigned long', size=8, is_signed=False) >>> prog['jiffies'] Object(prog, 'volatile unsigned long', address=0xffffffffbe405000) >>> prog.read(0xffffffffbe411e10, 16) b'swapper/0\x00\x00\x00\x00\x00\x00\x00' The :meth:`drgn.Program.type()`, :meth:`drgn.Program.variable()`, :meth:`drgn.Program.constant()`, and :meth:`drgn.Program.function()` methods look up those various things in a program. :meth:`drgn.Program.read()` reads memory from the program's address space. The :meth:`[] ` operator looks up a variable, constant, or function:: >>> prog['jiffies'] == prog.variable('jiffies') True It is usually more convenient to use the ``[]`` operator rather than the ``variable()``, ``constant()``, or ``function()`` methods unless the program has multiple objects with the same name, in which case the methods provide more control. Objects ^^^^^^^ Variables, constants, functions, and computed values are all called *objects* in drgn. Objects are represented by the :class:`drgn.Object` class. An object may exist in the memory of the program (a *reference*):: >>> Object(prog, 'int', address=0xffffffffc09031a0) Or, an object may be a constant or temporary computed value (a *value*):: >>> Object(prog, 'int', value=4) What makes drgn scripts expressive is that objects can be used almost exactly like they would be in the program's own source code. For example, structure members can be accessed with the dot (``.``) operator, arrays can be subscripted with ``[]``, arithmetic can be performed, and objects can be compared:: >>> print(prog['init_task'].comm[0]) (char)115 >>> print(repr(prog['init_task'].nsproxy.mnt_ns.mounts + 1)) Object(prog, 'unsigned int', value=34) >>> prog['init_task'].nsproxy.mnt_ns.pending_mounts > 0 False Python doesn't have all of the operators that C or C++ do, so some substitutions are necessary: * Instead of ``*ptr``, dereference a pointer with :meth:`ptr[0] `. * Instead of ``ptr->member``, access a member through a pointer with :meth:`ptr.member `. * Instead of ``&var``, get the address of a variable with :meth:`var.address_of_() `. A common use case is converting a ``drgn.Object`` to a Python value so it can be used by a standard Python library. There are a few ways to do this: * The :meth:`drgn.Object.value_()` method gets the value of the object with the directly corresponding Python type (i.e., integers and pointers become ``int``, floating-point types become ``float``, booleans become ``bool``, arrays become ``list``, structures and unions become ``dict``). * The :meth:`drgn.Object.string_()` method gets a null-terminated string as ``bytes`` from an array or pointer. * The :class:`int() `, :class:`float() `, and :class:`bool() ` functions do an explicit conversion to that Python type. Objects have several attributes; the most important are :attr:`drgn.Object.prog_` and :attr:`drgn.Object.type_`. The former is the :class:`drgn.Program` that the object is from, and the latter is the :class:`drgn.Type` of the object. Note that all attributes and methods of the ``Object`` class end with an underscore (``_``) in order to avoid conflicting with structure or union members. The ``Object`` attributes and methods always take precedence; use :meth:`drgn.Object.member_()` if there is a conflict. References vs. Values """"""""""""""""""""" The main difference between reference objects and value objects is how they are evaluated. References are read from the program's memory every time they are evaluated; values simply return the stored value (:meth:`drgn.Object.read_()` reads a reference object and returns it as a value object):: >>> import time >>> jiffies = prog['jiffies'] >>> jiffies.value_() 4391639989 >>> time.sleep(1) >>> jiffies.value_() 4391640290 >>> jiffies2 = jiffies.read_() >>> jiffies2.value_() 4391640291 >>> time.sleep(1) >>> jiffies2.value_() 4391640291 >>> jiffies.value_() 4391640593 References have a :attr:`drgn.Object.address_` attribute, which is the object's address as a Python ``int``. This is slightly different from the :meth:`drgn.Object.address_of_()` method, which returns the address as a ``drgn.Object``. Of course, both references and values can have a pointer type; ``address_`` refers to the address of the pointer object itself, and :meth:`drgn.Object.value_()` refers to the value of the pointer (i.e., the address it points to):: >>> address = prog['jiffies'].address_ >>> type(address) >>> print(hex(address)) 0xffffffffbe405000 >>> jiffiesp = prog['jiffies'].address_of_() >>> jiffiesp Object(prog, 'volatile unsigned long *', value=0xffffffffbe405000) >>> print(hex(jiffiesp.value_())) 0xffffffffbe405000 .. _absent-objects: Absent Objects """""""""""""" In addition to reference objects and value objects, objects may also be *absent*. >>> Object(prog, "int").value_() Traceback (most recent call last): File "", line 1, in _drgn.ObjectAbsentError: object absent This represents an object whose value or address is not known. For example, this can happen if the object was optimized out of the program by the compiler. Any attempt to operate on an absent object results in a :exc:`drgn.ObjectAbsentError` exception, although basic information including its type may still be accessed. Helpers ^^^^^^^ Some programs have common data structures that you may want to examine. For example, consider linked lists in the Linux kernel: .. code-block:: c struct list_head { struct list_head *next, *prev; }; #define list_for_each(pos, head) \ for (pos = (head)->next; pos != (head); pos = pos->next) When working with these lists, you'd probably want to define a function: .. code-block:: python3 def list_for_each(head): pos = head.next while pos != head: yield pos pos = pos.next Then, you could use it like so for any list you need to look at:: >>> for pos in list_for_each(head): ... do_something_with(pos) Of course, it would be a waste of time and effort for everyone to have to define these helpers for themselves, so drgn includes a collection of helpers for many use cases. See :doc:`helpers`. .. _validators: Validators """""""""" Validators are a special category of helpers that check the consistency of a data structure. In general, helpers assume that the data structures that they examine are valid. Validators do not make this assumption and do additional (potentially expensive) checks to detect broken invariants, corruption, etc. Validators raise :class:`drgn.helpers.ValidationError` if the data structure is not valid or :class:`drgn.FaultError` if the data structure is invalid in a way that causes a bad memory access. They have names prefixed with ``validate_``. For example, :func:`drgn.helpers.linux.list.validate_list()` checks the consistency of a linked list in the Linux kernel (in particular, the consistency of the ``next`` and ``prev`` pointers):: >>> validate_list(prog["my_list"].address_of_()) drgn.helpers.ValidationError: (struct list_head *)0xffffffffc029e460 next 0xffffffffc029e000 has prev 0xffffffffc029e450 :func:`drgn.helpers.linux.list.validate_list_for_each_entry()` does the same checks while also returning the entries in the list for further validation: .. code-block:: python3 def validate_my_list(prog): for entry in validate_list_for_each_entry( "struct my_entry", prog["my_list"].address_of_(), "list", ): if entry.value < 0: raise ValidationError("list contains negative entry") Other Concepts -------------- In addition to the core concepts above, drgn provides a few additional abstractions. Threads ^^^^^^^ The :class:`drgn.Thread` class represents a thread. :meth:`drgn.Program.threads()`, :meth:`drgn.Program.thread()`, :meth:`drgn.Program.main_thread()`, and :meth:`drgn.Program.crashed_thread()` can be used to find threads:: >>> for thread in prog.threads(): ... print(thread.tid) ... 39143 39144 >>> print(prog.main_thread().tid) 39143 >>> print(prog.crashed_thread().tid) 39144 Stack Traces ^^^^^^^^^^^^ drgn represents stack traces with the :class:`drgn.StackTrace` and :class:`drgn.StackFrame` classes. :func:`drgn.stack_trace()`, :meth:`drgn.Program.stack_trace()`, and :meth:`drgn.Thread.stack_trace()` return the call stack for a thread. The :meth:`[] ` operator looks up an object in the scope of a ``StackFrame``:: >>> trace = stack_trace(115) >>> trace #0 context_switch (./kernel/sched/core.c:4683:2) #1 __schedule (./kernel/sched/core.c:5940:8) #2 schedule (./kernel/sched/core.c:6019:3) #3 schedule_hrtimeout_range_clock (./kernel/time/hrtimer.c:2148:3) #4 poll_schedule_timeout (./fs/select.c:243:8) #5 do_poll (./fs/select.c:961:8) #6 do_sys_poll (./fs/select.c:1011:12) #7 __do_sys_poll (./fs/select.c:1076:8) #8 __se_sys_poll (./fs/select.c:1064:1) #9 __x64_sys_poll (./fs/select.c:1064:1) #10 do_syscall_x64 (./arch/x86/entry/common.c:50:14) #11 do_syscall_64 (./arch/x86/entry/common.c:80:7) #12 entry_SYSCALL_64+0x7c/0x15b (./arch/x86/entry/entry_64.S:113) #13 0x7f3344072af7 >>> trace[5] #5 at 0xffffffff8a5a32d0 (do_sys_poll+0x400/0x578) in do_poll at ./fs/select.c:961:8 (inlined) >>> prog['do_poll'] (int (struct poll_list *list, struct poll_wqueues *wait, struct timespec64 *end_time)) >>> trace[5]['list'] *(struct poll_list *)0xffffacca402e3b50 = { .next = (struct poll_list *)0x0, .len = (int)1, .entries = (struct pollfd []){}, } Symbols ^^^^^^^ The symbol table of a program is a list of identifiers along with their address and size. drgn represents symbols with the :class:`drgn.Symbol` class, which is returned by :meth:`drgn.Program.symbol()`. Types ^^^^^ drgn automatically obtains type definitions from the program. Types are represented by the :class:`drgn.Type` class and created by various factory functions like :meth:`drgn.Program.int_type()`:: >>> prog.type('int') prog.int_type(name='int', size=4, is_signed=True) You won't usually need to work with types directly, but see :ref:`api-reference-types` if you do. Platforms ^^^^^^^^^ Certain operations and objects in a program are platform-dependent; drgn allows accessing the platform that a program runs with the :class:`drgn.Platform` class. Command Line Interface ---------------------- The drgn CLI is basically a wrapper around the drgn library which automatically creates a :class:`drgn.Program`. The CLI can be run in interactive mode or script mode. Script Mode ^^^^^^^^^^^ Script mode is useful for reusable scripts. Simply pass the path to the script along with any arguments: .. code-block:: console $ cat script.py import sys from drgn.helpers.linux import find_task pid = int(sys.argv[1]) uid = find_task(pid).cred.uid.val.value_() print(f'PID {pid} is being run by UID {uid}') $ sudo drgn script.py 601 PID 601 is being run by UID 1000 It's even possible to run drgn scripts directly with the proper `shebang `_:: $ cat script2.py #!/usr/bin/env drgn mounts = prog['init_task'].nsproxy.mnt_ns.mounts.value_() print(f'You have {mounts} filesystems mounted') $ sudo ./script2.py You have 36 filesystems mounted .. _interactive-mode: Interactive Mode ^^^^^^^^^^^^^^^^ Interactive mode uses the Python interpreter's `interactive mode `_ and adds a few nice features, including: * History * Tab completion * Automatic import of relevant helpers * Pretty printing of objects and types The default behavior of the Python `REPL `_ is to print the output of :func:`repr()`. For :class:`drgn.Object` and :class:`drgn.Type`, this is a raw representation:: >>> print(repr(prog['jiffies'])) Object(prog, 'volatile unsigned long', address=0xffffffffbe405000) >>> print(repr(prog.type('atomic_t'))) prog.typedef_type(name='atomic_t', type=prog.struct_type(tag=None, size=4, members=(TypeMember(prog.type('int'), name='counter', bit_offset=0),))) The standard :func:`print()` function uses the output of :func:`str()`. For drgn objects and types, this is a representation in programming language syntax:: >>> print(prog['jiffies']) (volatile unsigned long)4395387628 >>> print(prog.type('atomic_t')) typedef struct { int counter; } atomic_t In interactive mode, the drgn CLI automatically uses ``str()`` instead of ``repr()`` for objects and types, so you don't need to call ``print()`` explicitly:: $ sudo drgn >>> prog['jiffies'] (volatile unsigned long)4395387628 >>> prog.type('atomic_t') typedef struct { int counter; } atomic_t Next Steps ---------- Refer to the :doc:`api_reference`. Look through the :doc:`helpers`. Read some :doc:`case_studies`. Browse through the `tools `_. Check out the `community contributions `_.