User Guide

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 for whatever you are debugging.

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

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

You can use various predefined helpers:

>>> len(list(bpf_prog_for_each()))
>>> 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']:

>>> 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_())

Core Concepts

The most important interfaces in drgn are programs, objects, and helpers.


A program being debugged is represented by an instance of the 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)
>>>, 16)

The drgn.Program.type(), drgn.Program.variable(), drgn.Program.constant(), and drgn.Program.function() methods look up those various things in a program. reads memory from the program’s address space. The [] operator looks up a variable, constant, or function:

>>> prog['jiffies'] == prog.variable('jiffies')

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.


Variables, constants, functions, and computed values are all called objects in drgn. Objects are represented by the 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])
>>> 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

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 ptr[0].

  • Instead of ptr->member, access a member through a pointer with ptr.member.

  • Instead of &var, get the address of a variable with 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 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 drgn.Object.string_() method gets a null-terminated string as bytes from an array or pointer.

  • The int(), float(), and bool() functions do an explicit conversion to that Python type.

Objects have several attributes; the most important are drgn.Object.prog_ and drgn.Object.type_. The former is the drgn.Program that the object is from, and the latter is the 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 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 (drgn.Object.read_() reads a reference object and returns it as a value object):

>>> import time
>>> jiffies = prog['jiffies']
>>> jiffies.value_()
>>> time.sleep(1)
>>> jiffies.value_()
>>> jiffies2 = jiffies.read_()
>>> jiffies2.value_()
>>> time.sleep(1)
>>> jiffies2.value_()
>>> jiffies.value_()

References have a drgn.Object.address_ attribute, which is the object’s address as a Python int. This is slightly different from the 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 drgn.Object.value_() refers to the value of the pointer (i.e., the address it points to):

>>> address = prog['jiffies'].address_
>>> type(address)
<class 'int'>
>>> print(hex(address))
>>> jiffiesp = prog['jiffies'].address_of_()
>>> jiffiesp
Object(prog, 'volatile unsigned long *', value=0xffffffffbe405000)
>>> print(hex(jiffiesp.value_()))

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 "<console>", line 1, in <module>
_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 drgn.ObjectAbsentError exception, although basic information including its type may still be accessed.


Some programs have common data structures that you may want to examine. For example, consider linked lists in the Linux kernel:

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:

def list_for_each(head):
    pos =
    while pos != head:
        yield pos
        pos =

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 Helpers.


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 drgn.helpers.ValidationError if the data structure is not valid or 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, 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

drgn.helpers.linux.list.validate_list_for_each_entry() does the same checks while also returning the entries in the list for further validation:

def validate_my_list(prog):
     for entry in validate_list_for_each_entry(
         "struct my_entry",
         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.


The drgn.Thread class represents a thread. drgn.Program.threads(), drgn.Program.thread(), drgn.Program.main_thread(), and drgn.Program.crashed_thread() can be used to find threads:

>>> for thread in prog.threads():
...     print(thread.tid)
>>> print(prog.main_thread().tid)
>>> print(prog.crashed_thread().tid)

Stack Traces

drgn represents stack traces with the drgn.StackTrace and drgn.StackFrame classes. drgn.stack_trace(), drgn.Program.stack_trace(), and drgn.Thread.stack_trace() return the call stack for a thread. The [] 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))<absent>
>>> trace[5]['list']
*(struct poll_list *)0xffffacca402e3b50 = {
        .next = (struct poll_list *)0x0,
        .len = (int)1,
        .entries = (struct pollfd []){},


The symbol table of a program is a list of identifiers along with their address and size. drgn represents symbols with the drgn.Symbol class, which is returned by drgn.Program.symbol().


drgn automatically obtains type definitions from the program. Types are represented by the drgn.Type class and created by various factory functions like 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 Types if you do.


Certain operations and objects in a program are platform-dependent; drgn allows accessing the platform that a program runs with the drgn.Platform class.

Command Line Interface

The drgn CLI is basically a wrapper around the drgn library which automatically creates a 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:

$ cat
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 601
PID 601 is being run by UID 1000

It’s even possible to run drgn scripts directly with the proper shebang:

$ cat
#!/usr/bin/env drgn

mounts = prog['init_task'].nsproxy.mnt_ns.mounts.value_()
print(f'You have {mounts} filesystems mounted')
$ sudo ./
You have 36 filesystems mounted

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 repr(). For drgn.Object and 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 print() function uses the output of 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 API Reference. Look through the Helpers. Read some Case Studies. Browse through the tools. Check out the community contributions.