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. The program must have debugging symbols available.

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

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

>>> trace = prog.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'

Core Concepts

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

Programs

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)
>>> prog.read(0xffffffffbe411e10, 16)
b'swapper/0\x00\x00\x00\x00\x00\x00\x00'

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

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_()
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 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))
0xffffffffbe405000
>>> jiffiesp = prog['jiffies'].address_of_()
>>> jiffiesp
Object(prog, 'volatile unsigned long *', value=0xffffffffbe405000)
>>> print(hex(jiffiesp.value_()))
0xffffffffbe405000

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.

Helpers

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 = 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 Helpers.

Other Concepts

In addition to the core concepts above, drgn provides a few additional abstractions.

Stack Traces

drgn represents stack traces with the drgn.StackTrace and drgn.StackFrame classes. drgn.Program.stack_trace() returns the call stack for a thread. The [] operator looks up an object in the scope of a StackFrame:

>>> trace = prog.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 []){},
}

Symbols

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().

Types

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.

Platforms

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 script.py
import sys
from drgn.helpers.linux import find_task

pid = int(sys.argv[1])
uid = find_task(prog, 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 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 official examples and tools.