Advanced Usage¶

The 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). 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. drgn.program_from_core_dump(), drgn.program_from_kernel(), and drgn.program_from_pid() correspond to the -c, -k, and -p command line options, respectively; they return a 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.

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 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. The drgn.cli.run_interactive() function allows you to run the same drgn CLI once you’ve created a drgn.Program, so it’s easy to make a custom program which allows interactive debugging.

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”:

import drgn
import os
import sys
from drgn.cli import run_interactive


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

    def read_file(address, count, offset, physical):
        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))
run_interactive(prog, banner_func=lambda _: "BTRFS debugger")

drgn.Program.add_type_finder() and drgn.Program.add_object_finder() are the equivalent methods for plugging in types and objects.

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 drgn.MissingDebugInfoError. Any additional errors are truncated. The default is 5; -1 is unlimited.
DRGN_PREFER_ORC_UNWINDER: Whether to prefer using ORC 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_SYS_MODULE: Whether drgn should use /sys/module to find information about 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.

Linux Kernel Special Objects¶

When debugging the Linux kernel, there are some special drgn.Objects accessible with drgn.Program.object() and drgn.Program[]. Some of these are available even without debugging information, thanks to metadata called “vmcoreinfo” which is present in kernel core dumps. These special objects include:

UTS_RELEASE

Object type: const char []

This corresponds to the UTS_RELEASE macro in the Linux kernel source code. This is the exact kernel release (i.e., the output of uname -r).

To use this as a Python string, you must convert it:

>>> release = prog["UTS_RELEASE"].string_().decode("ascii")

This is available without debugging information.

PAGE_SIZE

Object type: unsigned long

PAGE_SHIFT

Object type: unsigned int

PAGE_MASK

Object type: unsigned long

These correspond to the macros of the same name in the Linux kernel source code. The page size is the smallest contiguous unit of physical memory which can be allocated or mapped by the kernel.

>>> prog['PAGE_SIZE']
(unsigned long)4096
>>> prog['PAGE_SHIFT']
(int)12
>>> prog['PAGE_MASK']
(unsigned long)18446744073709547520
>>> 1 << prog['PAGE_SHIFT'] == prog['PAGE_SIZE']
True
>>> ~(prog['PAGE_SIZE'] - 1) == prog['PAGE_MASK']
True

These are available without debugging information.

jiffies

Object type: volatile unsigned long

This is a counter of timer ticks. It is actually an alias of jiffies_64 on 64-bit architectures, or the least significant 32 bits of jiffies_64 on 32-bit architectures. Since this alias is defined via the linker, drgn handles it specially.

This is not available without debugging information.

vmemmap

Object type: struct page *

This is a pointer to the “virtual memory map”, an array of struct page for each physical page of memory. While the purpose and implementation details of this array are beyond the scope of this documentation, it is enough to say that it is represented in the kernel source in an architecture-dependent way, frequently as a macro or constant. The definition provided by drgn ensures that users can access it without resorting to architecture-specific logic.

This is not available without debugging information.

VMCOREINFO

Object type: const char []

This is the data contained in the vmcoreinfo note, which is present either as an ELF note in /proc/kcore or ELF vmcores, or as a special data section in kdump-formatted vmcores. The vmcoreinfo note contains critical data necessary for interpreting the kernel image, such as KASLR offsets and data structure locations.

In the Linux kernel, this data is normally stored in a variable called vmcoreinfo_data. However, drgn reads this information from ELF note or from the diskdump header. It is possible (in rare cases, usually with vmcores created by hypervisors) for a vmcore to contain vmcoreinfo which differs from the data in vmcoreinfo_data, so it is important to distinguish the contents. For that reason, we use the name VMCOREINFO to distinguish it from the kernel variable vmcoreinfo_data.