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.
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
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))
drgn.Program.add_type_finder()
and
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
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_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.