Write extensions

Extending GEF

GEF intends to provide a battery-included, quickly installable and crazy fast debugging environment sitting on top of GDB.

But it most importantly provides all the primitives required to allow hackers to quickly create their own commands. This page intends to summarize how to create advanced GDB commands in moments using GEF as a library.

A dedicated repository was born to host external scripts. This repo is open to all for contributions, no restrictions and the most valuable ones will be integrated into gef.py.

Quick start

Here is the most basic skeleton for creating a new GEF command named newcmd:

@register
class NewCommand(GenericCommand):
    """Dummy new command."""
    _cmdline_ = "newcmd"
    _syntax_  = f"{_cmdline_}"
    # optionally
    # _examples_ = [f"{_cmdline_} arg1 ...", ]
    # _aliases_ = ["alias_to_cmdline", ]

    @only_if_gdb_running         # not required, ensures that the debug session is started
    def do_invoke(self, argv):
        # let's say we want to print some info about the architecture of the current binary
        print(f"gef.arch={gef.arch}")
        # or showing the current $pc
        print(f"gef.arch.pc={gef.arch.pc:#x}")

Loading it in GEF is as easy as

gef➤  source /path/to/newcmd.py
[+] Loading 'NewCommand'

The new command is now loaded and part of GEF and can be invoked as such:

gef➤  newcmd
gef.arch=<__main__.X86_64 object at 0x7fd5583571c0>
gef.arch.pc=0x55555555a7d0

Yes, that's it! Check out the complete API to see what else GEF offers.

Detailed explanation

Our new command must be a class that inherits from GEF's GenericCommand. The only requirements are:

• a _cmdline_ attribute (the command to type on the GDB prompt).
• a _syntax_ attribute, which GEF will use to auto-generate the help menu.
• a method do_invoke(self, args) which will be executed when the command is invoked. args is a list of the command line args provided when invoked.

We make GEF aware of this new command by registering it in the __main__ section of the script, by invoking the global function register_external_command().

Now you have a new GEF command which you can load, either from cli:

gef➤  source /path/to/newcmd.py

or add to your ~/.gdbinit:

echo source /path/to/newcmd.py >> ~/.gdbinit

Customizing context panes

Sometimes you want something similar to a command to run on each break-like event and display itself as a part of the GEF context. This can be achieved using the following function register_external_context_pane().

Here is a simple example of how to make a custom context pane:

__start_time__ = int(time.time())
def wasted_time_debugging():
    gef_print("You have wasted {} seconds!".format(int(time.time()) - __start_time__))

def wasted_time_debugging_title():
    return "wasted:time:debugging:{}".format(int(time.time()) - __start_time__)

register_external_context_pane("wasted_time_debugging", wasted_time_debugging, wasted_time_debugging_title)

Loading it in GEF is as easy as loading a command

gef➤  source /path/to/custom_context_pane.py

It can even be included in the same file as a Command. Now on each break you will notice a new pane near the bottom of the context. The order can be modified in the GEF context config.

Context Pane API

The API demonstrated above requires very specific argument types:

def register_external_context_pane(
    name: str,
    context_callback: Callable[None,[]],
    context_callback_title: Callable[str, []],
    condition_callback: Optional[Callable[bool, []]] = None
) -> None:

• name: a string that will be used as the panes setting name
• context_callback: a function that uses gef_print() to print content in the pane
• context_callback_title: a function that returns the title string or None to hide the title
• condition_callback (optional): a function that returns a boolean deciding whether this context pane should be shown

API

Some of the most important parts of the API for creating new commands are mentioned (but not limited to) below. To see the full help of a function, open GDB and GEF, and use the embedded Python interpreter's help command.

For example:

gef➤  pi help(Architecture)

or even from outside GDB:

gdb -q -ex 'pi help(hexdump)' -ex quit

The GEF API aims to provide a simpler and more Pythonic approach to GDB's.

Some basic examples:

• read the memory

gef ➤  pi print(hexdump( gef.memory.read(parse_address("$pc"), length=0x20 )))
0x0000000000000000     f3 0f 1e fa 31 ed 49 89 d1 5e 48 89 e2 48 83 e4    ....1.I..^H..H..
0x0000000000000010     f0 50 54 4c 8d 05 66 0d 01 00 48 8d 0d ef 0c 01    .PTL..f...H.....

• get access to the memory layout

gef ➤ pi print('\n'.join([ f"{x.page_start:#x} -> {x.page_end:#x}" for x in gef.memory.maps]))
0x555555554000 -> 0x555555558000
0x555555558000 -> 0x55555556c000
0x55555556c000 -> 0x555555575000
0x555555576000 -> 0x555555577000
0x555555577000 -> 0x555555578000
0x555555578000 -> 0x55555559a000
0x7ffff7cd8000 -> 0x7ffff7cda000
0x7ffff7cda000 -> 0x7ffff7ce1000
0x7ffff7ce1000 -> 0x7ffff7cf2000
0x7ffff7cf2000 -> 0x7ffff7cf7000
[...]

The API also offers a number of decorators to simplify the creation of new/existing commands, such as:

• @only_if_gdb_running to execute only if a GDB session is running.
• @only_if_gdb_target_local to check if the target is local i.e. not debugging using GDB remote.
• and many more...

Reference

For a complete reference of the API offered by GEF, visit docs/api/gef.md.

Parsing command arguments

@parse_arguments( {"required_argument_1": DefaultValue1, ...}, {"--optional-argument-1": DefaultValue1, ...} )

This decorator aims to facilitate the argument passing to a command. If added, it will use the argparse module to parse arguments, and will store them in the kwargs["arguments"] of the calling function (therefore the function must have *args, **kwargs added to its signature). Argument type is inferred directly from the default value except for boolean, where a value of True corresponds to argparse's store_true action. For more details on argparse, refer to its Python documentation.

Values given for the parameters also allow list of arguments being past. This can be useful in the case where the number of exact option values is known in advance. This can be achieved simply by using a type of tuple or list for the default value. parse_arguments will determine the type of what to expect based on the first default value of the iterable, so make sure it's not empty. For instance:

@parse_arguments( {"instructions": ["nop", "int3", "hlt"], }, {"--arch": "x64", } )

Argument flags are also supported, allowing to write simpler version of the flag such as

@parse_arguments( {}, {("--long-argument", "-l"): value, } )

A basic example would be as follow:

class MyCommand(GenericCommand):
    [...]

    @parse_arguments({"foo": [1,]}, {"--bleh": "", ("--blah", "-l): False})
    def do_invoke(self, argv, *args, **kwargs):
      args = kwargs["arguments"]
      if args.foo == 1: ...
      if args.blah == True: ...

When the user enters the following command:

gef➤ mycommand --blah 3 14 159 2653

The function MyCommand!do_invoke() can use the command line argument value

args.foo == [3, 14, 159, 2653] # a List(int) from user input
args.bleh == "" # the default value
args.blah == True # set to True because user input declared the option (would have been False otherwise)

Adding new architectures

Support for new architectures can be added by inheriting from the Architecture class. Examples can be found in gef-extras.

Sometimes architectures can more precisely determine whether they apply to the current target by looking at the architecture determined by gdb. For these cases the custom architecture may implement the supports_gdb_arch() static function to signal that they should be used instead of the default. The function receives only one argument:

• gdb_str (of type str) which is the architecture name as reported by GDB.

The function must return:

• True if the current Architecture class supports the target binary; False otherwise.
• None to simply ignore this check and let GEF try to determine the architecture.

One example is the ARM Cortex-M architecture which in some cases should be used over the generic ARM one:

@staticmethod
def supports_gdb_arch(gdb_arch: str) -> Optional[bool]:
    return bool(re.search("^armv.*-m$", gdb_arch))