GDB Python unwinder and frame decorator prototype

frametest_unwind.py

#
# GDB Python unwinder and frame decorator prototype
#
# This is a very basic demonstration for how to get GDB to unwind the
# "frametest" example crash in https://github.com/ben-cohen/corelibgen and
# apply a frame decorator to the backtrace.
#
# It needs a recent version of gdb and only works on i386 or x86_64.
# You need to manually fill in the PC value of the crash at "XXX" below!
#
# References:
#    https://sourceware.org/gdb/current/onlinedocs/gdb/Python-API.html
#    https://github.com/tromey/spidermonkey-unwinder
#    https://tromey.com/blog/?p=927
#
# Ben Cohen, June 2019
#

import itertools
import gdb
from gdb.unwinder import Unwinder
from gdb.FrameDecorator import FrameDecorator

# XXX: The PC value of the crash
CRASH_PC = 0x7f83a1c586a5

arch = gdb.selected_inferior().architecture().name()
if arch == "i386:x86-64":
    SP_REGISTER = 'rsp'
    PC_REGISTER = 'rip'
    BP_REGISTER = 'rbp'
    WORD_SIZE = 8
elif arch == "i386":
    SP_REGISTER = 'esp'
    PC_REGISTER = 'eip'
    BP_REGISTER = 'ebp'
    WORD_SIZE = 8


class FrameId(object):
    def __init__(self, sp, pc):
        self.sp = sp
        self.pc = pc


class SymValueWrapper():
    def __init__(self, symbol, value):
        self.sym = symbol
        self.val = value

    def value(self):
        return self.val

    def symbol(self):
        return self.sym


class TestFrameDecorator(FrameDecorator):
    def __init__(self, base):
        super(TestFrameDecorator, self).__init__(base)
        self.base = base

    def function(self):
        # Make a fake function name
        return "crash_frame"

    def frame_args(self):
        # Make fake function arguments
        args = []
        args.append(SymValueWrapper("foo", 42))
    
        return args


class TestFrameFilter():
    def __init__(self):
        self.name = "Foo"
        self.priority = 100
        self.enabled = True

        gdb.frame_filters[self.name] = self

    def decorate_crashed_frame(self, frame):
        if frame.inferior_frame().pc() == CRASH_PC:
            return TestFrameDecorator(frame)
        else:
            return frame

    def filter(self, frame_iter):
        return itertools.imap(self.decorate_crashed_frame, frame_iter)


class TestUnwinder(Unwinder):
    def __init__(self):
        super(TestUnwinder, self).__init__("Test")

    def __call__(self, pending_frame):
        pc = pending_frame.read_register(PC_REGISTER)

        if pc != CRASH_PC:
            return None

        sp = pending_frame.read_register(SP_REGISTER)
        unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc))

        # This is specific for unwinding the "frametest" example!
        caller_sp = sp + 0x12345678
        caller_bp = gdb.parse_and_eval('*(long*)%s'%caller_sp)
        caller_sp += WORD_SIZE
        caller_pc = gdb.parse_and_eval('*(long*)%s'%caller_sp)
        caller_sp += WORD_SIZE
        unwind_info.add_saved_register(SP_REGISTER, caller_sp)
        unwind_info.add_saved_register(PC_REGISTER, caller_pc)
        unwind_info.add_saved_register(BP_REGISTER, caller_bp)

        return unwind_info


unwinder = TestUnwinder()
gdb.unwinder.register_unwinder(None, unwinder, replace=True)
filter = TestFrameFilter()