GDB调试–Python

你还在用GDB调试程序吗？

如果是，那么我们是同道中人。但是你知道GDB有一个很强大的功能，Python scripting嘛？

如果是的，那么恭喜你，你是一个大牛。

本文主要讲述如何使用Python来提高你的gdb调试技能， 让你从繁重的重复的工作里面挣脱出来呼吸新鲜空气。

首先，第一件事，使用gdb7.x以上的版本，最好9.x的。因为Python的支持是从gdb7.0（2009年？）开始的。

进入正题

gdb本来就支持自定义脚本辅助调试，为什么还要用Python脚本呢？因为自定义脚本的语法比较老，不如写Python欢快。如果你喜欢用原来的自定义脚本方法，那也是可以的。

借助Python，你可以将难看的数据变得好看，

借助Python，你可以将重复的工作变成一个命令，

借助Python，你可以更快的调试bug，

借助Python，你可以装逼，哈哈哈

……

将难看的数据变得好看

以下面的代码为例

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#include <map>
#include <iostream>
#include <string>
using namespace  std;

int main() {
    std::map<string, string> lm;
    lm["good"] = "heart";
    // 查看map 里面内容
    std::cout<<lm["good"];
}

当代码运行到std<<cout时, 你想查看map里面的内容，如果没有python和自定义的脚本，print lm看到的是

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$2 = {_M_t = {
    _M_impl = {<std::allocator<std::_Rb_tree_node<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >> = {<__gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >> = {<No data fields>}, <No data fields>}, <std::_Rb_tree_key_compare<std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >> = {
        _M_key_compare = {<std::binary_function<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool>> = {<No data fields>}, <No data fields>}}, <std::_Rb_tree_header> = {_M_header = {
          _M_color = std::_S_red, _M_parent = 0x55555556eeb0, 
          _M_left = 0x55555556eeb0, _M_right = 0x55555556eeb0}, 
        _M_node_count = 1}, <No data fields>}}}

但是当你在gdb9.2里面输入print lm的时候，你看到的将是

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(gdb) p lm
$3 = std::map with 1 element = {["good"] = "heart"}

map里面有什么一清二楚。这是因为gdb9.x自带了一系列标准库的Python pretty priniter。 如果你使用的是gdb7.x，那么你可以手动的导入这些pretty printer实现同样的效果。具体步骤如下：

  1. 下载pretty printer: svn co svn://http://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python
  2. 在gdb里面输入(将路径改成你下载的路径)：
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python
import sys
sys.path.insert(0, '/home/maude/gdb_printers/python')
from libstdcxx.v6.printers import register_libstdcxx_printers
register_libstdcxx_printers (None)
end

这样你就可以放心使用了~

详细请看：

https://sourceware.org/gdb/wiki/STLSupport

https://codeyarns.com/2014/07/17/how-to-enable-pretty-printing-for-stl-in-gdb/

将重复的工作变成一个命令

比如在调试的时候，你知道当前栈指向一个字符串，但是你不知道具体在哪里，你想遍历这个栈将它找出来，那么你可以借助Python自定义一个命令”stackwalk”，这个命令可以直接Python代码遍历栈，将字符串找出来。

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#####################################################
# Usage: to load this to gdb run:
# (gdb) source ..../path/to/<script_file>.py

import gdb

class StackWalk(gdb.Command):
    def __init__(self):
        # This registers our class as "StackWalk"
        super(StackWalk, self).__init__("stackwalk", gdb.COMMAND_DATA)

    def invoke(self, arg, from_tty):
        # When we call "StackWalk" from gdb, this is the method
        # that will be called.
        print("Hello from StackWalk!")
        # get the register
        rbp = gdb.parse_and_eval('$rbp')
        rsp = gdb.parse_and_eval('$rsp')
        ptr = rsp
        ppwc = gdb.lookup_type('wchar_t').pointer().pointer()
        while ptr < rbp:
            try:
                print('pointer is {}'.format(ptr))
                print(gdb.execute('wc_print {}'.format(ptr.cast(ppwc).dereference())))
                print('===')
            except:
                pass
            ptr += 8
        

# This registers our class to the gdb runtime at "source" time.
StackWalk()


更快的调试bug

当你调试多线程的时候，你发现callstack 一堆，而且好多都是重复的，如果它们可以自动去重或者折叠多好，这样你只需要关注一小部分。好消息！Python可以让你用一个命令就可以轻松搞定。而且已经有人写好了相应的代码，你只需要导入即可。详细介绍请看https://fy.blackhats.net.au/blog/html/2017/08/04/so_you_want_to_script_gdb_with_python.html 

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# From https://fy.blackhats.net.au/blog/html/2017/08/04/so_you_want_to_script_gdb_with_python.html
#####################################################
#
# Usage: to load this to gdb run:
# (gdb) source ..../path/to/debug_naughty.py
#
# To have this automatically load, you need to put the script
# in a path related to your binary. If you make /usr/sbin/foo,
# You can ship this script as:
# /usr/share/gdb/auto-load/ <PATH TO BINARY>
# /usr/share/gdb/auto-load/usr/sbin/foo
#
# This will trigger gdb to autoload the script when you start
# to acces a core or the live binary from this location.
#

import gdb


class StackFold(gdb.Command):
    def __init__(self):
        super(StackFold, self).__init__("stackfold", gdb.COMMAND_DATA)

    def invoke(self, arg, from_tty):
        # An inferior is the 'currently running applications'. In this case we only
        # have one.
        stack_maps = {}
        # This creates a dict where each element is keyed by backtrace.
        # Then each backtrace contains an array of "frames"
        #
        inferiors = gdb.inferiors()
        for inferior in inferiors:
            for thread in inferior.threads():
                try:
                    # Change to our threads context
                    thread.switch()
                    # Get the thread IDS
                    (tpid, lwpid, tid) = thread.ptid
                    gtid = thread.num
                    # Take a human readable copy of the backtrace, we'll need this for display later.
                    o = gdb.execute('bt', to_string=True)
                    # Build the backtrace for comparison
                    backtrace = []
                    gdb.newest_frame()
                    cur_frame = gdb.selected_frame()
                    while cur_frame is not None:
                        if cur_frame.name() is not None:
                            backtrace.append(cur_frame.name())

                        cur_frame = cur_frame.older()
                    # Now we have a backtrace like ['pthread_cond_wait@@GLIBC_2.3.2', 'lazy_thread', 'start_thread', 'clone']
                    # dicts can't use lists as keys because they are non-hashable, so we turn this into a string.
                    # Remember, C functions can't have spaces in them ...
                    s_backtrace = ' '.join(backtrace)
                    # Let's see if it exists in the stack_maps
                    if s_backtrace not in stack_maps:
                        stack_maps[s_backtrace] = []
                    # Now lets add this thread to the map.
                    stack_maps[s_backtrace].append({'gtid': gtid, 'tpid' : tpid, 'bt': o} )
                except Exception as e:
                    print(e)
        # Now at this point we have a dict of traces, and each trace has a "list" of pids that match. Let's display them
        for smap in stack_maps:
            # Get our human readable form out.
            o = stack_maps[smap][0]['bt']
            for t in stack_maps[smap]:
                # For each thread we recorded
                print("Thread %s (LWP %s))" % (t['gtid'], t['tpid']))
            print(o)

# This registers our class to the gdb runtime at "source" time.
StackFold()

等等！还有好多，毕竟Python图灵完备，只要GDB提供相应的API,你想要啥都能实现。

会了这些，你就可以向新手装逼去了

References:

1 https://undo.io/resources/gdb-watchpoint/python-gdb/

2 https://codeyarns.com/2014/07/17/how-to-enable-pretty-printing-for-stl-in-gdb/