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[docker+gdb] Debug PHP source code and see strval function C implementation
- 藏色散人forward
- 2019-10-09 13:21:262891browse
php strval 函数的作用很简单，就是你给他一个值，他给你返回字符串类型。
算是一个比较简单的函数了，我们来通过 gdb 来一探究竟。
通过本文，你可以窥探下
● gdb 的简单使用
● gdb gui 模式初探
● 看看平时写的 PHP 代码在 C 语言里的样子
● 对使用 gdb 调试 php 代码有个初步了解
● 对了，文末有一些截图，不要错过
采购食材
● 电脑一台
● docker 和 docker-compose
gdb 也好， PHP 也好，都打包成 docker 镜像啦，开袋即食，甚好。
备菜环节
1、使用 docker 拉取环境
# 拉取准备好的环境 git clone https://github.com/rovast/docker-examples.git # 进入项目 cd docker-examples/gdb-php-src/ # 启动，会经历一个漫长又不太漫长的等待，看你网速 docker-compose up -d
关于容器内的环境，大家可以看看 dockerfile
其实很简单，就是基于 gcc 官方镜像构建，然后增加了 vim gdb，并且下载了 php7.0.0 的源码，按照 debug 参数进行编译
显示如下
Creating network "gdb-php-src_default" with the default driver Creating gdb-php-src ... done
2、进入容器
docker exec -it gdb-php-src bash ### 显示下面的东西，表示你已经进入到容器内了 #### root@71a98d1bc1a6:/home#
我们看看容器内的环境（php 以及 gdb）
### 我们在容器内看看环境 root@71a98d1bc1a6:/home# ls php-7.0.0 start.md root@71a98d1bc1a6:/home# gdb -v GNU gdb (Debian 7.12-6) 7.12.0.20161007-git Copyright (C) 2016 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>. Find the GDB manual and other documentation resources online at: <http://www.gnu.org/software/gdb/documentation/>. For help, type "help". Type "apropos word" to search for commands related to "word". root@71a98d1bc1a6:/home# php -v PHP 7.0.0 (cli) (built: Apr 17 2019 13:33:30) ( NTS DEBUG ) Copyright (c) 1997-2015 The PHP Group Zend Engine v3.0.0, Copyright (c) 1998-2015 Zend Technologies root@71a98d1bc1a6:/home#
开火（请在容器内操作）
1、新建测试文件
root@71a98d1bc1a6:/home# vi test.php
输入以下内容
<?php strval(1234);
这个文件干的事情就比较简单了，就是把 -1234 [整形] 转换为 -1234 [字符串]
2、开始调试，进入 gdb
接下来车速较快，各位按步骤跟上
输入 gdb php，开始调试
root@71a98d1bc1a6:/home# gdb php GNU gdb (Debian 7.12-6) 7.12.0.20161007-git Copyright (C) 2016 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>. Find the GDB manual and other documentation resources online at: <http://www.gnu.org/software/gdb/documentation/>. For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from php...done. (gdb)
3、打一些断点（敲命令时可以用 tab 补全）
(gdb) b zend_long_to_str Breakpoint 1 at 0x810423: file /home/php-7.0.0/Zend/zend_operators.c, line 2743. (gdb) b zend_print_ulong_to_buf Breakpoint 2 at 0x5f387b: zend_print_ulong_to_buf. (13 locations) (gdb)
这里在关键函数 zend_long_to_str 和 zend_print_ulong_to_buf 打了断点。
b 在 gdb 中是 breakpoint 缩写，后面可以加函数名，或者当前文件的行号都是可以的
4、执行，查看断点值
(gdb) r test.php # 执行我们刚才的那个 PHP 文件 Starting program: /usr/local/bin/php test.php [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". warning: File "/usr/local/lib64/libstdc++.so.6.0.25-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load". To enable execution of this file add add-auto-load-safe-path /usr/local/lib64/libstdc++.so.6.0.25-gdb.py line to your configuration file "/root/.gdbinit". To completely disable this security protection add set auto-load safe-path / line to your configuration file "/root/.gdbinit". For more information about this security protection see the "Auto-loading safe path" section in the GDB manual. E.g., run from the shell: info "(gdb)Auto-loading safe path" Breakpoint 1, zend_long_to_str (num=-1234) at /home/php-7.0.0/Zend/zend_operators.c:2743 2743 char *res = zend_print_long_to_buf(buf + sizeof(buf) - 1, num); (gdb)
看的好像不明了嘛， ctrl + x 后再按 a 进入 gui 模式看看
┌──/home/php-7.0.0/Zend/zend_operators.c────────────────────────────────────────────────────────────────────────────────────────────────┐
│2731 ZEND_API void ZEND_FASTCALL zend_locale_sprintf_double(zval *op ZEND_FILE_LINE_DC) /* {{{ */ │
│2732 { │
│2733 zend_string *str; │
│2734 │
│2735 str = zend_strpprintf(0, "%.*G", (int) EG(precision), (double)Z_DVAL_P(op)); │
│2736 ZVAL_NEW_STR(op, str); │
│2737 } │
│2738 /* }}} */ │
│2739 │
│2740 ZEND_API zend_string* ZEND_FASTCALL zend_long_to_str(zend_long num) /* {{{ */ │
│2741 { │
│2742 char buf[MAX_LENGTH_OF_LONG + 1]; │
B+>│2743 char *res = zend_print_long_to_buf(buf + sizeof(buf) - 1, num); │
│2744 return zend_string_init(res, buf + sizeof(buf) - 1 - res, 0); │
│2745 } │
│2746 /* }}} */ │
│2747 │
│2748 ZEND_API zend_uchar ZEND_FASTCALL is_numeric_str_function(const zend_string *str, zend_long *lval, double *dval) /* {{{ */ { │
│2749 return is_numeric_string_ex(ZSTR_VAL(str), ZSTR_LEN(str), lval, dval, -1, NULL); │
│2750 } │
│2751 /* }}} */ │
│2752 │
│2753 ZEND_API zend_uchar ZEND_FASTCALL _is_numeric_string_ex(const char *str, size_t length, zend_long *lval, double *dval, int allo│
│2754 { │
│2755 const char *ptr; │
│2756 int digits = 0, dp_or_e = 0; │
│2757 double local_dval = 0.0; │
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
multi-thre Thread 0x7ffff7fe37 In: zend_long_to_str L2743 PC: 0x810423
(gdb)有点意思了，函数在 2743 行断住了，我们来看看 num 的值
(gdb) p num $1 = -1234 (gdb)
对嘛，这个就是我们要处理的值，我们全速运行到 zend_print_long_to_buf 里看看
(gdb) c Continuing.
显示如下
(gdb) c
┌──/home/php-7.0.0/Zend/zend_operators.h────────────────────────────────────────────────────────────────────────────────────────────────┐
│781 else \ │
│782 ZEND_TRY_BINARY_OP2_OBJECT_OPERATION(opcode) │
│783 │
│784 #define ZEND_TRY_UNARY_OBJECT_OPERATION(opcode) \ │
│785 if (UNEXPECTED(Z_TYPE_P(op1) == IS_OBJECT) \ │
│786 && UNEXPECTED(Z_OBJ_HANDLER_P(op1, do_operation)) \ │
│787 && EXPECTED(SUCCESS == Z_OBJ_HANDLER_P(op1, do_operation)(opcode, result, op1, NULL))) { \ │
│788 return SUCCESS; \ │
│789 } │
│790 │
│791 /* buf points to the END of the buffer */ │
│792 static zend_always_inline char *zend_print_ulong_to_buf(char *buf, zend_ulong num) { │
B+>│793 *buf = '\0'; │
│794 do { │
│795 *--buf = (char) (num % 10) + '0'; │
│796 num /= 10; │
│797 } while (num > 0); │
│798 return buf; │
│799 } │
│800 │
│801 /* buf points to the END of the buffer */ │
│802 static zend_always_inline char *zend_print_long_to_buf(char *buf, zend_long num) { │
│803 if (num < 0) { │
│804 char *result = zend_print_ulong_to_buf(buf, ~((zend_ulong) num) + 1); │
│805 *--result = '-'; │
│806 return result; │
│807 } else { │
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
multi-thre Thread 0x7ffff7fe37 In: zend_print_ulong_to_buf L793 PC: 0x8041fd接下来，我们来进行一些单步调试，看看内存里的值
Breakpoint 1, zend_long_to_str (num=-1234) at /home/php-7.0.0/Zend/zend_operators.c:2743 (gdb) c #-------------> 表示继续执行 Continuing. Breakpoint 2, zend_print_ulong_to_buf (buf=0x7fffffffafe4 "", num=1234) at /home/php-7.0.0/Zend/zend_operators.h:793 (gdb) b 798 #-------------> 798行打个断点 Breakpoint 6 at 0x5f38e2: /home/php-7.0.0/Zend/zend_operators.h:798. (13 locations) (gdb) c Continuing. Breakpoint 6, zend_print_ulong_to_buf (buf=0x7fffffffafe0 "1234", num=0) at /home/php-7.0.0/Zend/zend_operators.h:798 (gdb) p buf #-------------> 查看 buf 的值 $2 = 0x7fffffffafe0 "1234" (gdb) x/10c buf #-------------> 查看 buf 位置开始，连续 10 个以 char 为单位的内存值 0x7fffffffafe0: 49 '1' 50 '2' 51 '3' 52 '4' 0 '\000' 0 '\000' 0 '\000' 0 '\000' 0x7fffffffafe8: 0 '\000' 65 'A' (gdb)
我们看到，函数返回的 buf 是字符串类型的 '1234'
我们看看函数 zend_print_ulong_to_buf，其实就是从高位到低位，按个取模（除以 10，取整数部分），然后塞到 buf 缓冲区。
比较有意思的是，buf 初始化的时候指向的是缓冲区的末尾，所以填充的时候高位在最后，然后逐步往前填充低位。
最后结束的时候，buf 就是我们需要的字符串类容了
消化
其实，本文就是使用 gdb 调试了 PHP 代码，仅此而已。
更多的是给大家提供了一个直接上手玩玩的机会，你所需要的只是个 docker，然后动动手调试，很有意思。
动手试试吧，甚至，去看 C 源码吧！
附录
手摸手带你看 strval 函数 C 实现
![[docker+gdb] Debug PHP source code and see strval function C implementation](https://img.php.cn/upload/image/951/627/163/1570598362469082.png "1570598362469082.png")
![[docker+gdb] Debug PHP source code and see strval function C implementation](https://img.php.cn/upload/image/465/818/113/1570598372754507.png "1570598372754507.png")
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