mach-o格式分析

拼搏现实的明天。 2022-07-18 04:09 227阅读 0赞

文章出处：[http://turingh.github.io/2016/03/07/mach-o%E6%96%87%E4%BB%B6%E6%A0%BC%E5%BC%8F%E5%88%86%E6%9E%90/][http_turingh.github.io_2016_03_07_mach-o_E6_96_87_E4_BB_B6_E6_A0_BC_E5_BC_8F_E5_88_86_E6_9E_90]

`mach-o`格式是OS X系统上的可执行文件格式，类似于windows的`PE`与linux的`ELF`，如果不彻底搞清楚`mach-o`的格式与相关知识，去做其他研究，无异于建造空中阁楼。

每个Mach-O文件斗包含一个Mach-O头，然后是载入命令(Load Commands),最后是数据块(Data)。

接下来就对整个Mach-O的格式做出详细的分析。

# 0x01 Mach-O格式简单介绍 #

Mach-O文件的格式如下图所示：

[![Mach-O][]][Mach-O]

又如下几个部分组成：

*  Header：保存了Mach-O的一些基本信息，包括了平台、文件类型、LoadCommands的个数等等。
 *  LoadCommands：这一段紧跟Header，加载Mach-O文件时会使用这里的数据来确定内存的分布。
 *  Data：每一个segment的具体数据都保存在这里，这里包含了具体的代码、数据等等。

# 0x02 Headers #

## 2.1 数据结构 ##

Headers的定义可以在开源的内核代码中找到。

根据`mach_header`与`mach_header_64`的定义，很明显可以看出，Headers的主要作用就是帮助系统迅速的定位Mach-O文件的运行环境，文件类型。

[![Header][]][Header]

## 2.2 实例 ##

使用工具分析一个mach-o文件来具体的看一下Mach-O Headers。

通过otool可以得到Mach header的具体的情况，但是可读性略微有一点差。

还有一个工具是MachOview可以看的更清楚一点。

[![Header][Header 1]][Header 1]

*  MagicNumber的值为0xFEEDFACF所以该文件是一个64位平台上的文件
 *  CPU Type和CPU SubType也很容易理解，运行在X86\_64的CPU平台上
 *  File Type标示了该文件是一个可执行文件，后面具体分析
 *  Flags标示了这个MachO文件的四个特性，后面具体分析

## 2.3 具体参数 ##

### 2.3.1 FileType ###

因为Mach-O文件不仅仅用来实现可执行文件，同时还用来实现了其他内容

*  内核扩展
 *  库文件
 *  CoreDump
 *  …

他的源码定义如下：

解释一下一些常用到的文件类型。

<table style="margin:20px 0px; padding:0px; border:1px solid rgb(222,222,222); outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; border-collapse:collapse; border-spacing:0px; width:1007px; color:rgb(255,255,255)"> 
 <thead style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px; background:rgb(45,45,45)"> 
 <th style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-weight:normal; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; text-align:left"> File Type</th> 
 <th style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-weight:normal; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; text-align:left"> 用处</th> 
 <th style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-weight:normal; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; text-align:left"> 例子</th> 
 </tr> 
 </thead> 
 <tbody style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> MH_OBJECT</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 编译过程中产生的*.obj文件</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> gcc -c xxx.c 生成xxx.o文件</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> MH_EXECUTABLE</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 可执行二进制文件</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> /usr/bin/git</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> MH_CORE</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> CoreDump</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 崩溃时的Dump文件</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> MH_DYLIB</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 动态库</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> /usr/lib/里面的那些库文件</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> MH_DYLINKER</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 连接器linker</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> /usr/lib/dyld文件</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> MH_KEXT_BUNDLE</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 内核扩展文件</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 自己开发的简单内核模块</td> 
 </tr> 
 </tbody> 
</table>

### 2.3.2 flags ###

Mach-O headers还包含了一些很重要的dyld的加载参数。代码中的定义如下：

同样简单的介绍几个比较重要的。

## 2.4 Headers小结 ##

[![Headers][]][Headers]

# 0x03 Load Commands #

这是load\_command的数据结构

Load Commands 直接就跟在Header后面，所有command占用内存的总和在Mach-O Header里面已经给出了。在加载过Header之后就是通过解析LoadCommand来加载接下来的数据了。我简单的看了一下内核中是如何解析macho数据的，抛开内核的实现细节，逻辑其实也十分简单。

<table style="margin:20px 0px; padding:0px; border:none; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; border-collapse:collapse; border-spacing:0px; width:auto; color:rgb(255,255,255)"> 
 <tbody style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
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 <td style="margin:0px; padding:0px; border:none; outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px 20px 0px 0px; border:none; outline:0px; font-weight:inherit; font-style:inherit; font-family:Menlo,Consolas,&quot;Source Code Pro&quot;,Inconsolata,Monaco,monospace; font-size:10px; vertical-align:baseline; line-height:22.4px; overflow:auto; color:rgb(153,153,153); text-align:right; background:rgb(0,43,54)">1
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</pre> </td> 
 <td style="margin:0px; padding:0px; border:none; outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> <pre style="margin-top:0px; margin-bottom:0px; padding:0px; border:none; outline:0px; font-weight:inherit; font-style:inherit; font-family:Menlo,Consolas,&quot;Source Code Pro&quot;,Inconsolata,Monaco,monospace; font-size:10px; vertical-align:baseline; line-height:22.4px; overflow:auto; background:rgb(0,43,54)">static
load_return_t
parse_machfile(
	struct vnode 		*vp, 
	vm_map_t		map,
	thread_t		thread,
	struct mach_header	*header,
	off_t			file_offset,
	off_t			macho_size,
	int			depth,
	int64_t			aslr_offset,
	int64_t			dyld_aslr_offset,
	load_result_t		*result
)
{
 
	[...] //此处省略大量初始化与检测

		/*
		 * Loop through each of the load_commands indicated by the
		 * Mach-O header; if an absurd value is provided, we just
		 * run off the end of the reserved section by incrementing
		 * the offset too far, so we are implicitly fail-safe.
		 */
		offset = mach_header_sz;
		ncmds = header-&gt;ncmds;

		while (ncmds--) {
 
			/*
			 *	Get a pointer to the command.
			 */
			lcp = (struct load_command *)(addr + offset);
			//lcp设为当前要解析的cmd的地址
			oldoffset = offset;
			//oldoffset是从macho文件内存开始的地方偏移到当前command的偏移量
			offset += lcp-&gt;cmdsize;
			//重新计算offset，再加上当前command的长度，offset的值为文件内存起始地址到下一个command的偏移量
			/*
			 * Perform prevalidation of the struct load_command
			 * before we attempt to use its contents. Invalid
			 * values are ones which result in an overflow, or
			 * which can not possibly be valid commands, or which
			 * straddle or exist past the reserved section at the
			 * start of the image.
			 */
			if (oldoffset &gt; offset ||
			 lcp-&gt;cmdsize &lt; sizeof(struct load_command) ||
			 offset &gt; header-&gt;sizeofcmds + mach_header_sz) {
 
				ret = LOAD_BADMACHO;
				break;
			}
			//做了一个检测，与如何加载进入内存无关

			/*
			 * Act on struct load_command's for which kernel
			 * intervention is required.
			 */
			switch(lcp-&gt;cmd) {
 
			case LC_SEGMENT:
				[...]
				ret = load_segment(lcp,
				 header-&gt;filetype,
				 control,
				 file_offset,
				 macho_size,
				 vp,
				 map,
				 slide,
				 result);
				break;
			case LC_SEGMENT_64:
				[...]
				ret = load_segment(lcp,
				 header-&gt;filetype,
				 control,
				 file_offset,
				 macho_size,
				 vp,
				 map,
				 slide,
				 result);
				break;
			case LC_UNIXTHREAD:
				if (pass != 1)
					break;
				ret = load_unixthread(
						 (struct thread_command *) lcp,
						 thread,
						 slide,
						 result);
				break;
			case LC_MAIN:
				if (pass != 1)
					break;
				if (depth != 1)
					break;
				ret = load_main(
						 (struct entry_point_command *) lcp,
						 thread,
						 slide,
						 result);
				break;
			case LC_LOAD_DYLINKER:
				if (pass != 3)
					break;
				if ((depth == 1) &amp;&amp; (dlp == 0)) {
 
					dlp = (struct dylinker_command *)lcp;
					dlarchbits = (header-&gt;cputype &amp; CPU_ARCH_MASK);
				} else {
 
					ret = LOAD_FAILURE;
				}
				break;
			case LC_UUID:
				if (pass == 1 &amp;&amp; depth == 1) {
 
					ret = load_uuid((struct uuid_command *) lcp,
							(char *)addr + mach_header_sz + header-&gt;sizeofcmds,
							result);
				}
				break;
			case LC_CODE_SIGNATURE:
				[...]
				ret = load_code_signature(
					(struct linkedit_data_command *) lcp,
					vp,
					file_offset,
					macho_size,
					header-&gt;cputype,
					result);
				[...]
				break;
#if CONFIG_CODE_DECRYPTION
			case LC_ENCRYPTION_INFO:
			case LC_ENCRYPTION_INFO_64:
				if (pass != 3)
					break;
				ret = set_code_unprotect(
					(struct encryption_info_command *) lcp,
					addr, map, slide, vp, file_offset,
					header-&gt;cputype, header-&gt;cpusubtype);
				if (ret != LOAD_SUCCESS) {
 
					printf("proc %d: set_code_unprotect() error %d "
					 "for file \"%s\"\n",
					 p-&gt;p_pid, ret, vp-&gt;v_name);
					/* 
					 * Don't let the app run if it's 
					 * encrypted but we failed to set up the
					 * decrypter. If the keys are missing it will
					 * return LOAD_DECRYPTFAIL.
					 */
					 if (ret == LOAD_DECRYPTFAIL) {
 
						/* failed to load due to missing FP keys */
						proc_lock(p);
						p-&gt;p_lflag |= P_LTERM_DECRYPTFAIL;
						proc_unlock(p);
					 }
					 psignal(p, SIGKILL);
				}
				break;
#endif
			default:
				/* Other commands are ignored by the kernel */
				ret = LOAD_SUCCESS;
				break;
			}
			if (ret != LOAD_SUCCESS)
				break;
		}
		if (ret != LOAD_SUCCESS)
			break;
	}

	[...] //此处略去加载之后的处理代码
}
</pre> </td> 
 </tr> 
 </tbody> 
</table>

### 3.1cmdsize字段 ###

这里主要看while循环刚刚进入的时候几行代码,来理解是如何通过load\_command的cmd字段来解析Macho文件的数据。

### 3.2 cmd字段 ###

从这一段代码可以看出，根据cmd字段的类型不同，使用了不同的函数来加载。简单的列出一张表看一看在内核代码中不同的command类型都有哪些作用。

<table style="margin:20px 0px; padding:0px; border:1px solid rgb(222,222,222); outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; border-collapse:collapse; border-spacing:0px; width:1007px; color:rgb(255,255,255)"> 
 <thead style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px; background:rgb(45,45,45)"> 
 <th style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-weight:normal; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; text-align:left"> Command类型</th> 
 <th style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-weight:normal; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; text-align:left"> 处理函数</th> 
 <th style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-weight:normal; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; text-align:left"> 用途</th> 
 </tr> 
 </thead> 
 <tbody style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline"> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> LC_SEGMENT；LC_SEGMENT_64</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> load_segment</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 将segment中的数据加载并映射到进程的内存空间去</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> LC_LOAD_DYLINKER</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> load_dylinker</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 调用/usr/lib/dyld程序</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> LC_UUID</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> load_uuid</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 加载128-bit的唯一ID</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> LC_THREAD</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> load_thread</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 开启一个MACH线程，但是不分配栈空间。</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> LC_UNIXTHREAD</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> load_unixthread</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 开启一个UNIX线程</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> LC_CODE_SIGNATURE</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> load_code_signature</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 进行数字签名</td> 
 </tr> 
 <tr style="margin:0px; padding:0px; border:0px; outline:0px; font-weight:inherit; font-style:inherit; font-family:inherit; vertical-align:baseline; height:35px"> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> LC_ENCRYPTION_INFO</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> set_code_unprotect</td> 
 <td style="margin:0px; padding:0px 10px; border:1px solid rgb(2,2,2); outline:0px; font-style:inherit; font-family:inherit; font-size:16px; vertical-align:middle; height:35px"> 加密二进制文件</td> 
 </tr> 
 </tbody> 
</table>

# 0x04 Segment&Section #

加载数据时，主要加载的就是LC\_SEGMET活着LC\_SEGMENT\_64。其他的Segment的用途在上一节已经简单的介绍了，这里不做深究。

LCSEGMENT以及LC\_SEGMENT\_64的数据结构是这样的。

可以看出，这里大部分的数据是用来帮助内核将Segment映射到虚拟内存的。主要要关注的是`nsects`

字段，标示了Segment中有多少secetion。section是具体有用的数据存放的地方。

Section的数据结构如下：

除了同样有帮助内存映射的变量外，在了解Mach-O格式的时候，只需要知道不同的Section有着不同的作用就可以了。

因为section类型已经是最小的分类了，还有更多复杂section段就不一一例举了，遇到没见过的section类型可以自行查找Apple文档。

# 0x05 小结 #

通过对Mach-O格式的仔细分析，可以更好的理解Mach-O文件的加载过程，为研究dyld或者其他OS X系统下的模块打好基础。

# 参考 #

1.mach-o文件加载的全过程(1)

[http://dongaxis.github.io/2015/01/01/mac-o%E6%96%87%E4%BB%B6%E5%8A%A0%E8%BD%BD%E7%9A%84%E5%85%A8%E8%BF%87%E7%A8%8B-1/][http_dongaxis.github.io_2015_01_01_mac-o_E6_96_87_E4_BB_B6_E5_8A_A0_E8_BD_BD_E7_9A_84_E5_85_A8_E8_BF_87_E7_A8_8B-1]

2.Mach-O 可执行文件

[http://objccn.io/issue-6-3/][http_objccn.io_issue-6-3]

3.iPhone Mach-O文件格式与代码签名

[http://zhiwei.li/text/2012/02/15/iphone-mach-o%E6%96%87%E4%BB%B6%E6%A0%BC%E5%BC%8F%E4%B8%8E%E4%BB%A3%E7%A0%81%E7%AD%BE%E5%90%8D/][http_zhiwei.li_text_2012_02_15_iphone-mach-o_E6_96_87_E4_BB_B6_E6_A0_BC_E5_BC_8F_E4_B8_8E_E4_BB_A3_E7_A0_81_E7_AD_BE_E5_90_8D]

4.Dynamic Linking of Imported Functions in Mach-O

[http://www.codeproject.com/Articles/187181/Dynamic-Linking-of-Imported-Functions-in-Mach-O][http_www.codeproject.com_Articles_187181_Dynamic-Linking-of-Imported-Functions-in-Mach-O]

5.otool详解Mach-o文件头部

[http://www.mc2lab.com/?p=68][http_www.mc2lab.com_p_68]

# PS: #

更多文章可以在我的学习分享博客[http://BLOGIMAGE/][http_BLOGIMAGE]

希望可以多多交流，不足之处还希望大家可以给与指正：）

**本文标题:**[mach-o格式分析][http_turingh.github.io_2016_03_07_mach-o_E6_96_87_E4_BB_B6_E6_A0_BC_E5_BC_8F_E5_88_86_E6_9E_90]

**文章作者:**[mrh][]

**发布时间:**2016年03月07日 - 11时18分

**最后更新:**2016年03月23日 - 23时37分

**原始链接:**[http://turingh.github.io/2016/03/07/mach-o文件格式分析/][http_turingh.github.io_2016_03_07_mach-o_E6_96_87_E4_BB_B6_E6_A0_BC_E5_BC_8F_E5_88_86_E6_9E_90]

**许可协议:** ["署名-非商用-相同方式共享 3.0"][-_-_ 3.0] 转载请保留原文链接及作者。

[http_turingh.github.io_2016_03_07_mach-o_E6_96_87_E4_BB_B6_E6_A0_BC_E5_BC_8F_E5_88_86_E6_9E_90]: http://turingh.github.io/2016/03/07/mach-o%E6%96%87%E4%BB%B6%E6%A0%BC%E5%BC%8F%E5%88%86%E6%9E%90/
[Mach-O]: /images/20220717/a42fa3b489704b73a5bec7a2b778ac4a.png
[Header]: /images/20220717/8954fc5071274180a68bb062d65c8d4c.png
[Header 1]: /images/20220717/8357767d40f3402ca5d9b48c60ae46b5.png
[Headers]: /images/20220717/e839b57dc4ea4e2cafbbe2b848b217b5.png
[http_dongaxis.github.io_2015_01_01_mac-o_E6_96_87_E4_BB_B6_E5_8A_A0_E8_BD_BD_E7_9A_84_E5_85_A8_E8_BF_87_E7_A8_8B-1]: http://dongaxis.github.io/2015/01/01/mac-o%E6%96%87%E4%BB%B6%E5%8A%A0%E8%BD%BD%E7%9A%84%E5%85%A8%E8%BF%87%E7%A8%8B-1/
[http_objccn.io_issue-6-3]: http://objccn.io/issue-6-3/
[http_zhiwei.li_text_2012_02_15_iphone-mach-o_E6_96_87_E4_BB_B6_E6_A0_BC_E5_BC_8F_E4_B8_8E_E4_BB_A3_E7_A0_81_E7_AD_BE_E5_90_8D]: http://zhiwei.li/text/2012/02/15/iphone-mach-o%E6%96%87%E4%BB%B6%E6%A0%BC%E5%BC%8F%E4%B8%8E%E4%BB%A3%E7%A0%81%E7%AD%BE%E5%90%8D/
[http_www.codeproject.com_Articles_187181_Dynamic-Linking-of-Imported-Functions-in-Mach-O]: http://www.codeproject.com/Articles/187181/Dynamic-Linking-of-Imported-Functions-in-Mach-O
[http_www.mc2lab.com_p_68]: http://www.mc2lab.com/?p=68
[http_BLOGIMAGE]: http://blogimage/
[mrh]: http://turingh.github.io/
[-_-_ 3.0]: http://creativecommons.org/licenses/by-nc-sa/3.0/cn/