Mach-O 探究

/*
 * The 32-bit mach header appears at the very beginning of the object file for
 * 32-bit architectures.
 */
struct mach_header {
	uint32_t	magic;		/* mach magic number identifier */
	cpu_type_t	cputype;	/* cpu specifier */
	cpu_subtype_t	cpusubtype;	/* machine specifier */
	uint32_t	filetype;	/* type of file */
	uint32_t	ncmds;		/* number of load commands */
	uint32_t	sizeofcmds;	/* the size of all the load commands */
	uint32_t	flags;		/* flags */
};

/*
 * The 64-bit mach header appears at the very beginning of object files for
 * 64-bit architectures.
 */
struct mach_header_64 {
	uint32_t	magic;		/* mach magic number identifier */
	cpu_type_t	cputype;	/* cpu specifier */
	cpu_subtype_t	cpusubtype;	/* machine specifier */
	uint32_t	filetype;	/* type of file */
	uint32_t	ncmds;		/* number of load commands */
	uint32_t	sizeofcmds;	/* the size of all the load commands */
	uint32_t	flags;		/* flags */
	uint32_t	reserved;	/* reserved */
};

#define	MH_MAGIC	0xfeedface	/* the mach magic number */ 32位
#define MH_MAGIC_64 0xfeedfacf /* the 64-bit mach magic number */ 64位

#define	MH_NOUNDEFS	0x1		/* Target 文件中没有带未定义的符号，常为静态二进制文件 */
#define MH_SPLIT_SEGS	0x20  /* Target 文件中的只读 Segment 和可读写 Segment 分开  */
#define MH_TWOLEVEL	0x80		/* 该 Image 使用二级命名空间(two name space binding)绑定方案 */
#define MH_FORCE_FLAT	0x100 /* 使用扁平命名空间(flat name space binding)绑定（与 MH_TWOLEVEL 互斥） */
#define MH_WEAK_DEFINES	0x8000 /* 二进制文件使用了弱符号 */
#define MH_BINDS_TO_WEAK 0x10000 /* 二进制文件链接了弱符号 */
#define MH_ALLOW_STACK_EXECUTION 0x20000/* 允许 Stack 可执行 */
#define	MH_PIE 0x200000  /* 加载程序在随机的地址空间，只在 MH_EXECUTE中使用 */
#define MH_NO_HEAP_EXECUTION 0x1000000 /* 将 Heap 标记为不可执行，可防止 heap spray 攻击 */

/*
 * The load commands directly follow the mach_header.  The total size of all
 * of the commands is given by the sizeofcmds field in the mach_header.  All
 * load commands must have as their first two fields cmd and cmdsize.  
 * The cmd
 * field is filled in with a constant for that command type.  
 * Each command type
 * has a structure specifically for it.  
 * The cmdsize field is the size in bytes
 * of the particular load command structure plus anything that follows it that
 * is a part of the load command (i.e. section structures, strings, etc.). 
 *  To
 * advance to the next load command the cmdsize can be added to the offset or
 * pointer of the current load command.  
 * The cmdsize for 32-bit architectures
 * MUST be a multiple of 4 bytes and for 64-bit architectures MUST be a multiple
 * of 8 bytes (these are forever the maximum alignment of any load commands).
 * The padded bytes must be zero.  All tables in the object file must also
 * follow these rules so the file can be memory mapped.  Otherwise the pointers
 * to these tables will not work well or at all on some machines.  With all
 * padding zeroed like objects will compare byte for byte.
 */
struct load_command {
	uint32_t cmd;		/* type of load command */
	uint32_t cmdsize;	/* total size of command in bytes */
};

/*
 * The segment load command indicates that a part of this file is to be
 * mapped into the task's address space.  The size of this segment in memory,
 * vmsize, maybe equal to or larger than the amount to map from this file,
 * filesize.  The file is mapped starting at fileoff to the beginning of
 * the segment in memory, vmaddr.  The rest of the memory of the segment,
 * if any, is allocated zero fill on demand.  The segment's maximum virtual
 * memory protection and initial virtual memory protection are specified
 * by the maxprot and initprot fields.  If the segment has sections then the
 * section structures directly follow the segment command and their size is
 * reflected in cmdsize.
 */
struct segment_command { /* for 32-bit architectures */
	uint32_t	cmd;		/* LC_SEGMENT */
	uint32_t	cmdsize;	/* includes sizeof section structs */
	char		segname[16];	/* segment name */
	uint32_t	vmaddr;		/* memory address of this segment */
	uint32_t	vmsize;		/* memory size of this segment */
	uint32_t	fileoff;	/* file offset of this segment */
	uint32_t	filesize;	/* amount to map from the file */
	vm_prot_t	maxprot;	/* maximum VM protection */
	vm_prot_t	initprot;	/* initial VM protection */
	uint32_t	nsects;		/* number of sections in segment */
	uint32_t	flags;		/* flags */
};

/*
 * The 64-bit segment load command indicates that a part of this file is to be
 * mapped into a 64-bit task's address space.  If the 64-bit segment has
 * sections then section_64 structures directly follow the 64-bit segment
 * command and their size is reflected in cmdsize.
 */
struct segment_command_64 { /* for 64-bit architectures */
	uint32_t	cmd;		/* LC_SEGMENT_64 */
	uint32_t	cmdsize;	/* includes sizeof section_64 structs */
	char		segname[16];	/* segment name */
	uint64_t	vmaddr;		/* memory address of this segment */
	uint64_t	vmsize;		/* memory size of this segment */
	uint64_t	fileoff;	/* file offset of this segment */
	uint64_t	filesize;	/* amount to map from the file */
	vm_prot_t	maxprot;	/* maximum VM protection */
	vm_prot_t	initprot;	/* initial VM protection */
	uint32_t	nsects;		/* number of sections in segment */
	uint32_t	flags;		/* flags */
};

typedef struct objc_image_info {
    uint32_t version; // currently 0
    uint32_t flags;

#if __cplusplus >= 201103L
  private:
  // 位移枚举 
    enum : uint32_t {
        IsReplacement       = 1<<0,  // used for Fix&Continue, now ignored
        SupportsGC          = 1<<1,  // 是否支持垃圾回收
        RequiresGC          = 1<<2,  // 镜像是否需要回收
        OptimizedByDyld     = 1<<3,  // image is from an optimized shared cache
        CorrectedSynthesize = 1<<4,  // used for an old workaround, now ignored
        IsSimulated         = 1<<5,  // image compiled for a simulator platform
        HasCategoryClassProperties  = 1<<6,  // class properties in category_t

        SwiftVersionMaskShift = 8,
        SwiftVersionMask    = 0xff << SwiftVersionMaskShift  // Swift ABI version

    };
   public:
    enum : uint32_t {
        SwiftVersion1   = 1,
        SwiftVersion1_2 = 2,
        SwiftVersion2   = 3,
        SwiftVersion3   = 4
    };

  public:
    bool isReplacement()   const { return flags & IsReplacement; }
    bool supportsGC()      const { return flags & SupportsGC; }
    bool requiresGC()      const { return flags & RequiresGC; }
    bool optimizedByDyld() const { return flags & OptimizedByDyld; }
    bool hasCategoryClassProperties() const { return flags & HasCategoryClassProperties; }
    bool containsSwift()   const { return (flags & SwiftVersionMask) != 0; }
    uint32_t swiftVersion() const { return (flags & SwiftVersionMask) >> SwiftVersionMaskShift; }
#endif
} objc_image_info;

bool ImageLoaderMachO::doInitialization(const LinkContext& context)
{
	CRSetCrashLogMessage2(this->getPath());

	// mach-o has -init and static initializers
	doImageInit(context);
	doModInitFunctions(context);
	
	CRSetCrashLogMessage2(NULL);
	
	return (fHasDashInit || fHasInitializers);
}

void ImageLoaderMachO::doImageInit(const LinkContext& context)
{
	if ( fHasDashInit ) {
		// mach-o文件中指令的个数
		const uint32_t cmd_count = ((macho_header*)fMachOData)->ncmds;
		// 遍历指令
		for (uint32_t i = 0; i < cmd_count; ++i) {
			switch (cmd->cmd) {
				case LC_ROUTINES_COMMAND:
					// 获取macho_routines_command的init_address
					Initializer func = (Initializer)(((struct macho_routines_command*)cmd)->init_address + fSlide);
					// 执行-init方法
					func(context.argc, context.argv, context.envp, context.apple, &context.programVars);
					break;
			}
			// 计算下一个指令((char*)cmd)+cmd->cmdsize
			cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
		}
	}
}

void ImageLoaderMachO::doModInitFunctions(const LinkContext& context)
{
	if ( fHasInitializers ) {
	   // mach-o文件中指令的个数
		const uint32_t cmd_count = ((macho_header*)fMachOData)->ncmds;
		// 遍历所有的指令
		for (uint32_t i = 0; i < cmd_count; ++i) {
		   // 如果指令是Mach-o中的LC_SEGMENT_COMMAND
			if ( cmd->cmd == LC_SEGMENT_COMMAND ) {
			     // 从sectionsStart到sectionsEnd
					for (const struct macho_section* sect=sectionsStart; sect < sectionsEnd; ++sect) {
					const uint8_t type = sect->flags & SECTION_TYPE;
					if ( type == S_MOD_INIT_FUNC_POINTERS ) {
						
						for (size_t i=0; i < count; ++i) {
							if ( context.verboseInit )
								dyld::log("dyld: calling initializer function %p in %s\n", func, this->getPath());
							// 执行initializer方法
							func(context.argc, context.argv, context.envp, context.apple, &context.programVars);
						}
					}
				}
			}
			// 根据指令的地址+指令大小获取到下一个指令
			cmd = (const struct load_command*)(((char*)cmd)+cmd->cmdsize);
		}
	}
}