fishhook 用于替换 iOS 程序中动态库的符号，常被用来 hook 系统中的 C 函数。
fishhook 的实现得益于 iOS 的动态链接机制，相关信息可以参考：iOS 系统的延迟绑定机制，而在此之前，你还需要了解 Mach-O 文件，详情可参考这篇文章：Mach-O 文件探索。
对于程序引用的动态库中的函数，链接器会将它的地址放在 __la_symbol_ptr 中，而对于动态库的全局数据，则是放在 __nl_symbol_ptr 中，所以整个 fishhook 的核心工作就是替换 __la_symbol_ptr 以及 __nl_symbol_ptr 的内容。
下面是一段官方示例代码，它告诉我们如何用正确的姿势来使用 fishhook：
#import <UIKit/UIKit.h>
#import "AppDelegate.h"
#import "fishhook.h"
static int (*orig_close)(int);
static int (*orig_open)(const char *, int, ...);
int my_close(int fd) {
printf("Calling real close(%d)\n", fd);
return orig_close(fd);
}
int my_open(const char *path, int oflag, ...) {
va_list ap = {0};
mode_t mode = 0;
if ((oflag & O_CREAT) != 0) {
// mode only applies to O_CREAT
va_start(ap, oflag);
mode = va_arg(ap, int);
va_end(ap);
printf("Calling real open('%s', %d, %d)\n", path, oflag, mode);
return orig_open(path, oflag, mode);
} else {
printf("Calling real open('%s', %d)\n", path, oflag);
return orig_open(path, oflag, mode);
}
}
int main(int argc, char * argv[]) {
@autoreleasepool {
rebind_symbols((struct rebinding[2]){{"close", my_close, (void *)&orig_close}, {"open", my_open, (void *)&orig_open}}, 2);
// Open our own binary and print out first 4 bytes (which is the same
// for all Mach-O binaries on a given architecture)
int fd = open(argv[0], O_RDONLY);
uint32_t magic_number = 0;
read(fd, &magic_number, 4);
printf("Mach-O Magic Number: %x \n", magic_number);
close(fd);
return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
}
}
上述代码的作用就是将系统的 open 函数以及 close 函数替换成我们自己的实现，整个过程由 rebind_symbols 函数完成。
int rebind_symbols(struct rebinding rebindings[], size_t rebindings_nel) {
int retval = prepend_rebindings(&_rebindings_head, rebindings, rebindings_nel);
if (retval < 0) {
return retval;
}
// If this was the first call, register callback for image additions (which is also invoked for
// existing images, otherwise, just run on existing images
if (!_rebindings_head->next) {
_dyld_register_func_for_add_image(_rebind_symbols_for_image);
} else {
uint32_t c = _dyld_image_count();
for (uint32_t i = 0; i < c; i++) {
_rebind_symbols_for_image(_dyld_get_image_header(i), _dyld_get_image_vmaddr_slide(i));
}
}
return retval;
}
rebind_symbols 函数的处理流程如下：
使用
prepend_rebindings函数来处理我们传入的绑定信息，fishhook 会将它们组织成一个链表，并用_rebindings_head指向链表的头部，接下来会判断用户是否是第一次进行符号替换。如果是第一次替换符号，则调用
_dyld_register_func_for_add_image注册回调函数_rebind_symbols_for_image，之后程序每次加载动态库时都会去调用_rebind_symbols_for_image。当注册成功时，对于那些已经加载的动态库也会触发回调，所以不用担心注册的时机。如果不是第一次替换符号，那么就遍历程序已经加载的动态库，对其调用
_rebind_symbols_for_image函数。
_rebind_symbols_for_image 函数如下，它实际上是调用 rebind_symbols_for_image ：
static void _rebind_symbols_for_image(const struct mach_header *header,
intptr_t slide) {
rebind_symbols_for_image(_rebindings_head, header, slide);
}
rebind_symbols_for_image 的代码：
static void rebind_symbols_for_image(struct rebindings_entry *rebindings,
const struct mach_header *header,
intptr_t slide) {
Dl_info info;
if (dladdr(header, &info) == 0) {
return;
}
segment_command_t *cur_seg_cmd;
segment_command_t *linkedit_segment = NULL;
struct symtab_command* symtab_cmd = NULL;
struct dysymtab_command* dysymtab_cmd = NULL;
uintptr_t cur = (uintptr_t)header + sizeof(mach_header_t);
for (uint i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
cur_seg_cmd = (segment_command_t *)cur;
if (cur_seg_cmd->cmd == LC_SEGMENT_ARCH_DEPENDENT) {
if (strcmp(cur_seg_cmd->segname, SEG_LINKEDIT) == 0) {
linkedit_segment = cur_seg_cmd;
}
} else if (cur_seg_cmd->cmd == LC_SYMTAB) {
symtab_cmd = (struct symtab_command*)cur_seg_cmd;
} else if (cur_seg_cmd->cmd == LC_DYSYMTAB) {
dysymtab_cmd = (struct dysymtab_command*)cur_seg_cmd;
}
}
if (!symtab_cmd || !dysymtab_cmd || !linkedit_segment ||
!dysymtab_cmd->nindirectsyms) {
return;
}
// Find base symbol/string table addresses
uintptr_t linkedit_base = (uintptr_t)slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;
nlist_t *symtab = (nlist_t *)(linkedit_base + symtab_cmd->symoff);
char *strtab = (char *)(linkedit_base + symtab_cmd->stroff);
// Get indirect symbol table (array of uint32_t indices into symbol table)
uint32_t *indirect_symtab = (uint32_t *)(linkedit_base + dysymtab_cmd->indirectsymoff);
cur = (uintptr_t)header + sizeof(mach_header_t);
for (uint i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
cur_seg_cmd = (segment_command_t *)cur;
if (cur_seg_cmd->cmd == LC_SEGMENT_ARCH_DEPENDENT) {
if (strcmp(cur_seg_cmd->segname, SEG_DATA) != 0 &&
strcmp(cur_seg_cmd->segname, SEG_DATA_CONST) != 0) {
continue;
}
for (uint j = 0; j < cur_seg_cmd->nsects; j++) {
section_t *sect =
(section_t *)(cur + sizeof(segment_command_t)) + j;
if ((sect->flags & SECTION_TYPE) == S_LAZY_SYMBOL_POINTERS) {
perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
}
if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
}
}
}
}
}
rebind_symbols_for_image 函数看上去有点长，但是如果你静下心来观察的话，发现它还是没有超过80行。。。。。。不过不要紧，我们慢慢往下看。
函数中经常出现的变量类型如 mach_header、segment_command、symtab_command 等都可以在系统的 loader.h 文件中找到相应的定义，所以这里不做过多的解释，想要了解更多的话请参考：Mach-O 文件探索。
rebind_symbols_for_image 首先会在程序的 load commands 中寻找符号表、动态符号表以及 LINKEDIT 段所对应的加载命令，它们包含程序动态链接过程的关键信息：
Dl_info info;
if (dladdr(header, &info) == 0) {
return;
}
segment_command_t *cur_seg_cmd;
segment_command_t *linkedit_segment = NULL;
struct symtab_command* symtab_cmd = NULL;
struct dysymtab_command* dysymtab_cmd = NULL;
uintptr_t cur = (uintptr_t)header + sizeof(mach_header_t);
for (uint i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
cur_seg_cmd = (segment_command_t *)cur;
if (cur_seg_cmd->cmd == LC_SEGMENT_ARCH_DEPENDENT) {
if (strcmp(cur_seg_cmd->segname, SEG_LINKEDIT) == 0) {
linkedit_segment = cur_seg_cmd;
}
} else if (cur_seg_cmd->cmd == LC_SYMTAB) {
symtab_cmd = (struct symtab_command*)cur_seg_cmd;
} else if (cur_seg_cmd->cmd == LC_DYSYMTAB) {
dysymtab_cmd = (struct dysymtab_command*)cur_seg_cmd;
}
}
if (!symtab_cmd || !dysymtab_cmd || !linkedit_segment ||
!dysymtab_cmd->nindirectsyms) {
return;
}
在确认上述信息齐全之后，会去计算它们的虚拟地址：
// Find base symbol/string table addresses
uintptr_t linkedit_base = (uintptr_t)slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;
nlist_t *symtab = (nlist_t *)(linkedit_base + symtab_cmd->symoff);
char *strtab = (char *)(linkedit_base + symtab_cmd->stroff);
// Get indirect symbol table (array of uint32_t indices into symbol table)
uint32_t *indirect_symtab = (uint32_t *)(linkedit_base + dysymtab_cmd->indirectsymoff);
值得一提的是这行代码：
uintptr_t linkedit_base = (uintptr_t)slide + linkedit_segment->vmaddr - linkedit_segment->fileoff;
因为地址空间加载随机化的缘故，系统在加载程序时，会在其原有的地址空间上进行偏移操作，而这个 slide 正是偏移的大小，所以 linkedit_base 代表的是程序被加载后的基地址。
接下来，rebind_symbols_for_image 会在 __DATA 段中寻找类型为 S_LAZY_SYMBOL_POINTERS 以及 S_NON_LAZY_SYMBOL_POINTERS 的节，二者分别包含我们要替换的 __la_symbol_ptr 以及 __nl_symbol_ptr，然后调用 perform_rebinding_with_section 函数：
cur = (uintptr_t)header + sizeof(mach_header_t);
for (uint i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
cur_seg_cmd = (segment_command_t *)cur;
if (cur_seg_cmd->cmd == LC_SEGMENT_ARCH_DEPENDENT) {
if (strcmp(cur_seg_cmd->segname, SEG_DATA) != 0 &&
strcmp(cur_seg_cmd->segname, SEG_DATA_CONST) != 0) {
continue;
}
for (uint j = 0; j < cur_seg_cmd->nsects; j++) {
section_t *sect =
(section_t *)(cur + sizeof(segment_command_t)) + j;
if ((sect->flags & SECTION_TYPE) == S_LAZY_SYMBOL_POINTERS) {
perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
}
if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
}
}
}
}
perform_rebinding_with_section 函数也是整个 fishhook 的核心代码，负责完成符号的替换：
static void perform_rebinding_with_section(struct rebindings_entry *rebindings,
section_t *section,
intptr_t slide,
nlist_t *symtab,
char *strtab,
uint32_t *indirect_symtab) {
uint32_t *indirect_symbol_indices = indirect_symtab + section->reserved1;
void **indirect_symbol_bindings = (void **)((uintptr_t)slide + section->addr);
for (uint i = 0; i < section->size / sizeof(void *); i++) {
uint32_t symtab_index = indirect_symbol_indices[i];
if (symtab_index == INDIRECT_SYMBOL_ABS || symtab_index == INDIRECT_SYMBOL_LOCAL ||
symtab_index == (INDIRECT_SYMBOL_LOCAL | INDIRECT_SYMBOL_ABS)) {
continue;
}
uint32_t strtab_offset = symtab[symtab_index].n_un.n_strx;
char *symbol_name = strtab + strtab_offset;
if (strnlen(symbol_name, 2) < 2) {
continue;
}
struct rebindings_entry *cur = rebindings;
while (cur) {
for (uint j = 0; j < cur->rebindings_nel; j++) {
if (strcmp(&symbol_name[1], cur->rebindings[j].name) == 0) {
if (cur->rebindings[j].replaced != NULL &&
indirect_symbol_bindings[i] != cur->rebindings[j].replacement) {
*(cur->rebindings[j].replaced) = indirect_symbol_bindings[i];
}
indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
goto symbol_loop;
}
}
cur = cur->next;
}
symbol_loop:;
}
}
这里最关键的就是下面两行代码的解读：
uint32_t *indirect_symbol_indices = indirect_symtab + section->reserved1;
void **indirect_symbol_bindings = (void **)((uintptr_t)slide + section->addr);
indirect_symtab 是动态符号表的地址，表中包含动态符号在符号表中的索引。那么 section->reserved1 又是代表什么呢？这里的 section 实际上是指 __DATA 段中包含 __la_symbol_ptr 以及 __nl_symbol_ptr 的 section，它们会在 reserved1 字段中记录自身所包含的动态符号在 indirect_symtab 的起始索引，因此通过 indirect_symbol_indices 便可以得到 section 所包含的动态符号在符号表中的索引信息。
indirect_symbol_bindings 则是代表程序偏移后的相关 section 的虚拟地址，fishhook 会在其中寻找指向目标动态符号的指针，然后将其指向我们自己的符号。
uint32_t symtab_index = indirect_symbol_indices[i];
if (symtab_index == INDIRECT_SYMBOL_ABS || symtab_index == INDIRECT_SYMBOL_LOCAL ||
symtab_index == (INDIRECT_SYMBOL_LOCAL | INDIRECT_SYMBOL_ABS)) {
continue;
}
uint32_t strtab_offset = symtab[symtab_index].n_un.n_strx;
char *symbol_name = strtab + strtab_offset;
if (strnlen(symbol_name, 2) < 2) {
continue;
}
symtab_index 表示动态符号在符号表中的索引，据此我们可以得到动态符号的名字，接下来就是遍历用户传来的绑定信息，若是和动态符号相匹配，则进行替换：
while (cur) {
for (uint j = 0; j < cur->rebindings_nel; j++) {
if (strcmp(&symbol_name[1], cur->rebindings[j].name) == 0) {
if (cur->rebindings[j].replaced != NULL &&
indirect_symbol_bindings[i] != cur->rebindings[j].replacement) {
*(cur->rebindings[j].replaced) = indirect_symbol_bindings[i];
}
indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
goto symbol_loop;
}
}
cur = cur->next;
}
symbol_loop:;
这里引用下 fishhook 官方的流程图：
最后的感慨：fishhook 真是有创意！
