iOS RE 4 beginners 3 - fishhook

关于

Fishhook是Facebook提供的利用MachO文件惰性加载原理,通过修改懒加载和非懒加载两个表的指针达到C函数HOOK的目的一个轻量级的hook库。理解这个工具和熟悉流程也是可以帮助更好的理解MachO文件格式 :)

原理图如下:

fishhook

源码阅读

核心其实就是rebind_symbols 这个接口,另一个 rebind_symbols_image 是指定macho中的symbol进行rebind,所以从 rebind_symbols函数看起就行了。

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FISHHOOK_VISIBILITY
int rebind_symbols(struct rebinding rebindings[], size_t rebindings_nel);

简单看下关键的调用路径:

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rebind_symbols(struct rebinding rebindings[], size_t rebindings_nel);
_rebind_symbols_for_image(_dyld_get_image_header(i), _dyld_get_image_vmaddr_slide(i));
rebind_symbols_for_image(_rebindings_head, header, slide);
perform_rebinding_with_section(...)

_rebindings_head 指向一个需要重绑定的符号的单项链表:

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struct rebinding {
const char *name;
void *replacement;
void **replaced;
};

struct rebindings_entry {
struct rebinding *rebindings;
size_t rebindings_nel;
struct rebindings_entry *next;
};

static struct rebindings_entry *_rebindings_head;
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); // now, cur points to LOAD_CMDs
// iter LOAD CMDs
for (uint i = 0; i < header->ncmds; i++, cur += cur_seg_cmd->cmdsize) {
cur_seg_cmd = (segment_command_t *)cur;
// find LINK_EDIT seg
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) {
// find SYMTAB CMD
symtab_cmd = (struct symtab_command*)cur_seg_cmd;
} else if (cur_seg_cmd->cmd == LC_DYSYMTAB) {
// find DYSYM CMD
dysymtab_cmd = (struct dysymtab_command*)cur_seg_cmd;
}
}

if (!symtab_cmd || !dysymtab_cmd || !linkedit_segment ||
!dysymtab_cmd->nindirectsyms) {
return;
}

...

// 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);
}
}
}
}
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) {
// if _DATA,CONST
const bool isDataConst = strcmp(section->segname, SEG_DATA_CONST) == 0;

//__la_symbol_ptr的reserved1字段标识了section描述的符号在符号表中开始的index
//动态符号表中第一个需要解析的符号 开始地址
uint32_t *indirect_symbol_indices = indirect_symtab + section->reserved1;
// section __la_symbol_ptr
void **indirect_symbol_bindings = (void **)((uintptr_t)slide + section->addr);
vm_prot_t oldProtection = VM_PROT_READ;
// chang memory protection to write && back old memery protection
if (isDataConst) {
oldProtection = get_protection(rebindings);
mprotect(indirect_symbol_bindings, section->size, PROT_READ | PROT_WRITE);
}

// Traverse section -> symtab
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;
}

// nlist_t
uint32_t strtab_offset = symtab[symtab_index].n_un.n_strx;
char *symbol_name = strtab + strtab_offset;
bool symbol_name_longer_than_1 = symbol_name[0] && symbol_name[1];
struct rebindings_entry *cur = rebindings;
while (cur) {
for (uint j = 0; j < cur->rebindings_nel; j++) {
// yes, it's target symbol to rebind!
if (symbol_name_longer_than_1 &&
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]; // backup old func
}
// do rebind, hook!
**indirect_symbol_bindings[i] = cur->rebindings[j].replacement;**
goto symbol_loop;
}
}
cur = cur->next;
}
symbol_loop:;
}
// restore protection
if (isDataConst) {
int protection = 0;
if (oldProtection & VM_PROT_READ) {
protection |= PROT_READ;
}
if (oldProtection & VM_PROT_WRITE) {
protection |= PROT_WRITE;
}
if (oldProtection & VM_PROT_EXECUTE) {
protection |= PROT_EXEC;
}
mprotect(indirect_symbol_bindings, section->size, protection);
}
}

调试

直接拿官方的demo编译出来调试分析流程:

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~/study/ios_re_link/fishhook  cat main.c
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdarg.h>
#include "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[])
{
getchar();
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 0;

}%
~/study/ios_re_link/fishhook  cat Makefile
all:
xcrun -sdk iphoneos clang main.c fishhook.c -o main -target arm64-apple-ios12.2
codesign -s "A64593A4DDFA3557CCEFF47FC8E688DCD3E6E455" --entitlements entitlements.xml -f main

push:
scp main root@10.2.5.0:/tmp

clean:
rm main
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* thread #1, queue = 'com.apple.main-thread', stop reason = step over
frame #0: 0x0000000100a3f6ac main`rebind_symbols_for_image(rebindings=0x00000001012005b0, header=0x0000000100a38000, slide=10715136) at fishhook.c:187:8
184 }
185 }
186
-> 187 if (!symtab_cmd || !dysymtab_cmd || !linkedit_segment ||
188 !dysymtab_cmd->nindirectsyms) {
189 return;
190 }
Target 0: (main) stopped.
(lldb) po symtab_cmd
0x0000000100a38440

(lldb) po dysymtab_cmd
0x0000000100a38458

(lldb) po linkedit_segment
0x0000000100a383c8

(lldb)

然后找到 LC_SEGMENT_64_DATA 处理 S_LAZY_SYMBOL_POINTERSS_NON_LAZY_SYMBOL_POINTERS

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(lldb) n
Process 2046 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = step over
frame #0: 0x0000000100a3f828 main`rebind_symbols_for_image(rebindings=0x00000001012005b0, header=0x0000000100a38000, slide=10715136) at fishhook.c:215:42
212 perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
213 }
214 if ((sect->flags & SECTION_TYPE) == S_NON_LAZY_SYMBOL_POINTERS) {
-> 215 perform_rebinding_with_section(rebindings, sect, slide, symtab, strtab, indirect_symtab);
216 }
217 }
218 }

seg

这里为了调试,重点关注 S_LAZY_SYMBOL_POINTERS 的处理

首先在rebind之前查看open符号

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(lldb) image lookup -va 0x0000000100a3fee0
Address: main[0x0000000100007ee0] (main.__TEXT.__stub_helper + 180)
Summary:
Module: file = "/private/var/tmp/main", arch = "arm64"

//....
Process 2046 resuming
Process 2046 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 4.1
frame #0: 0x0000000100a3fbc8 main`perform_rebinding_with_section(rebindings=0x00000001012005b0, section=0x0000000100a382d8, slide=10715136, symtab=0x0000000100a44210, strtab=" ", indirect_symtab=0x0000000100a44780) at fishhook.c:135:46
132 strcmp(&symbol_name[1], cur->rebindings[j].name) == 0) {
133 if (cur->rebindings[j].replaced != NULL &&
134 indirect_symbol_bindings[i] != cur->rebindings[j].replacement) {
-> 135 *(cur->rebindings[j].replaced) = indirect_symbol_bindings[i];
136 }
137 indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
138 goto symbol_loop;
Target 0: (main) stopped.
(lldb) p symbol_name
(char *) $20 = 0x0000000100a44937 "_open"
(lldb)

首先备份了原函数地址,确保hook后可以通过 orign_open调用到原本的函数。

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(lldb) p i
(uint) $24 = 13
(lldb) po indirect_symbol_bindings[13]
0x0000000100a3fee0

(lldb) image lookup -va 0x0000000100a3fee0
Address: main[0x0000000100007ee0] (main.__TEXT.__stub_helper + 180)
Summary:
Module: file = "/private/var/tmp/main", arch = "arm64"

(lldb)

之后找到函数指针,完成替换

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Process 2046 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = step over
frame #0: 0x0000000100a3fbfc main`perform_rebinding_with_section(rebindings=0x00000001012005b0, section=0x0000000100a382d8, slide=10715136, symtab=0x0000000100a44210, strtab=" ", indirect_symtab=0x0000000100a44780) at fishhook.c:137:41
134 indirect_symbol_bindings[i] != cur->rebindings[j].replacement) {
135 *(cur->rebindings[j].replaced) = indirect_symbol_bindings[i];
136 }
-> 137 indirect_symbol_bindings[i] = cur->rebindings[j].replacement;
138 goto symbol_loop;
139 }
140 }
Target 0: (main) stopped.
(lldb) image list |grep main
[ 0] EAE1AE51-465A-32E0-8B3F-195FE2480F4F 0x0000000100a38000 /private/var/tmp/main
/System/Volumes/Data/Users/muhe/study/ios_re_link/fishhook/main.dSYM/Contents/Resources/DWARF/main(0x0000000100a38000)
(lldb)
(lldb) x/20gx indirect_symbol_bindings
0x100a40018: 0x00000001d8642a68 0x0000000100a3fe50
0x100a40028: 0x0000000100a3fe5c 0x0000000100a3fe68
0x100a40038: 0x0000000100a3fe74 0x00000001d8581374
0x100a40048: 0x0000000100a3f224 0x00000001d8581694
0x100a40058: 0x0000000100a3fea4 0x00000001d860ae30
0x100a40068: 0x00000001d871060c 0x00000001d873dd30
0x100a40078: 0x0000000100a3fed4 0x0000000100a3f270
0x100a40088: 0x0000000100a3feec 0x0000000100a3fef8
0x100a40098: 0x00000001d873dfd0 0x0000000100a3ff10
0x100a400a8: 0x0000000100d04498 0x0000000100a3ff72
(lldb) x/gx 0x100a40078+8
0x100a40080: 0x0000000100a3f270
(lldb)

---
>>> hex(0x100a40080-0x0000000100a38000)
'0x8080'
>>>

func_ptr

如果调用 原本的函数会走什么流程?

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0x100a3f448 <+176>: bl     0x100a3fd9c               ; symbol stub for: close
-> 0x100a3f44c <+180>: adrp x8, 1
0x100a3f450 <+184>: ldr x8, [x8]
0x100a3f454 <+188>: ldr x8, [x8]
0x100a3f458 <+192>: ldur x10, [x29, #-0x8]
0x100a3f45c <+196>: subs x8, x8, x10
0x100a3f460 <+200>: b.ne 0x100a3f478 ; <+224> at main.c
0x100a3f464 <+204>: mov w8, #0x0
0x100a3f468 <+208>: mov x0, x8
0x100a3f46c <+212>: ldp x29, x30, [sp, #0x70]
0x100a3f470 <+216>: add sp, sp, #0x80 ; =0x80
0x100a3f474 <+220>: ret
0x100a3f478 <+224>: bl 0x100a3fd60 ; symbol stub for: __stack_chk_fail
(lldb) dis -a 0x100a3fd9c
main`close:
0x100a3fd9c <+0>: nop
0x100a3fda0 <+4>: ldr x16, #0x2a8 ; (void *)0x0000000100a3f224: my_close at /Users/muhe/study/ios_re_link/fishhook/main.c:10
0x100a3fda4 <+8>: br x16

引用

https://github.com/facebook/fishhook

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