[Anti-debug] ptrace and binary patch

ptrace可以在用戶端攔截或修改系統的syscall
使parent可以監視和控制clild的reg和img實現斷點與追蹤

在X86的系統要進行syscall是先把register的值設好，然後呼叫int80
而ptrace會再syscall之前先看process是否處於被追蹤(traced)的狀態
如果是的話，就把控制權給"追蹤的process"，因此此時可以進行觀察修改reg或設斷點
GDB背後的原理其實就是靠ptrace

舉例：parent先fork出一個child並且監視他的行為
   1. Child先允許別人可以追蹤他(PTRACE_TRACEME)
     if(child == 0){ptrace(PTRACE_TRACEME, 0, NULL, NULL); syscall...}
   2. Parent等待kernel呼叫，取得child reg的值(PTRACE_PEEKUSER)
     wait(NULL);
     eax = ptrace (PTRACE_PEEKUSER, child, 4 * ORIG_EAX, NULL);  
     ptrace(PTRACE_CONT, child, NULL, NULL);

anti-debug code

#include <stdio.h>
#include <sys/ptrace.h>
int main()
{
    if (ptrace(PTRACE_TRACEME, 0, 1, 0) < 0)
    {   
        printf("DEBUGGING...\n");
        return 1;
    }   
    printf("Hello\n");
    return 0;
}

因為已經被gdb traced了，所以在呼叫ptrace就會failed，
藉此來達成anti-debug的功能。

繞過方法1: LDPROLAD
ptrace.c
int ptrace(int i, int j, int k, int l)
{
  printf(" PTRACE CALLED!\n");
}

$ gcc -shared ptrace.c -o ptrace.so
(gdb) set environment LD_PRELOAD ./ptrace.so

繞過方法2: Binary Patch
objdump -D a.out | less
    /main

 804843c:       e8 0f ff ff ff          call   8048350 
 8048441:       85 c0                   test   %eax,%eax
 8048443:       79 13                   jns    8048458 

test
    Usage:  TEST    dest,src
    Modifies flags: CF(carry) OF(overflow) PF SF(negative or sign) ZF(zero)
    Performs a logical AND of the two operands updating the flags
    register "without saving the result".
 
test   %eax,%eax  (if eax < 0, set SF to 1 )
79 Jump if not sign (jns) positive
78 Jump if sign (js) negative

vim a.out
    :%! xxd
    /79
        change 79 to 78
    :%! xxd -r
ZZ

如此一來gdb和真實的環境就相反了
比較正確的改法是 jmp (without any condition)
http://ref.x86asm.net/coder32.html#xEB

vim a.out
    :%! xxd
    /79
        change 79 to eb
    :%! xxd -r
ZZ

804843c:       e8 0f ff ff ff          call   8048350 
8048441:       85 c0                   test   %eax,%eax
8048443:       eb 13                   jmp    8048458 

補充
strace: trace syscall and signal
ltrace: a lib call tracer
背後其實都是透過ptrace來實現的