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I am trying to recreate a segment of assembly back into the C code. Below is the progress I've made so far, but I'm getting stumped on a specific section.

00000000000010049 <mysteryFunc>:
    10049:  f3 0f 1e fa             endbr64 
    1004d:  55                      push   rbp
    1004e:  48 89 e5                mov    rbp,rsp
    10051:  48 89 7d d8             mov    QWORD PTR [rbp-0x28],rdi     ; This is the function call with a unsigned char ptr as input s
    
    10055:  66 c7 45 e8 00 00       mov    WORD PTR [rbp-0x18],0x0      ; Declaring h -- unsigned short h = 0;
    
    1005b:  c7 45 ec 00 00 00 00    mov    DWORD PTR [rbp-0x14],0x0     ; Declaring length -- unsigned long length = 0;
    10062:  48 8b 45 d8             mov    rax,QWORD PTR [rbp-0x28]     ; Declaring a temp char pointer -- unsigned char *temp = s;
    10066:  48 89 45 f0             mov    QWORD PTR [rbp-0x10],rax
    1006a:  eb 5f                   jmp    100cb <mysteryFunc+0x82>     ; This seems to be a goto call to the end of a do-while loop

    1006c:  48 8b 45 f0             mov    rax,QWORD PTR [rbp-0x10]     
    10070:  48 89 45 f8             mov    QWORD PTR [rbp-0x8],rax

    10074:  8b 45 ec                mov    eax,DWORD PTR [rbp-0x14]     ; if(!(length & 1)) {
    10077:  83 e0 01                and    eax,0x1
    1007a:  85 c0                   test   eax,eax
    1007c:  75 22                   jne    100a0 <mysteryFunc+0x57>

I don't know what instructions in C would cause this kind of repositioning.

1006c:  48 8b 45 f0             mov    rax,QWORD PTR [rbp-0x10]     ; What's happening here?
10070:  48 89 45 f8             mov    QWORD PTR [rbp-0x8],rax

The function does some processing of a string and checks for null within a do-while loop. I get the feeling it's shifting to a next character, but a lot of the ways I can think to do something like that in C don't produce an assembly instruction like that.

The function does some things in less than conventional ways, like a goto call to the end of a do-while loop to check if the input string is null, instead of just using a regular while loop.

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1006c:  48 8b 45 f0             mov    rax,QWORD PTR [rbp-0x10]

copies the value in rbp-0x10(local variable) to rax(register).

10070:  48 89 45 f8             mov    QWORD PTR [rbp-0x8],rax

copies rax to another local variable rbp-0x8. Its just creating a copy of a local variable which was a copy of the first argument to mysteryFunc.

The C-code could look something like this

int doSomething(char * s){
    char * start = s, * end = s;

sample here

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  • Thanks, that was it. Was a second char pointer declared in the same line as temp and assigned in the do-while loop. Apr 25 at 8:04
  • 1
    @enkrypted87 If this is what you wanted, you can accept this answer.
    – sudhackar
    Apr 25 at 8:08

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