Practical Reverse Engineering p. 79 #4

Question number 4 on page 79 of Practical Reverse Engineering is as follows:

Figure 2-10 shows another easy function.

Here is the function's disassembly:

Figure 2-10. Practical Reverse Engineering. © 2014 by Bruce Dang

The ARM processor is in Thumb state. This function takes a pointer of an unknown type. If the pointer is null, it returns 0. Otherwise it subtracts 8 bytes from the pointer and returns the value there.

DWORD sub8_ptr(void *r0)
{
    /* CBNZ R0, loc_100C3DA */
    if (r0 == NULL)
        return 0;       /* MOVS R0, #0 */

    return *(r0 - 8);   /* LDR.W R0, [R0,#–8]  */
}