5.2 Function Call

Before We Begin

• The name of the program in the following examples is "Testing.exe". This is why you will see "testing.*" prepended to some things.
• Parameters do not have to be set in any order. RCX can be set after EDX with no issue.
• Parameters don't have to be passed through the 64-bit registers. For example, you can pass a parameter through RCX, ECX, CX, etc.
• You'll see new instructions that are variants on other instructions you know. For example, MOVAPS is new, but it's just a variant on the MOV instruction. MOVAPS is used for moving single precision values around. Single-precision values are floats, and double-precision values are doubles. Unless I'm trying to figure out exactly what the data type of some value is, I usually ignore the variant and just focus on the main idea. In other words, I don't really care that MOVAPS works with single precision, for now I just care that it moves some data.

You should be able to follow this lesson without the binary, but if you want to compile your own version of the code we will be reversing here is the code:

#include <iostream>
int main() {
    printf("Printing with printf(). Here is some data: %d, %d, %f, %p, %d, %f, %f\n", 4, 2, 1.0f, (void*)0xFFF7865, 1498, 87.003f, 99123.34);
    return 0;
}

Analyzing The Function Call

Let's take a look at a function call. In this case the function being called is printf().

When this function is run, it prints the following to the console:
Printing with printf(). Here is some data: 4, 2, 1.000000, 000000000FFF7865, 1498, 87.002998, 99123.340000

Parameters Passed

We know these parameters are being passed to printf() because the parameters are set before CALL <testing.printf>.

1. The first parameter passed (RCX) is the memory address 0x7FF773B09D80. This is the address of the string "Printing with printf(). Here is some data: %d, %d, %f, %p, %d, %f, %f\n". This is our format string for printf(). Don't forget that this format string is a parameter to printf().
2. The second parameter passed (EDX) is the value of 0x4.
3. The third parameter (R8D) is set to RDX - 0x2 which is 0x2.
4. The fourth parameter (R9) is set to XMM3. XMM3 contains the value at memory address real@3ff000... This is not a memory address, this is just a "label" that x64dbg is giving the memory address. The real address is 0x7FF773B09DC8. This address contains 1 (as a double).

Now the registers used for passing parameters are used, so the program will have to use the stack.

5. The fifth parameter is put onto the stack with MOV QWORD PTR SS:[RSP + 0x20], 0xFFF7865. This is the value of "000000000FFF7865" that gets printed.
6. The sixth parameter is put onto the stack with MOV DWORD PTR SS:[RSP + 0x28], 0x5DA. 0x5DA is hexadecimal for 1498.
7. The seventh and eighth parameters are put onto the stack at the same time with MOVUPS XMMWORD PTR SS:[RSP + 0x30], XMM0. Remember that the XMM registers can hold two values. XMM0 contains both 87.002998 and 99123.34.

At this point the printf() function is called. printf() will now take these parameters and use them as it needs to.

There you go, that's a function call. If you can understand what was just covered then you shouldn't have much issue understanding any other function call you encounter. Remember, if you aren't reversing 64-bit Windows the calling convention might be different.

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