gcc error: -mpreferred-stack-boundary=2 is not between 4 and 12
I'm just starting to learn a bit of c and assembly to learn about buffer overflows (so go easy on me, this is my first dive into programming :)
At this point I'm really mostly following along with some training videos online and trying to get a grasp. The videos use an x32 system, what I'm doing it trying to learn it in both 32 and 64 bit at the same time so I have two virtual machines I compile similar code on (I replace syscall codes as needed for x64, etc). I'm learning about buffer overflows right now.
This tiny script is to force a return value of 20 after the program runs.
Anyway, so there is this code that on x32 I'm supposed to compile this way:
gcc -ggdb -mpreferred-stack-boundary=2 -o Code Code.c
This compiles fine. This is the code:
char shellcode = "\xbb\x14\x00\x00\x00"
ret = (int *)&ret +2;
(*ret) = (int)shellcode;
on x32 it works the same as the video, which is after it runs, if I check the return code, it shows as 20:
# echo $?
On the 64-bit machine, if I try to compile with the exact same command, I get error:
$ gcc -ggdb -mpreferred-stack-boundary=2 -o ShellCode ShellCode.c
ShellCode.c:1: error: -mpreferred-stack-boundary=2 is not between 4 and 12
Instead of -mpreferred-stack-boundary=2, I tested compiling with a setting of 4, and also all even numbers up to 12, and I also tested leaving that switch out completely I don't get an error at compile time when I test 4-12 or leaving this switch out, but I'm not getting a return code of 20 that I'm supposed to get if this script is working right. In fact I seem to get random return codes each time I run it.
On the 32-bit machine, I get a 20 every time.
I don't yet have a good handle of the math involved, so this could have to do with the +2 in the script too, I'm not sure.
Does anyone have any ideas about this error and what the significance of this switch actually is?