Stack directions

Jan 21, 2024

Writeups for this fancy problem #

The problem is:

How would you compute that the stack grows downwards/upwards (in C/C++).

Credits: LowLevelLearning (YouTube)

Inline Assembly #

The following code is applicable to a x64-bit machine.

As the function get_rsp has a constant stack frame, we can rely on the number to deduce whether the stack grows upwards/downwards.

uint64_t get_rsp() {
    asm("movq %rsp, %rax");
}

int main(int argc, char** argv) {
    uint64_t rsp1 = get_rsp();
    printf("rsp = %p\n", rsp1);
    asm("pushq $0xa");

    uint64_t rsp2 = get_rsp();
    printf("rsp = %p\n", rsp2);

    printf("Stack grows downwards, so : %d\n", rsp1 > rsp2);

    asm("popq %rax");
}

Recursion #

The following code is cross-platform. It uses recursion to obtain a new stack frame on top of the original stack frame and obtains the pointer of the old number and does comparision.

bool up_or_down(int* ptr) {
    int x = 1;
    if (ptr != NULL) {
        return &x < ptr;
    } else {
        return up_or_down(&x);
    }
}

int main(int argc, char** argv) {
    if (up_or_down(NULL)) {
        printf("Stack grows downwards");
    } else {
        printf("Stack grows upwards");
    }
}

Alloca #

The following code uses alloca to get two seperate regions on the stack at runtime.

int main(int argc, char** argv) {
    int* x = alloca(0xf);
    int* y = alloca(0xf);

    printf("x = %p, y = %p\n", x, y);

    if (&x > &y) {
        printf("Stack grows downwards!");
    } else {
        printf("Stack grows upwards");
    }
}