// alloc_sim.c - Vulnerable memory allocator
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_BLOCK 1024

void *my_malloc(size_t size) {
    size_t total = size + sizeof(size_t); // header
    if (total > MAX_BLOCK) return NULL;
    void *ptr = malloc(total);
    if (!ptr) return NULL;
    *(size_t *)ptr = size;
    return ptr + sizeof(size_t);
}

void my_free(void *p) {
    if (!p) return;
    size_t *header = (size_t *)(p - sizeof(size_t));
    free(header);
}

int main() {
    // Craft size that causes integer overflow: 0xFFFFFFFF - sizeof(size_t) + 1 wraps to small number
    size_t huge = 0xFFFFFFFF;  // 4GB - 1
    char *buf = my_malloc(huge - sizeof(size_t) + 1); // overflow: total becomes 0?
    if (buf) {
        // Write far beyond allocated buffer, heap overflow
        memset(buf, 'A', 1000);
        printf("Wrote 1000 bytes to tiny buffer\n");
        my_free(buf);
    }
    return 0;
}

gcc -o alloc_sim alloc_sim.c -fno-stack-protector
./alloc_sim