Obtain size of array via write permission check

Question

To obtain the length of a null terminated string,we simply write len = strlen(str) however,i often see here on SO posts saying that to get the size of an int array for example,you need to keep track of it on your own and that's what i do normally.But,i have a question,could we obtain the size by using some sort of write permission check,that checks if we have writing permissions to a block of memory? for example :

#include <stdio.h>

int getSize(int *arr);
bool permissionTo(int *ptr);

int main(void)
{
    int arr[3] = {1,2,3};
    int size = getSize(arr) * sizeof(int);
}

int getSize(int *arr)
{
    int *ptr = arr;
    int size = 0;
    while( permissionTo(ptr) )
    {
        size++;
        ptr++;
    }
    return size;
}
bool permissionTo(int *ptr)
{
    /*............*/
}

Answer 1

No, you can't. Memory permissions don't have this granularity on most, if not all, architectures.

Almost all CPU architectures manage memory in pages. On most things you'll run into today one page is 4kB. There's no practical way to control permissions on anything smaller than that.

Most memory management is done by your libc allocating a large:ish chunk of memory from the kernel and then handing out smaller chunks of it to individual malloc calls. This is done for performance (among other things) because creating, removing or modifying a memory mapping is an expensive operation especially on multiprocessor systems.

For the stack (as in your example), allocations are even simpler. The kernel knows that "this large area of memory will be used by the stack" and memory accesses to it just simply allocates the necessary pages to back it. All tracking your program does of stack allocations is one register.