Vulnerability hidden by type conversion

packets.c

#include <stdint.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

typedef struct packet_st {
    char *data;
    size_t length;
} packet_t;

void receive_packet(packet_t *p) {
    char *buffer_p = malloc(100);
    /* write assembly code to buffer_p
    ...
    */
    p->data = buffer_p;
    p->length = 0xFFFFFFFFFFFFFFFF;
}


int main(void) {
    packet_t p = {
        .data = NULL,
        .length = 0,
    };
    receive_packet(&p);

    char buffer[64];
    int bytesToCopy = p.length;

    if (bytesToCopy < 64) {
        memcpy(buffer, p.data, bytesToCopy);
    }
}