c - How to properly utilize masks to send index information to perf event output?

Question

According to the documentation for bpf_perf_event_output found here: http://man7.org/linux/man-pages/man7/bpf-helpers.7.html

"The flags are used to indicate the index in map for which the value must be put, masked with BPF_F_INDEX_MASK."

In the following code:

SEC("xdp_sniffer")
int xdp_sniffer_prog(struct xdp_md *ctx)
{

    void *data_end = (void *)(long)ctx->data_end;
    void *data = (void *)(long)ctx->data;

    if (data < data_end) {
        /* If we have reached here, that means this
        * is a useful packet for us. Pass on-the-wire
        * size and our cookie via metadata.
        */
        /* If we have reached here, that means this
        * is a useful packet for us. Pass on-the-wire
        * size and our cookie via metadata.
        */
        __u64 flags = BPF_F_INDEX_MASK;
        __u16 sample_size;
        int ret;
        struct S metadata;

        metadata.cookie = 0xdead;
        metadata.pkt_len = (__u16)(data_end - data);

        /* To minimize writes to disk, only
        * pass necessary information to userspace;
        * that is just the header info.
        */
        sample_size = min(metadata.pkt_len, SAMPLE_SIZE);
        flags |= (__u64)sample_size << 32;

        ret = bpf_perf_event_output(ctx, &my_map, flags,
                        &metadata, sizeof(metadata));
        if (ret)
            bpf_printk("perf_event_output failed: %d\n", ret);
    }
    return XDP_PASS;
}

It works as you would expect and stores the information for the given CPU number. However, suppose I want all packets to be sent to index 1.

I swap

__u64 flags = BPF_F_INDEX_MASK;

for

__u64 flags = 0x1ULL;

The code compiles correctly and throws no errors, however no packets get saved at all anymore. What am I doing wrong if I want all of the packets to be sent to index 1?