BPF hashmap not showing correct value when retrieving

Question

I'm trying to create a hashmap in BPF in kernel code, as follows:

struct bpf_map_def SEC("maps") hash_map = {
    .type = BPF_MAP_TYPE_HASH,
    .key_size = sizeof(__u32),
    .value_size = sizeof(__u32),
    .max_entries = 1000,
};

Then I just insert and retrieve from the map as follows:

__u32 key = src_ip;
__u32 init_val = dst_ip;
__u32 *value;

value = bpf_map_lookup_elem(&hash_map, &key);
if (value) {
    trace_printk("value found=%lu\n", value);
}

Where src_ip and dst_ip are the source and destination IPs from the packet I'm processing:

__u32 src_ip = ip->saddr;
__u32 dst_ip = ip->daddr;

However when I print the value obtained from the map I get:

value found=18446637648727971896

Whereas the actual src and dst addresses in the print that I have just above shows:

src= 763730773, dst= 2818323142

I just don't understand how to retrieve/store in the map correctly. Any pointers on that?

Thanks.

Answer 1

The lookup function returns a pointer to your value:

void *bpf_map_lookup_elem(struct bpf_map *map, const void *key);

Have you tried to dereference it? Something like:

trace_printk("value found=%lu\n", *value);