Re: [PATCH bpf-next v3 5/7] bpf: extract map key pointer calculation

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 9/6/25 00:19, Eduard Zingerman wrote:
> On Fri, 2025-09-05 at 17:45 +0100, Mykyta Yatsenko wrote:
>
> [...]
>
>> +static void *map_key_from_value(struct bpf_map *map, void *value, u32 *arr_idx)
> `arr_idx` is unused at every call site of this function.
> And even if it was used, why both set through pointer and return same value?
this function returns the pointer to map key; in case of array map, map 
key is an array index,
which is not stored anywhere in the map explicitly.
arr_idx is a container for the array map key, we need to pass it from 
the outside so that the
lifetime is long enough.
In case of hash map, we return the pointer to the actual key, stored in 
the map,
arr_idx is not needed then.
>
>> +{
>> +	if (map->map_type == BPF_MAP_TYPE_ARRAY) {
>> +		struct bpf_array *array = container_of(map, struct bpf_array, map);
>> +
>> +		*arr_idx = ((char *)value - array->value) / array->elem_size;
>> +		return arr_idx;
>> +	}
>> +	BUG_ON(map->map_type != BPF_MAP_TYPE_HASH && map->map_type != BPF_MAP_TYPE_LRU_HASH);
>> +	return (void *)value - round_up(map->key_size, 8);
>> +}
>> +
> [...]