bpf: extract map key pointer calculation

Calculation of the BPF map key, given the pointer to a value is
duplicated in a couple of places in helpers already, in the next patch
another use case is introduced as well.
This patch extracts that functionality into a separate function.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: mykyta5

kernel/bpf/helpers.c

@@ -1082,6 +1082,17 @@ const struct bpf_func_proto bpf_snprintf_proto = {
 	.arg5_type	= ARG_CONST_SIZE_OR_ZERO,
 };
 
+static void *map_key_from_value(struct bpf_map *map, void *value, u32 *arr_idx)
+{
+	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;
+	}
+	return (void *)value - round_up(map->key_size, 8);
+}
+
 struct bpf_async_cb {
 	struct bpf_map *map;
 	struct bpf_prog *prog;
@@ -1164,15 +1175,8 @@ static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer)
 	 * bpf_map_delete_elem() on the same timer.
 	 */
 	this_cpu_write(hrtimer_running, t);
-	if (map->map_type == BPF_MAP_TYPE_ARRAY) {
-		struct bpf_array *array = container_of(map, struct bpf_array, map);
 
-		/* compute the key */
-		idx = ((char *)value - array->value) / array->elem_size;
-		key = &idx;
-	} else { /* hash or lru */
-		key = value - round_up(map->key_size, 8);
-	}
+	key = map_key_from_value(map, value, &idx);
 
 	callback_fn((u64)(long)map, (u64)(long)key, (u64)(long)value, 0, 0);
 	/* The verifier checked that return value is zero. */
@@ -1198,15 +1202,7 @@ static void bpf_wq_work(struct work_struct *work)
 	if (!callback_fn)
 		return;
 
-	if (map->map_type == BPF_MAP_TYPE_ARRAY) {
-		struct bpf_array *array = container_of(map, struct bpf_array, map);
-
-		/* compute the key */
-		idx = ((char *)value - array->value) / array->elem_size;
-		key = &idx;
-	} else { /* hash or lru */
-		key = value - round_up(map->key_size, 8);
-	}
+	key = map_key_from_value(map, value, &idx);
 
         rcu_read_lock_trace();
         migrate_disable();