Re: [PATCH net-next 3/7] bpf: add array type of eBPF maps

On 11/04/2014 03:54 AM, Alexei Starovoitov wrote:

add new map type BPF_MAP_TYPE_ARRAY and its implementation

- optimized for fastest possible lookup()
   . in the future verifier/JIT may recognize lookup() with constant key
     and optimize it into constant pointer. Can optimize non-constant
     key into direct pointer arithmetic as well, since pointers and
     value_size are constant for the life of the eBPF program.
     In other words array_map_lookup_elem() may be 'inlined' by verifier/JIT
     while preserving concurrent access to this map from user space

- two main use cases for array type:
   . 'global' eBPF variables: array of 1 element with key=0 and value is a
     collection of 'global' variables which programs can use to keep the state
     between events
   . aggregation of tracing events into fixed set of buckets

- all array elements pre-allocated and zero initialized at init time

- key as an index in array and can only be 4 byte

- map_delete_elem() returns EINVAL, since elements cannot be deleted

- map_update_elem() replaces elements in an non-atomic way
   (for atomic updates hashtable type should be used instead)

Signed-off-by: Alexei Starovoitov <redacted>

...

+/* Called from syscall or from eBPF program */
+static int array_map_update_elem(struct bpf_map *map, void *key, void *value,
+				 u64 map_flags)
+{
+	struct bpf_array *array = container_of(map, struct bpf_array, map);
+	u32 index = *(u32 *)key;
+
+	if (map_flags > BPF_MAP_UPDATE_ONLY)
+		/* unknown flags */
+		return -EINVAL;
+
+	if (map_flags == BPF_MAP_CREATE_ONLY)
+		return -EINVAL;
+
+	if (index >= array->map.max_entries)
+		/* all elements were pre-allocated, cannot insert a new one */
+		return -E2BIG;
+
+	memcpy(array->value + array->elem_size * index, value, array->elem_size);

What would protect this from concurrent updates?

+	return 0;
+}