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

[Daniel Borkmann <dborkman@redhat.com>]

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 <ast@plumgrid.com>

...
> +/* 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;
> +}