bpf: introduce BPF syscall and maps

BPF syscall is a multiplexor for a range of different operations on eBPF.
This patch introduces syscall with single command to create a map.
Next patch adds commands to access maps.

'maps' is a generic storage of different types for sharing data between kernel
and userspace.

Userspace example:
/* this syscall wrapper creates a map with given type and attributes
 * and returns map_fd on success.
 * use close(map_fd) to delete the map
 */
int bpf_create_map(enum bpf_map_type map_type, int key_size,
                   int value_size, int max_entries)
{
    union bpf_attr attr = {
        .map_type = map_type,
        .key_size = key_size,
        .value_size = value_size,
        .max_entries = max_entries
    };

    return bpf(BPF_MAP_CREATE, &attr, sizeof(attr));
}

'union bpf_attr' is backwards compatible with future extensions.

More details in Documentation/networking/filter.txt and in manpage

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/bpf.h

@@ -62,4 +62,27 @@ struct bpf_insn {
 	__s32	imm;		/* signed immediate constant */
 };
 
+/* BPF syscall commands */
+enum bpf_cmd {
+	/* create a map with given type and attributes
+	 * fd = bpf(BPF_MAP_CREATE, union bpf_attr *, u32 size)
+	 * returns fd or negative error
+	 * map is deleted when fd is closed
+	 */
+	BPF_MAP_CREATE,
+};
+
+enum bpf_map_type {
+	BPF_MAP_TYPE_UNSPEC,
+};
+
+union bpf_attr {
+	struct { /* anonymous struct used by BPF_MAP_CREATE command */
+		__u32	map_type;	/* one of enum bpf_map_type */
+		__u32	key_size;	/* size of key in bytes */
+		__u32	value_size;	/* size of value in bytes */
+		__u32	max_entries;	/* max number of entries in a map */
+	};
+} __attribute__((aligned(8)));
+
 #endif /* _UAPI__LINUX_BPF_H__ */