ebpf: add prandom helper for packet sampling

This work is similar to commit 4cd3675ebf ("filter: added BPF
random opcode") and adds a possibility for packet sampling in eBPF.

Currently, this is only possible in classic BPF and useful to
combine sampling with f.e. packet sockets, possible also with tc.

Example function proto-type looks like:

  u32 (*prandom_u32)(void) = (void *)BPF_FUNC_get_prandom_u32;

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/helpers.c

@@ -11,6 +11,7 @@
  */
 #include <linux/bpf.h>
 #include <linux/rcupdate.h>
+#include <linux/random.h>
 
 /* If kernel subsystem is allowing eBPF programs to call this function,
  * inside its own verifier_ops->get_func_proto() callback it should return
@@ -87,3 +88,14 @@ const struct bpf_func_proto bpf_map_delete_elem_proto = {
 	.arg1_type = ARG_CONST_MAP_PTR,
 	.arg2_type = ARG_PTR_TO_MAP_KEY,
 };
+
+static u64 bpf_get_prandom_u32(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+	return prandom_u32();
+}
+
+const struct bpf_func_proto bpf_get_prandom_u32_proto = {
+	.func		= bpf_get_prandom_u32,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+};