ebpf: add prandom helper for packet sampling

This work is similar to commit 4cd3675ebf74 ("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>

2 files changed, 14 insertions, 0 deletions

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 50603aec766a..c1dbbb5d289b 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -661,6 +661,8 @@ const struct bpf_func_proto bpf_map_lookup_elem_proto __weak;
 const struct bpf_func_proto bpf_map_update_elem_proto __weak;
 const struct bpf_func_proto bpf_map_delete_elem_proto __weak;
 
+const struct bpf_func_proto bpf_get_prandom_u32_proto __weak;
+
 /* To execute LD_ABS/LD_IND instructions __bpf_prog_run() may call
  * skb_copy_bits(), so provide a weak definition of it for NET-less config.
  */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a3c7701a8b5e..95eb59a045ea 100644
--- a/kernel/bpf/helpers.c
+++ b/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,
+};