[RFC PATCH bpf-next 4/5] bpf: allow SO_BUSY_POLL in bpf_setsockopt()

[Cong Wang <xiyou.wangcong@gmail.com>]

Add SO_BUSY_POLL to the SOL_SOCKET allowlist in sol_socket_sockopt() so a
sock_ops or cgroup BPF program can enable busy polling on a socket (set
sk->sk_ll_usec) without an application setsockopt or the global
net.core.busy_read sysctl. SO_BUSY_POLL needs no CAP_NET_ADMIN in
sk_setsockopt(), so no privilege gating is added; the value is an int and
joins the existing optlen == sizeof(int) group.

This lets a BPF program opt specific flows into busy polling at the point
it has the context to decide. The TCP loopback splice path
(bpf_sock_splice_pair) uses it: the splice receiver busy-polls the ring
instead of parking, turning the latency-bound TCP_RR case into a large
win (numbers are in the splice busy-poll patch).

Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Cong Wang <cwang@multikernel.io>
---
 net/core/filter.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 9590877b0714..302dfaf03f39 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5325,6 +5325,7 @@ static int sol_socket_sockopt(struct sock *sk, int optname,
 	case SO_MAX_PACING_RATE:
 	case SO_BINDTOIFINDEX:
 	case SO_TXREHASH:
+	case SO_BUSY_POLL:
 	case SK_BPF_CB_FLAGS:
 		if (*optlen != sizeof(int))
 			return -EINVAL;
-- 
2.43.0