bpf: allow clearing all sock_ops callback flags

The helper function bpf_sock_ops_cb_flags_set() can be used to both
set and clear the sock_ops callback flags. However, its current
behavior is not consistent. BPF program may clear a flag if more than
one were set, or replace a flag with another one, but cannot clear all
flags.

This patch also updates the documentation to clarify the ability to
clear flags of this helper function.
Signed-off-by: Hoang Tran <hoang.tran@uclouvain.be>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -1737,12 +1737,19 @@ union bpf_attr {
  * 		error if an eBPF program tries to set a callback that is not
  * 		supported in the current kernel.
  *
- * 		The supported callback values that *argval* can combine are:
+ * 		*argval* is a flag array which can combine these flags:
  *
  * 		* **BPF_SOCK_OPS_RTO_CB_FLAG** (retransmission time out)
  * 		* **BPF_SOCK_OPS_RETRANS_CB_FLAG** (retransmission)
  * 		* **BPF_SOCK_OPS_STATE_CB_FLAG** (TCP state change)
  *
+ * 		Therefore, this function can be used to clear a callback flag by
+ * 		setting the appropriate bit to zero. e.g. to disable the RTO
+ * 		callback:
+ *
+ * 		**bpf_sock_ops_cb_flags_set(bpf_sock,**
+ * 			**bpf_sock->bpf_sock_ops_cb_flags & ~BPF_SOCK_OPS_RTO_CB_FLAG)**
+ *
  * 		Here are some examples of where one could call such eBPF
  * 		program:
  *

net/core/filter.c

@@ -4437,7 +4437,6 @@ BPF_CALL_2(bpf_sock_ops_cb_flags_set, struct bpf_sock_ops_kern *, bpf_sock,
 	if (!IS_ENABLED(CONFIG_INET) || !sk_fullsock(sk))
 		return -EINVAL;
 
-	if (val)
 	tcp_sk(sk)->bpf_sock_ops_cb_flags = val;
 
 	return argval & (~BPF_SOCK_OPS_ALL_CB_FLAGS);