sctp: add bpf_bypass_getsockopt proto callback

Implement ->bpf_bypass_getsockopt proto callback and filter out
SCTP_SOCKOPT_PEELOFF, SCTP_SOCKOPT_PEELOFF_FLAGS and SCTP_SOCKOPT_CONNECTX3
socket options from running eBPF hook on them.

SCTP_SOCKOPT_PEELOFF and SCTP_SOCKOPT_PEELOFF_FLAGS options do fd_install(),
and if BPF_CGROUP_RUN_PROG_GETSOCKOPT hook returns an error after success of
the original handler sctp_getsockopt(...), userspace will receive an error
from getsockopt syscall and will be not aware that fd was successfully
installed into a fdtable.

As pointed by Marcelo Ricardo Leitner it seems reasonable to skip
bpf getsockopt hook for SCTP_SOCKOPT_CONNECTX3 sockopt too.
Because internaly, it triggers connect() and if error is masked
then userspace will be confused.

This patch was born as a result of discussion around a new SCM_PIDFD interface:
https://lore.kernel.org/all/20230413133355.350571-3-aleksandr.mikhalitsyn@canonical.com/

Fixes: 0d01da6a ("bpf: implement getsockopt and setsockopt hooks")
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: linux-sctp@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Suggested-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -8281,6 +8281,22 @@ static int sctp_getsockopt(struct sock *sk, int level, int optname,
 	return retval;
 }
 
+static bool sctp_bpf_bypass_getsockopt(int level, int optname)
+{
+	if (level == SOL_SCTP) {
+		switch (optname) {
+		case SCTP_SOCKOPT_PEELOFF:
+		case SCTP_SOCKOPT_PEELOFF_FLAGS:
+		case SCTP_SOCKOPT_CONNECTX3:
+			return true;
+		default:
+			return false;
+		}
+	}
+
+	return false;
+}
+
 static int sctp_hash(struct sock *sk)
 {
 	/* STUB */
@@ -9650,6 +9666,7 @@ struct proto sctp_prot = {
 	.shutdown    =	sctp_shutdown,
 	.setsockopt  =	sctp_setsockopt,
 	.getsockopt  =	sctp_getsockopt,
+	.bpf_bypass_getsockopt	= sctp_bpf_bypass_getsockopt,
 	.sendmsg     =	sctp_sendmsg,
 	.recvmsg     =	sctp_recvmsg,
 	.bind        =	sctp_bind,
@@ -9705,6 +9722,7 @@ struct proto sctpv6_prot = {
 	.shutdown	= sctp_shutdown,
 	.setsockopt	= sctp_setsockopt,
 	.getsockopt	= sctp_getsockopt,
+	.bpf_bypass_getsockopt	= sctp_bpf_bypass_getsockopt,
 	.sendmsg	= sctp_sendmsg,
 	.recvmsg	= sctp_recvmsg,
 	.bind		= sctp_bind,