net: mptcp: fallback to TCP in case of any error

Specific MPTCP errors could happen but only one is detectable: if
ENOPROTOOPT errno is returned, it likely means MPTCP has been disable
via this sysctl knob: net.mptcp.enabled.

But because MPTCP could be blocked by the administrator using different
techniques (SELinux, etc.) making the socket creation returning other
errors, it looks better to always retry to create a "plain" TCP socket
when any errors are returned.

This work has been co-developed by Gregory Detal
<gregory.detal@tessares.net>.

Updates #56539

Change-Id: I94fb8448dae351e1d3135b4f182570979c6b36d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/471138
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

diff --git a/src/net/mptcpsock_linux.go b/src/net/mptcpsock_linux.go
index 4663d28b4b419785fcaa7b1291b606022d3ca923..15a788249890af812ccef966314f1c285e9f2551 100644 (file)
--- a/src/net/mptcpsock_linux.go
+++ b/src/net/mptcpsock_linux.go
@@ -44,19 +44,33 @@ func initMPTCPavailable() {
 }
 
 func (sd *sysDialer) dialMPTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
-       // Fallback to dialTCP if Multipath TCP isn't supported on this operating system.
-       if !supportsMultipathTCP() {
-               return sd.dialTCP(ctx, laddr, raddr)
+       if supportsMultipathTCP() {
+               if conn, err := sd.doDialTCPProto(ctx, laddr, raddr, _IPPROTO_MPTCP); err == nil {
+                       return conn, nil
+               }
        }
 
-       return sd.doDialTCPProto(ctx, laddr, raddr, _IPPROTO_MPTCP)
+       // Fallback to dialTCP if Multipath TCP isn't supported on this operating
+       // system. But also fallback in case of any error with MPTCP.
+       //
+       // Possible MPTCP specific error: ENOPROTOOPT (sysctl net.mptcp.enabled=0)
+       // But just in case MPTCP is blocked differently (SELinux, etc.), just
+       // retry with "plain" TCP.
+       return sd.dialTCP(ctx, laddr, raddr)
 }
 
 func (sl *sysListener) listenMPTCP(ctx context.Context, laddr *TCPAddr) (*TCPListener, error) {
-       // Fallback to listenTCP if Multipath TCP isn't supported on this operating system.
-       if !supportsMultipathTCP() {
-               return sl.listenTCP(ctx, laddr)
+       if supportsMultipathTCP() {
+               if dial, err := sl.listenTCPProto(ctx, laddr, _IPPROTO_MPTCP); err == nil {
+                       return dial, nil
+               }
        }
 
-       return sl.listenTCPProto(ctx, laddr, _IPPROTO_MPTCP)
+       // Fallback to listenTCP if Multipath TCP isn't supported on this operating
+       // system. But also fallback in case of any error with MPTCP.
+       //
+       // Possible MPTCP specific error: ENOPROTOOPT (sysctl net.mptcp.enabled=0)
+       // But just in case MPTCP is blocked differently (SELinux, etc.), just
+       // retry with "plain" TCP.
+       return sl.listenTCP(ctx, laddr)
 }