wgengine/magicsock: set UDP socket buffer sizes to 7MB

- At high data rates more buffer space is required in order to avoid
  packet loss during any cause of delay.
- On slower machines more buffer space is required in order to avoid
  packet loss while decryption & tun writing is underway.
- On higher latency network paths more buffer space is required in order
  to overcome BDP.
- On Linux set with SO_*BUFFORCE to bypass net.core.{r,w}mem_max.
- 7MB is the current default maximum on macOS 12.6
- Windows test is omitted, as Windows does not support getsockopt for
  these options.

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock_linux.go

@@ -12,6 +12,7 @@ import (
 	"io"
 	"net"
 	"net/netip"
+	"syscall"
 	"time"
 	"unsafe"
 
@@ -20,6 +21,8 @@ import (
 	"tailscale.com/envknob"
 	"tailscale.com/net/netns"
 	"tailscale.com/types/key"
+	"tailscale.com/types/logger"
+	"tailscale.com/types/nettype"
 )
 
 const (
@@ -288,3 +291,30 @@ func setBPF(conn net.PacketConn, filter []bpf.RawInstruction) error {
 	}
 	return nil
 }
+
+// trySetSocketBuffer attempts to set SO_SNDBUFFORCE and SO_RECVBUFFORCE which
+// can overcome the limit of net.core.{r,w}mem_max, but require CAP_NET_ADMIN.
+// It falls back to the portable implementation if that fails, which may be
+// silently capped to net.core.{r,w}mem_max.
+func trySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {
+	if c, ok := pconn.(*net.UDPConn); ok {
+		var errRcv, errSnd error
+		rc, err := c.SyscallConn()
+		if err == nil {
+			rc.Control(func(fd uintptr) {
+				errRcv = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_RCVBUFFORCE, socketBufferSize)
+				if errRcv != nil {
+					logf("magicsock: failed to force-set UDP read buffer size to %d: %v", socketBufferSize, errRcv)
+				}
+				errSnd = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_SNDBUFFORCE, socketBufferSize)
+				if errSnd != nil {
+					logf("magicsock: failed to force-set UDP write buffer size to %d: %v", socketBufferSize, errSnd)
+				}
+			})
+		}
+
+		if err != nil || errRcv != nil || errSnd != nil {
+			portableTrySetSocketBuffer(pconn, logf)
+		}
+	}
+}