conn: don't enable GRO on Linux < 5.12

Kernels below 5.12 are missing this:

    commit 98184612aca0a9ee42b8eb0262a49900ee9eef0d
    Author: Norman Maurer <norman_maurer@apple.com>
    Date:   Thu Apr 1 08:59:17 2021

        net: udp: Add support for getsockopt(..., ..., UDP_GRO, ..., ...);

        Support for UDP_GRO was added in the past but the implementation for
        getsockopt was missed which did lead to an error when we tried to
        retrieve the setting for UDP_GRO. This patch adds the missing switch
        case for UDP_GRO

        Fixes: e20cf8d3f1f7 ("udp: implement GRO for plain UDP sockets.")
        Signed-off-by: Norman Maurer <norman_maurer@apple.com>
        Reviewed-by: David Ahern <dsahern@kernel.org>
        Signed-off-by: David S. Miller <davem@davemloft.net>

That means we can't set the option and then read it back later. Given
how buggy UDP_GRO is in general on odd kernels, just disable it on older
kernels all together.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

conn/controlfns_linux.go

@@ -13,6 +13,35 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// Taken from go/src/internal/syscall/unix/kernel_version_linux.go
+func kernelVersion() (major, minor int) {
+	var uname unix.Utsname
+	if err := unix.Uname(&uname); err != nil {
+		return
+	}
+
+	var (
+		values    [2]int
+		value, vi int
+	)
+	for _, c := range uname.Release {
+		if '0' <= c && c <= '9' {
+			value = (value * 10) + int(c-'0')
+		} else {
+			// Note that we're assuming N.N.N here.
+			// If we see anything else, we are likely to mis-parse it.
+			values[vi] = value
+			vi++
+			if vi >= len(values) {
+				break
+			}
+			value = 0
+		}
+	}
+
+	return values[0], values[1]
+}
+
 func init() {
 	controlFns = append(controlFns,
 
@@ -57,5 +86,24 @@ func init() {
 			}
 			return err
 		},
+
+		// Attempt to enable UDP_GRO
+		func(network, address string, c syscall.RawConn) error {
+			// Kernels below 5.12 are missing 98184612aca0 ("net:
+			// udp: Add support for getsockopt(..., ..., UDP_GRO,
+			// ..., ...);"), which means we can't read this back
+			// later. We could pipe the return value through to
+			// the rest of the code, but UDP_GRO is kind of buggy
+			// anyway, so just gate this here.
+			major, minor := kernelVersion()
+			if major < 5 || (major == 5 && minor < 12) {
+				return nil
+			}
+
+			c.Control(func(fd uintptr) {
+				_ = unix.SetsockoptInt(int(fd), unix.IPPROTO_UDP, unix.UDP_GRO, 1)
+			})
+			return nil
+		},
 	)
 }