cherry-pick: 4670-invalid-arg-cap-check

Updates #4670.

Squashed commit of the following:

commit 9c32739eb92ef57c78a4dc3ec3c0f280aebf7182
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Aug 3 20:04:54 2022 +0300

    aghnet: imp port check for older linuxes

CHANGELOG.md

@@ -24,7 +24,13 @@ and this project adheres to
 
 - Go 1.18 support.  v0.109.0 will require at least Go 1.19 to build.
 
+### Fixed
+
+- `invalid argument` errors during update checks on older Linux kernels
+  ([#4670]).
+
 [#2993]: https://github.com/AdguardTeam/AdGuardHome/issues/2993
+[#4670]: https://github.com/AdguardTeam/AdGuardHome/issues/4670
 
 
 

internal/aghnet/net_linux.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/AdguardTeam/AdGuardHome/internal/aghos"
 	"github.com/AdguardTeam/golibs/errors"
+	"github.com/AdguardTeam/golibs/log"
 	"github.com/AdguardTeam/golibs/stringutil"
 	"github.com/google/renameio/maybe"
 	"golang.org/x/sys/unix"
@@ -22,17 +23,27 @@ import (
 const dhcpcdConf = "etc/dhcpcd.conf"
 
 func canBindPrivilegedPorts() (can bool, err error) {
-	cnbs, err := unix.PrctlRetInt(
+	res, err := unix.PrctlRetInt(
 		unix.PR_CAP_AMBIENT,
 		unix.PR_CAP_AMBIENT_IS_SET,
 		unix.CAP_NET_BIND_SERVICE,
 		0,
 		0,
 	)
+	if err != nil {
+		if errors.Is(err, unix.EINVAL) {
+			// Older versions of Linux kernel do not support this.  Print a
+			// warning and check admin rights.
+			log.Info("warning: cannot check capability cap_net_bind_service: %s", err)
+		} else {
+			return false, err
+		}
+	}
+
 	// Don't check the error because it's always nil on Linux.
 	adm, _ := aghos.HaveAdminRights()
 
-	return cnbs == 1 || adm, err
+	return res == 1 || adm, nil
 }
 
 // dhcpcdStaticConfig checks if interface is configured by /etc/dhcpcd.conf to