support setting network device names

this means that net devices in devices/foo/default.nix can be
specified by their sysfs paths (instead of by "eth0" and "eth1" that
may change from one kernel version to the next) and given mnenomic
names that are helpful for the hardware. Like "wan" and "lan[1..4]"

devices/qemu/default.nix

@@ -77,8 +77,14 @@
         networkInterfaces =
           let inherit (config.system.service.network) link;
           in {
-            wan = link.build { ifname = "eth0"; };
-            lan = link.build { ifname = "eth1"; };
+            wan = link.build {
+              devpath = "/devices/pci0000:00/0000:00:13.0/virtio0";
+              ifname = "wan";
+            };
+            lan = link.build {
+              devpath = "/devices/pci0000:00/0000:00:14.0/virtio1";
+              ifname = "lan";
+            };
 
             wlan_24 = link.build {
               ifname = "wlan0";

modules/network/default.nix

@@ -68,7 +68,21 @@ in {
         ifname = mkOption {
           type = types.str;
           example = "eth0";
+          description = ''
+            Device name as used by the kernel (as seen in "ip link"
+            or "ifconfig" output). If devpath is also specified, the
+            device will be renamed to the name provided.
+          '';
         };
+        devpath = mkOption {
+          type = types.nullOr types.str;
+          default = null;
+          example = "/devices/platform/soc/soc:internal-regs/f1070000.ethernet";
+          description = ''
+            Path to the sysfs node of the device. If you provide this
+            and the ifname option, the device will be renamed to the
+            name given by ifname.
+          ''; };
         # other "ip link add" options could go here as well
         mtu = mkOption {
           type = types.nullOr types.int;

modules/network/link.nix

@@ -4,13 +4,27 @@
 , serviceFns
 , lib
 }:
-{ifname, mtu} :
+{
+  ifname
+, devpath ? null
+, mtu} :
+# if devpath is supplied, we rename the interface at that
+# path to have the specified name.
 let
   inherit (liminix.services) longrun oneshot;
   inherit (lib) concatStringsSep;
   name = "${ifname}.link";
-  up = liminix.networking.ifup name ifname;
+  rename = if devpath != null
+           then ''
+             oldname=$(cd /sys${devpath} && cd net/ && echo *)
+             ip link set ''${oldname} name ${ifname}
+           ''
+           else "";
 in oneshot {
-  inherit name up;
+  inherit name;
+  up = ''
+    ${rename}
+    ${liminix.networking.ifup name ifname}
+  '';
   down = "ip link set down dev ${ifname}";
 }