nixos filesystems: unify early filesystems handling

A new internal config option `fileSystems.<name>.early` is added to indicate
that the filesystem needs to be loaded very early (i.e. in initrd). They are
transformed to a shell script in `system.build.earlyMountScript` with calls to
an undefined `specialMount` function, which is expected to be caller-specific.
This option is used by stage-1, stage-2 and activation script to set up and
remount those filesystems. Options for them are updated according to systemd
defaults.

nixos/modules/security/hidepid.nix

···
   config = mkIf config.security.hideProcessInformation {
     users.groups.proc.gid = config.ids.gids.proc;
 
-    systemd.services.hidepid = {
-      wantedBy = [ "local-fs.target" ];
-      after = [ "systemd-remount-fs.service" ];
-      before = [ "local-fs-pre.target" "local-fs.target" "shutdown.target" ];
-      wants = [ "local-fs-pre.target" ];
-
-      serviceConfig = {
-        Type = "oneshot";
-        RemainAfterExit = true;
-        ExecStart = ''${pkgs.utillinux}/bin/mount -o remount,hidepid=2,gid=${toString config.ids.gids.proc} /proc'';
-        ExecStop = ''${pkgs.utillinux}/bin/mount -o remount,hidepid=0,gid=0 /proc'';
-      };
-
-      unitConfig = {
-        DefaultDependencies = false;
-        Conflicts = "shutdown.target";
-      };
-    };
+    fileSystems."/proc".options = [ "hidepid=2" "gid=${toString config.ids.gids.proc}" ];
   };
 }

nixos/modules/system/activation/activation-script.nix

···
 
     system.activationScripts.tmpfs =
       ''
-        ${pkgs.utillinux}/bin/mount -o "remount,size=${config.boot.devSize}" none /dev
-        ${pkgs.utillinux}/bin/mount -o "remount,size=${config.boot.devShmSize}" none /dev/shm
-        ${pkgs.utillinux}/bin/mount -o "remount,size=${config.boot.runSize}" none /run
+        specialMount() {
+          local device="$1"
+          local mountPoint="$2"
+          local options="$3"
+          local fsType="$4"
+
+          ${pkgs.utillinux}/bin/mount -t "$fsType" -o "remount,$options" "$device" "$mountPoint"
+        }
+        source ${config.system.build.earlyMountScript}
       '';
 
   };

nixos/modules/system/boot/stage-1-init.sh

···
 echo "<<< NixOS Stage 1 >>>"
 echo
 
-
-# Mount special file systems.
+# Make several required directories.
 mkdir -p /etc/udev
 touch /etc/fstab # to shut up mount
-touch /etc/mtab # to shut up mke2fs
+ln -s /proc/mounts /etc/mtab # to shut up mke2fs
 touch /etc/udev/hwdb.bin # to shut up udev
 touch /etc/initrd-release
-mkdir -p /proc
-mount -t proc proc /proc
-mkdir -p /sys
-mount -t sysfs sysfs /sys
-mount -t devtmpfs -o "size=@devSize@" devtmpfs /dev
-mkdir -p /run
-mount -t tmpfs -o "mode=0755,size=@runSize@" tmpfs /run
-mkdir /dev/pts
-mount -t devpts devpts /dev/pts
+
+# Mount special file systems.
+specialMount() {
+  local device="$1"
+  local mountPoint="$2"
+  local options="$3"
+  local fsType="$4"
+
+  mkdir -m 0755 -p "$mountPoint"
+  mount -n -t "$fsType" -o "$options" "$device" "$mountPoint"
+}
+source @earlyMountScript@
 
 # Log the script output to /dev/kmsg or /run/log/stage-1-init.log.
 mkdir -p /tmp

nixos/modules/system/boot/stage-1.nix

···
 
     inherit udevRules extraUtils modulesClosure;
 
-    inherit (config.boot) resumeDevice devSize runSize;
+    inherit (config.boot) resumeDevice;
+
+    inherit (config.system.build) earlyMountScript;
 
     inherit (config.boot.initrd) checkJournalingFS
       preLVMCommands preDeviceCommands postDeviceCommands postMountCommands preFailCommands kernelModules;

nixos/modules/system/boot/stage-2-init.sh

···
 # Likewise, stage 1 mounts /proc, /dev and /sys, so if we don't have a
 # stage 1, we need to do that here.
 if [ ! -e /proc/1 ]; then
-    mkdir -m 0755 -p /proc
-    mount -n -t proc proc /proc
-    mkdir -m 0755 -p /dev
-    mount -t devtmpfs devtmpfs /dev
-    mkdir -m 0755 -p /sys
-    mount -t sysfs sysfs /sys
+    specialMount() {
+        local device="$1"
+        local mountPoint="$2"
+        local options="$3"
+        local fsType="$4"
+
+        mkdir -m 0755 -p "$mountPoint"
+        mount -n -t "$fsType" -o "$options" "$device" "$mountPoint"
+    }
+    source @earlyMountScript@
 fi
 
 
···
 
 
 # More special file systems, initialise required directories.
-if ! mountpoint -q /dev/shm; then
-    mkdir -m 0755 /dev/shm
-    mount -t tmpfs -o "rw,nosuid,nodev,size=@devShmSize@" tmpfs /dev/shm
-fi
-mkdir -m 0755 -p /dev/pts
 [ -e /proc/bus/usb ] && mount -t usbfs usbfs /proc/bus/usb # UML doesn't have USB by default
 mkdir -m 01777 -p /tmp
 mkdir -m 0755 -p /var /var/log /var/lib /var/db
···
 # Also get rid of temporary GC roots.
 rm -rf /nix/var/nix/gcroots/tmp /nix/var/nix/temproots
 
-
-# Create a tmpfs on /run to hold runtime state for programs such as
-# udev (if stage 1 hasn't already done so).
-if ! mountpoint -q /run; then
-    rm -rf /run
-    mkdir -m 0755 -p /run
-    mount -t tmpfs -o "mode=0755,size=@runSize@" tmpfs /run
-fi
 
 # Create a ramfs on /run/keys to hold secrets that shouldn't be
 # written to disk (generally used for NixOps, harmless elsewhere).

nixos/modules/system/boot/stage-2.nix

···
     src = ./stage-2-init.sh;
     shellDebug = "${pkgs.bashInteractive}/bin/bash";
     isExecutable = true;
-    inherit (config.boot) devShmSize runSize;
     inherit (config.nix) readOnlyStore;
     inherit (config.networking) useHostResolvConf;
-    ttyGid = config.ids.gids.tty;
+    inherit (config.system.build) earlyMountScript;
     path =
       [ pkgs.coreutils
         pkgs.utillinux

nixos/modules/tasks/filesystems.nix

···
 
   prioOption = prio: optionalString (prio != null) " pri=${toString prio}";
 
+  specialFSTypes = [ "proc" "sysfs" "tmpfs" "devtmpfs" "devpts" ];
+
   fileSystemOpts = { name, config, ... }: {
 
     options = {
···
         description = "Disable running fsck on this filesystem.";
       };
 
+      early = mkOption {
+        default = false;
+        type = types.bool;
+        internal = true;
+        description = ''
+	  Mount this filesystem very early during boot. At the moment of
+	  mounting no disks are exposed, so this option is primarily for
+          special file systems.
+        '';
+      };
+
     };
 
     config = {
       mountPoint = mkDefault name;
-      device = mkIf (config.fsType == "tmpfs") (mkDefault config.fsType);
+      device = mkIf (elem config.fsType specialFSTypes) (mkDefault config.fsType);
       options = mkIf config.autoResize [ "x-nixos.autoresize" ];
 
       # -F needed to allow bare block device without partitions
···
 
   };
 
+  # Makes sequence of `specialMount device mountPoint options fsType` commands.
+  # `systemMount` should be defined in the sourcing script.
+  makeSpecialMounts = mounts:
+    pkgs.writeText "mounts.sh" (concatMapStringsSep "\n" (mount: ''
+      specialMount "${mount.device}" "${mount.mountPoint}" "${concatStringsSep "," mount.options}" "${mount.fsType}"
+    '') mounts);
+
 in
 
 {
···
           "/bigdisk".label = "bigdisk";
         }
       '';
-      type = types.loaOf types.optionSet;
-      options = [ fileSystemOpts ];
+      type = types.loaOf (types.submodule fileSystemOpts);
       description = ''
         The file systems to be mounted.  It must include an entry for
         the root directory (<literal>mountPoint = "/"</literal>).  Each
···
       { assertion = ! (fileSystems' ? "cycle");
         message = "The ‘fileSystems’ option can't be topologically sorted: mountpoint dependency path ${ls " -> " fileSystems'.cycle} loops to ${ls ", " fileSystems'.loops}";
       }
+      { assertion = all (x: !x.early || (x.label == null && !x.autoFormat && !x.autoResize)) fileSystems;
+        message = "Early filesystems don't support mounting by label, auto formatting and resizing";
+      }
     ];
 
     # Export for use in other modules
     system.build.fileSystems = fileSystems;
+    system.build.earlyMountScript = makeSpecialMounts (filter (fs: fs.early) fileSystems);
 
     boot.supportedFilesystems = map (fs: fs.fsType) fileSystems;
 
···
             + " " + (if skipCheck fs then "0" else
                      if fs.mountPoint == "/" then "1" else "2")
             + "\n"
-        ) fileSystems}
+        ) (filter (fs: !fs.early) fileSystems)}
 
         # Swap devices.
         ${flip concatMapStrings config.swapDevices (sw:
···
           };
 
       in listToAttrs (map formatDevice (filter (fs: fs.autoFormat) fileSystems));
+
+    # Sync mount options with systemd's src/core/mount-setup.c: mount_table.
+    fileSystems = mapAttrs (n: fs: fs // { early = true; }) {
+      "/proc" = { fsType = "proc"; options = [ "nosuid" "noexec" "nodev" ]; };
+      "/sys" = { fsType = "sysfs"; options = [ "nosuid" "noexec" "nodev" ]; };
+      "/run" = { fsType = "tmpfs"; options = [ "nosuid" "nodev" "strictatime" "mode=755" "size=${config.boot.runSize}" ]; };
+      "/dev" = { fsType = "devtmpfs"; options = [ "nosuid" "strictatime" "mode=755" "size=${config.boot.devSize}" ]; };
+      "/dev/shm" = { fsType = "tmpfs"; options = [ "nosuid" "nodev" "strictatime" "mode=1777" "size=${config.boot.devShmSize}" ]; };
+      "/dev/pts" = { fsType = "devpts"; options = [ "nosuid" "noexec" "mode=620" "gid=${toString config.ids.gids.tty}" ]; };
+    };
 
   };