/mnt/persist/private: split waiting on the keyfile out of the mount process

modules/persist/stores/private/default.nix

@@ -1,13 +1,3 @@
-# TODO: this can be simplified.
-# mounting /mnt/persist/private consists of two steps:
-# - way for a password to appear at /run/gocryptfs/private.key
-# - try mounting the filesystem, using that password
-# and retry if that fails.
-# because fs mounts lack a retry capability, that whole thing is encapsulated inside one step,
-# and then there's dancing to ensure that `test -L /mnt/persist/private` doesn't hang before the store has been mounted.
-# if i can break these apart, the implementation should look a lot less confusing.
-#
-# i can probably use `echo $passwd | gocryptfs-xray -dumpmasterkey /nix/persist/private/gocryptfs.conf` for the above
 { config, lib, pkgs, sane-lib, utils, ... }:
 
 let
@@ -17,34 +7,18 @@ let
 in
 lib.mkIf config.sane.persist.enable
 {
-  sane.programs."mount.fuse3.gocryptfs-private" = {
+  sane.programs."provision-private-key" = {
     packageUnwrapped = pkgs.static-nix-shell.mkBash {
-      pname = "mount.fuse3.gocryptfs-private";
+      pname = "provision-private-key";
       srcRoot = ./.;
       pkgs = [
         "coreutils-full"
+        "gocryptfs"
         "inotify-tools"
-        "libfuse-sane"
       ];
     };
-    # sandbox.method = "landlock";  #< trickier than it looks to enable sandboxing of any kind here.
-    sandbox.autodetectCliPaths = "existing";
-    sandbox.capabilities = [
-      "sys_admin"
-      "chown"
-      "dac_override"
-      "dac_read_search"
-      "fowner"
-      "lease"
-      "mknod"
-      "setgid"
-      "setuid"
-    ];
-    sandbox.extraPaths = [
-      "/dev/fuse"
-      "/run/gocryptfs"
-    ];
-    suggestedPrograms = [ "gocryptfs-private" ];
+    sandbox.method = "bwrap";
+    sandbox.autodetectCliPaths = "parent";
   };
   sane.programs.gocryptfs-private = {
     packageUnwrapped = pkgs.static-nix-shell.mkBash {
@@ -91,7 +65,7 @@ lib.mkIf config.sane.persist.enable
 
   fileSystems."${origin}" = {
     device = "gocryptfs-private#${backing}";
-    fsType = "fuse3.gocryptfs-private";
+    fsType = "fuse3.sane";
     options = [
       # "auto"
       "nofail"
@@ -117,7 +91,7 @@ lib.mkIf config.sane.persist.enable
     wantedBy = [ "local-fs.target" ];
     mount.depends = [
       config.sane.fs."${backing}".unit
-      config.sane.fs."/run/gocryptfs".unit
+      config.sane.fs."/run/gocryptfs/private.key".unit
     ];
     # unitConfig.DefaultDependencies = "no";
     mount.mountConfig.TimeoutSec = "infinity";
@@ -154,9 +128,15 @@ lib.mkIf config.sane.persist.enable
     user = config.sane.defaultUser;  #< must be user-writable so i can unlock it.
     mode = "0770";
   };
+  sane.fs."/run/gocryptfs/private.key".generated.command = [
+    "${lib.getExe config.sane.programs.provision-private-key.package}"
+    "/run/gocryptfs/private.key"
+    "${backing}/gocryptfs.conf"
+  ];
 
-  sane.programs."mount.fuse3.gocryptfs-private".enableFor.system = true;
-  system.fsPackages = [ pkgs.libfuse-sane config.sane.programs."mount.fuse3.gocryptfs-private".package ];
+  sane.programs."gocryptfs-private".enableFor.system = true;
+  sane.programs."provision-private-key".enableFor.system = true;
+  system.fsPackages = [ pkgs.libfuse-sane ];
 
   sane.user.services.gocryptfs-private = {
     description = "wait for /mnt/persist/private to be mounted";

modules/persist/stores/private/gocryptfs-private

@@ -1,4 +1,6 @@
 #!/usr/bin/env nix-shell
 #!nix-shell -i bash -p bash -p gocryptfs
 
-exec gocryptfs --sanebox-path /run/gocryptfs/private.key "$@"
+passfile=/run/gocryptfs/private.key
+gocryptfs --sanebox-path "$passfile" "$@"
+rm "$passfile"

modules/persist/stores/private/mount.fuse3.gocryptfs-private

@@ -1,66 +0,0 @@
-#!/usr/bin/env nix-shell
-#!nix-shell -i bash -p bash -p coreutils-full -p inotify-tools -p libfuse-sane
-
-# backing=$1
-# facing=$2
-mountArgs=("$@")
-passdir=/run/gocryptfs
-passfile="$passdir/private.key"
-
-waitForPassfileOnce() {
-  local timeout=$1
-  if [ -f "$passfile" ]; then
-    return 0
-  else
-    # wait for some file to be created inside the directory.
-    # inotifywait returns 0 if the file was created. 1 or 2 if timeout was hit or it was interrupted by a different event.
-    inotifywait --timeout "$timeout" --event create "$passdir"
-    return 1  #< maybe it was created; we'll pick that up immediately, on next check
-  fi
-}
-waitForPassfile() {
-  # there's a race condition between testing the path and starting `inotifywait`.
-  # therefore, use a retry loop. exponential backoff to decrease the impact of the race condition,
-  # especially near the start of boot to allow for quick reboots even if/when i hit the race.
-  for timeout in 4 4 8 8 8 8 16 16 16 16 16 16 16 16; do
-    if waitForPassfileOnce "$timeout"; then
-      return 0
-    fi
-  done
-  while true; do
-    if waitForPassfileOnce 30; then
-      return 0
-    fi
-  done
-}
-tryOpenStore() {
-  # try to open the store (blocking), if it fails, then delete the passfile because the user probably entered the wrong password
-  echo "mounting with ${mountArgs[*]}"
-  # gocryptfs will unlock the store, and *then* fork into the background.
-  # so when it returns, the files are either immediately accessible, or the mount failed (likely due to a bad password
-  # if ! SANEBOX_APPEND="--sanebox-path $passfile" gocryptfs "${mountArgs[@]}"; then
-  #   echo "failed mount (transient failure)"
-  #   rm -f "$passfile"
-  #   return 1
-  # fi
-  #
-  # the actual `mount` operation blocks fs operations until it succeeds or fails.
-  # hence, we don't want to be inside `mount` while waiting for the password, else we block the world.
-  # instead, do this funky recursive thing, where we call ourself after the password is provided.
-  # TODO: better would be to move the password procurement out of the mount altogether, into some .unit file,
-  # but that's tricky because the unit file wouldn't have a great way of validating the password.
-  if ! mount.fuse3.sane "${mountArgs[@]}"; then
-    echo "failed mount (transient failure)"
-    rm -f "$passfile"
-    return 1
-  fi
-}
-
-waitForPassfile
-while ! tryOpenStore; do
-  waitForPassfile
-done
-echo "mounted"
-# mount is complete (successful), and backgrounded.
-# remove the passfile even on successful mount, for vague safety reasons (particularly if the user were to explicitly unmount the private store).
-rm -f "$passfile"

modules/persist/stores/private/provision-private-key

@@ -0,0 +1,46 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p bash -p coreutils-full -p gocryptfs -p inotify-tools
+
+passfile="$1"  # e.g. /run/gocryptfs/private.key
+conffile="$2"  # e.g. /nix/persist/private/gocryptfs.conf
+passdir=$(dirname "$passfile")
+
+waitForPassfileOnce() {
+  local timeout=$1
+  if [ -f "$passfile" ]; then
+    return 0
+  else
+    # wait for some file to be created inside the directory.
+    # inotifywait returns 0 if the file was created. 1 or 2 if timeout was hit or it was interrupted by a different event.
+    inotifywait --timeout "$timeout" --event create "$passdir"
+    return 1  #< maybe it was created; we'll pick that up immediately, on next check
+  fi
+}
+waitForPassfile() {
+  # there's a race condition between testing the path and starting `inotifywait`.
+  # therefore, use a retry loop. exponential backoff to decrease the impact of the race condition,
+  # especially near the start of boot to allow for quick reboots even if/when i hit the race.
+  for timeout in 4 4 8 8 8 8 16 16 16 16 16 16 16 16; do
+    if waitForPassfileOnce "$timeout"; then
+      return 0
+    fi
+  done
+  while true; do
+    if waitForPassfileOnce 30; then
+      return 0
+    fi
+  done
+}
+validatePassword() {
+  if ! cat "$passfile" | gocryptfs-xray -dumpmasterkey "$conffile" > /dev/null; then
+    echo "failed key validation"
+    rm -f "$passfile"
+    return 1
+  fi
+}
+
+waitForPassfile
+while ! validatePassword; do
+  waitForPassfile
+done
+echo "key provisioned"