nixos/binfmt: Initial binfmt_msc support.

nixos/modules/module-list.nix

@@ -694,6 +694,7 @@
   ./services/x11/xserver.nix
   ./system/activation/activation-script.nix
   ./system/activation/top-level.nix
+  ./system/boot/binfmt.nix
   ./system/boot/coredump.nix
   ./system/boot/emergency-mode.nix
   ./system/boot/grow-partition.nix

nixos/modules/system/boot/binfmt.nix

@@ -0,0 +1,139 @@
+{ config, lib, ... }:
+let
+  inherit (lib) mkOption types optionalString;
+
+  cfg = config.boot.binfmtMiscRegistrations;
+
+  makeBinfmtLine = name: { recognitionType, offset, magicOrExtension
+                         , mask, preserveArgvZero, openBinary
+                         , matchCredentials, fixBinary, ...
+                         }: let
+    type = if recognitionType == "magic" then "M" else "E";
+    offset' = toString offset;
+    mask' = toString mask;
+    interpreter = "/run/binfmt/${name}";
+    flags = if !(matchCredentials -> openBinary)
+              then throw "boot.binfmtMiscRegistrations.${name}: you can't specify openBinary = false when matchCredentials = true."
+            else optionalString preserveArgvZero "P" +
+                 optionalString (openBinary && !matchCredentials) "O" +
+                 optionalString matchCredentials "C" +
+                 optionalString fixBinary "F";
+  in ":${name}:${type}:${offset'}:${magicOrExtension}:${mask'}:${interpreter}:${flags}";
+
+  binfmtFile = builtins.toFile "binfmt_nixos.conf"
+    (lib.concatStringsSep "\n" (lib.mapAttrsToList makeBinfmtLine cfg));
+
+  activationSnippet = name: { interpreter, ... }:
+    "ln -sf ${interpreter} /run/binfmt/${name}";
+  activationScript = ''
+    mkdir -p -m 0755 /run/binfmt
+    ${lib.concatStringsSep "\n" (lib.mapAttrsToList activationSnippet cfg)}
+  '';
+in {
+  options = {
+    boot.binfmtMiscRegistrations = mkOption {
+      default = {};
+
+      description = ''
+        Extra binary formats to register with the kernel.
+        See https://www.kernel.org/doc/html/latest/admin-guide/binfmt-misc.html for more details.
+      '';
+
+      type = types.attrsOf (types.submodule ({ config, ... }: {
+        options = {
+          recognitionType = mkOption {
+            default = "magic";
+            description = "Whether to recognize executables by magic number or extension.";
+            type = types.enum [ "magic" "extension" ];
+          };
+
+          offset = mkOption {
+            default = null;
+            description = "The byte offset of the magic number used for recognition.";
+            type = types.nullOr types.int;
+          };
+
+          magicOrExtension = mkOption {
+            description = "The magic number or extension to match on.";
+            type = types.str;
+          };
+
+          mask = mkOption {
+            default = null;
+            description =
+              "A mask to be ANDed with the byte sequence of the file before matching";
+            type = types.nullOr types.str;
+          };
+
+          interpreter = mkOption {
+            description = ''
+              The interpreter to invoke to run the program.
+
+              Note that the actual registration will point to
+              /run/binfmt/''${name}, so the kernel interpreter length
+              limit doesn't apply.
+            '';
+            type = types.path;
+          };
+
+          preserveArgvZero = mkOption {
+            default = false;
+            description = ''
+              Whether to pass the original argv[0] to the interpreter.
+
+              See the description of the 'P' flag in the kernel docs
+              for more details;
+            '';
+            type = types.bool;
+          };
+
+          openBinary = mkOption {
+            default = config.matchCredentials;
+            description = ''
+              Whether to pass the binary to the interpreter as an open
+              file descriptor, instead of a path.
+            '';
+            type = types.bool;
+          };
+
+          matchCredentials = mkOption {
+            default = false;
+            description = ''
+              Whether to launch with the credentials and security
+              token of the binary, not the interpreter (e.g. setuid
+              bit).
+
+              See the description of the 'C' flag in the kernel docs
+              for more details.
+
+              Implies/requires openBinary = true.
+            '';
+            type = types.bool;
+          };
+
+          fixBinary = mkOption {
+            default = false;
+            description = ''
+              Whether to open the interpreter file as soon as the
+              registration is loaded, rather than waiting for a
+              relevant file to be invoked.
+
+              See the description of the 'F' flag in the kernel docs
+              for more details.
+            '';
+            type = types.bool;
+          };
+        };
+      }));
+    };
+  };
+
+  config = lib.mkIf (cfg != {}) {
+    environment.etc."binfmt.d/nixos.conf".source = binfmtFile;
+    system.activationScripts.binfmt = activationScript;
+    systemd.additionalUpstreamSystemUnits =
+      [ "proc-sys-fs-binfmt_misc.automount"
+        "proc-sys-fs-binfmt_misc.mount"
+      ];
+  };
+}