bunpen: dbus_proxy: implement restrict_dbus, totally untested function to shell out to xdg-dbus-proxy

pkgs/by-name/bunpen/Makefile

@@ -4,6 +4,7 @@ all: bunpen
 IP ?= $(shell which ip)
 IPTABLES ?= $(shell which iptables)
 PASTA ?= $(shell which pasta)
+XDG_DBUS_PROXY ?= $(shell which xdg-dbus-proxy)
 
 config/runtime_deps.ha: config/runtime_deps.ha.in
 	test -x "$(IP)"
@@ -12,6 +13,7 @@ config/runtime_deps.ha: config/runtime_deps.ha.in
 	sed -e "s:@IP@:$(IP):" \
 	    -e "s:@IPTABLES@:$(IPTABLES):" \
 	    -e "s:@PASTA@:$(PASTA):" \
+	    -e "s:@XDG_DBUS_PROXY@:$(XDG_DBUS_PROXY):" \
 	    $^ > $@
 	cat $@
 

pkgs/by-name/bunpen/config/runtime_deps.ha.in

@@ -4,3 +4,4 @@
 export const IP: str = "@IP@";
 export const IPTABLES: str = "@IPTABLES@";
 export const PASTA: str = "@PASTA@";
+export const XDG_DBUS_PROXY: str = "@XDG_DBUS_PROXY@";

pkgs/by-name/bunpen/package.nix

@@ -8,6 +8,7 @@
   stdenv,
   util-linux,
   which,
+  xdg-dbus-proxy,
 }: stdenv.mkDerivation {
   pname = "bunpen";
   version = "0.1.0";
@@ -23,6 +24,7 @@
     "IP=${lib.getExe' iproute2 "ip"}"
     "IPTABLES=${lib.getExe' iptables "iptables"}"
     "PASTA=${lib.getExe' passt "pasta"}"
+    "XDG_DBUS_PROXY=${lib.getExe xdg-dbus-proxy}"
   ];
 
   nativeCheckInputs = [

pkgs/by-name/bunpen/restrict/dbus_proxy.ha

@@ -14,6 +14,9 @@
 //    other dbus names.
 //    - own the dbus name `org.baz.Bap`.
 //
+// `man xdg-dbus-proxy` provides this example invocation:
+// `xdg-dbus-proxy --fd=26 unix:path=/run/usr/1000/bus /run/usr/1000/.dbus-proxy/session-bus-proxy --filter --own=org.gnome.ghex.* --talk=ca.desrt.dconf --call=org.freedesktop.portal.*=* --broadcast=org.freedesktop.portal.*=@/org/freedesktop/portal/*`
+//
 // proof-of-concept minimal:
 // ```sh
 // $ xdg-dbus-proxy unix:path=/run/user/colin/dbus/bus /run/user/colin/dbus/proxy-1 --filter &
@@ -55,35 +58,61 @@
 //    - wildcards are supported above as expected (e.g. `'--call=org.gnome.Calls=org.gtk.Application.*@*'`
 // - best is to use exclusively `--call=...` rules (and `--own`), no --see or --talk
 
-// use config;
+use config;
 use errors;
-// use errors::ext;
+use errors::ext;
 use strings;
 use os;
-// use os::exec;
-//
-// // instantiate a `xdg-dbus-proxy` process,
-// //   which talks to the bus on the sandboxed process's behalf,
-// //   selectively forwarding traffic based on the passed policy.
-// // this can be called from within an existing sandbox.
-// //   the dbus bus should be reachable from the present namespace.
-// //   xdg-dbus-proxy will run in this namespace,
-// //
-// // `man xdg-dbus-proxy` provides this example invocation:
-// // `xdg-dbus-proxy --fd=26 unix:path=/run/usr/1000/bus /run/usr/1000/.dbus-proxy/session-bus-proxy --filter --own=org.gnome.ghex.* --talk=ca.desrt.dconf --call=org.freedesktop.portal.*=* --broadcast=org.freedesktop.portal.*=@/org/freedesktop/portal/*`
-// fn setup_dbus_proxy(dbus: restrict::dbus_subset) void = {
-//   let dbus_socket = errors::ext::check("setup_dbus_proxy: get_dbus_socket", get_dbus_socket());
-//   match (os::exec::fork()) {
-//     case let child_pid: os::exec::process =>
-//       // parent process: continue execution
-//       void;
-//     case void =>
-//       // child process: TODO: spawn xdg-dbus-proxy
-//       void;
-//     case let e: os::exec::error =>
-//       errors::ext::check("setup_dbus_proxy: fork", e);
-//   };
-// };
+use os::exec;
+use ps;
+use rt::ext;
+
+export type dbus_resources = struct {
+  talk: []str,
+  own: []str,
+};
+
+// given an existing, unfiltered bus at the fs path `upstream`,
+// proxy the bus into the fs path indicated by `downstream`.
+// clients attaching to the `downstream` path are provided access only to the
+// resources indicated by `dbus`.
+fn dbus_restrict(dbus: dbus_resources, upstream: str, downstream: str) void = {
+  match (ps::fork_and_die_with_parent()) {
+    case let child_pid: os::exec::process =>
+      let proxy_args: []str = [
+        "xdg-dbus-proxy",
+        "--filter",
+      ];
+      for (let spec .. dbus.talk) {
+        append(proxy_args, "--talk");
+        append(proxy_args, spec);
+      };
+      for (let spec .. dbus.own) {
+        append(proxy_args, "--own");
+        append(proxy_args, spec);
+      };
+      errors::ext::check(
+        "dbus_restrict: invoke xdg-dbus-proxy",
+        rt::ext::execvpe_or_default(
+          config::XDG_DBUS_PROXY,
+          "xdg-dbus-proxy",
+          proxy_args,
+          os::getenvs(),
+        ),
+      );
+    case void =>
+      // xdg-dbus-proxy provides no explicit synchronization mechanism;
+      // we have to assume that once we fork, it starts listening "reasonably quickly".
+      // TODO: i could probably hack some synchronization in here:
+      // - `write` a HELLO message and block until it's read.
+      // - invoke with `--log` and parse the output.
+      // - or maybe pre-create the unix socket _before_ forking here, and
+      //   somehow pass that into xdg-dbus-proxy instead of letting it create
+      //   its own socket.
+      void;
+  };
+};
+
 
 fn parse_dbus_address(address: (str | void)) (str | errors::invalid) = {
   // dbus address is specified like: