bunpen: add prototype xdg-dbus-proxy to git

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

@@ -0,0 +1,99 @@
+// vim: set shiftwidth=2 :
+//
+// xdg-dbus-proxy sits between a dbus client and a dbus bus and selectively
+// forwards messages between the two.
+//
+// intended xdg-dbus-proxy use looks like:
+// 1. invoke `xdg-dbus-proxy unix:path=/unfiltered/bus /filtered/bus --filter --talk=org.foo.Bar --own=org.baz.Bap`
+// 2. in a different process, restrict mount namespace so that `/unfiltered/bus` isn't viewable.
+//    set the environment variable: DBUS_SESSION_BUS_ADDRESS=unix:path=/filtered/bus.
+//    execute your dbus client.
+// 3. the resulting setup is one where the client may:
+//    - talk to whatever owns `org.foo.Bar`.
+//    - see `org.foo.Bar` on the bus, and see the bus itself, but not see any
+//    other dbus names.
+//    - own the dbus name `org.baz.Bap`.
+//
+// proof-of-concept minimal:
+// ```sh
+// $ xdg-dbus-proxy unix:path=/run/user/colin/dbus/bus /run/user/colin/dbus/proxy-1 --filter &
+// $ DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/colin/dbus/proxy-1 busctl --user list
+// # should show *only* the bus, and no clients
+// ```
+//
+// proof-of-concept TALK:
+// ```sh
+// $ xdg-dbus-proxy unix:path=/run/user/colin/dbus/bus /run/user/colin/dbus/proxy-1 --talk=org.gnome.Calls --filter &
+// $ DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/colin/dbus/proxy-1 gdbus call --session --dest=org.gnome.Calls --object-path /org/gnome/Calls --method org.gtk.Application.Activate '[]'
+// # the gnome-calls window should now be highlighted (assuming that's how it responds to an activation request)
+// # take away `--talk=org.gnome.Calls`, and the final step should get EPERM instead
+// ```
+//
+// proof-of-concept CALL (more highly locked down):
+// ```sh
+// $ xdg-dbus-proxy unix:path=/run/user/colin/dbus/bus /run/user/colin/dbus/proxy-1 --filter '--call=org.gnome.Calls=org.gtk.Application.Activate@/org/gnome/Calls' '--call=org.gnome.Calls=org.freedesktop.DBus.Introspectable.Introspect@/org/gnome/Calls'
+// $ DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/colin/dbus/proxy-1 gdbus call --session --dest=org.gnome.Calls --object-path /org/gnome/Calls --method org.gtk.Application.Activate '[]'
+// # works.
+// # remove either of the `--call` options, and it fails
+// ```
+//
+// EDGE-CASES / FAQ:
+// - run `xdg-dbus-proxy` with `--log` argument to dump intercepted traffiic
+// - without `--filter`, the proxy allows ALL traffic (useless!)
+// - with `--filter`, and no other flags, proxy denies all messages (except messages to the bus)
+// - `--talk=org.gnome.Calls` implies `--see=org.gnome.Calls`
+// - `--own=org.gnome.Calls` implies `--talk=org.gnome.Calls` (and --see=...)
+// - `--talk=org.gnome.Calls` allows calling *any* method on the object which owns org.gnome.Calls
+// - `--call=NAME=INTERFACE.METHOD@/PATH` implies some *subset* of `--talk=NAME`, as necessary,
+//    as well it implies `--see=NAME`.
+//    - 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 errors;
+// use errors::ext;
+// 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);
+//   };
+// };
+// 
+// fn get_dbus_socket() (str | errors::error) = {
+//   // dbus address is specified like:
+//   // DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/colin/dbus/bus
+//   return match (os::getenv("DBUS_SESSION_BUS_ADDRESS")) {
+//     case void => yield errors::invalid;
+//     case let value: str => 
+//       let expected_prefix = "unix:path=";
+//       if (!strings::hasprefix(value, expected_prefix)) {
+//         yield errors::invalid;
+//       };
+//       value = strings::sub(value, len(expected_prefix));
+//       if (!strings::hasprefix(value, "/")) {
+//         // expect the dbus bus address to be an absolute path
+//         // TODO: consider parsing this as an actual path?
+//         yield errors::invalid;
+//       };
+//       yield value;
+//   };
+// };