shim: new shim implementation

This implementation of shim accepts configuration as a gob stream over a unix socket, with support for mediating access to wayland via WAYLAND_SOCKET fd. All configuration is now included in the payload, and child is started inside bwrap configured with supplied bwrap.Config.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/shim/parent.go

@@ -0,0 +1,90 @@
+package shim
+
+import (
+	"encoding/gob"
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"syscall"
+
+	"git.ophivana.moe/cat/fortify/internal/verbose"
+)
+
+// called in the parent process
+
+func ServeConfig(socket string, payload *Payload, wl string, done chan struct{}) (*net.UnixConn, error) {
+	var ws *net.UnixConn
+	if payload.WL {
+		if f, err := net.DialUnix("unix", nil, &net.UnixAddr{Name: wl, Net: "unix"}); err != nil {
+			return nil, err
+		} else {
+			verbose.Println("connected to wayland at", wl)
+			ws = f
+		}
+	}
+
+	if c, err := net.ListenUnix("unix", &net.UnixAddr{Name: socket, Net: "unix"}); err != nil {
+		return nil, err
+	} else {
+		verbose.Println("configuring shim on socket", socket)
+		if err = os.Chmod(socket, 0777); err != nil {
+			fmt.Println("fortify: cannot change permissions of shim setup socket:", err)
+		}
+
+		go func() {
+			var conn *net.UnixConn
+			if conn, err = c.AcceptUnix(); err != nil {
+				fmt.Println("fortify: cannot accept connection from shim:", err)
+			} else {
+				if err = gob.NewEncoder(conn).Encode(*payload); err != nil {
+					fmt.Println("fortify: cannot stream shim payload:", err)
+					return
+				}
+
+				if payload.WL {
+					// get raw connection
+					var rc syscall.RawConn
+					if rc, err = ws.SyscallConn(); err != nil {
+						fmt.Println("fortify: cannot obtain raw wayland connection:", err)
+						return
+					} else {
+						go func() {
+							// pass wayland socket fd
+							if err = rc.Control(func(fd uintptr) {
+								if _, _, err = conn.WriteMsgUnix(nil, syscall.UnixRights(int(fd)), nil); err != nil {
+									fmt.Println("fortify: cannot pass wayland connection to shim:", err)
+									return
+								}
+								_ = conn.Close()
+
+								// block until shim exits
+								<-done
+								verbose.Println("releasing wayland connection")
+							}); err != nil {
+								fmt.Println("fortify: cannot obtain wayland connection fd:", err)
+							}
+						}()
+					}
+				} else {
+					_ = conn.Close()
+				}
+			}
+			if err = c.Close(); err != nil {
+				fmt.Println("fortify: cannot close shim socket:", err)
+			}
+			if err = os.Remove(socket); err != nil && !errors.Is(err, os.ErrNotExist) {
+				fmt.Println("fortify: cannot remove dangling shim socket:", err)
+			}
+		}()
+		return ws, nil
+	}
+}
+
+// Try runs shim and stops execution if FORTIFY_SHIM is set.
+func Try() {
+	if s, ok := os.LookupEnv(EnvShim); ok {
+		shim(s)
+	}
+	panic("unreachable")
+}