WIP TOFU socket ownership

client/cmd/peercred_linux.go

@@ -0,0 +1,35 @@
+//go:build linux
+
+package cmd
+
+import (
+	"fmt"
+	"net"
+
+	"golang.org/x/sys/unix"
+)
+
+// peerUID returns the uid of the process on the other end of a unix socket
+// connection, read from the kernel via SO_PEERCRED.
+func peerUID(c net.Conn) (int, error) {
+	uc, ok := c.(*net.UnixConn)
+	if !ok {
+		return 0, fmt.Errorf("connection is not a unix socket: %T", c)
+	}
+	raw, err := uc.SyscallConn()
+	if err != nil {
+		return 0, fmt.Errorf("raw conn: %w", err)
+	}
+
+	var cred *unix.Ucred
+	var credErr error
+	if err := raw.Control(func(fd uintptr) {
+		cred, credErr = unix.GetsockoptUcred(int(fd), unix.SOL_SOCKET, unix.SO_PEERCRED)
+	}); err != nil {
+		return 0, fmt.Errorf("getsockopt control: %w", err)
+	}
+	if credErr != nil {
+		return 0, fmt.Errorf("SO_PEERCRED: %w", credErr)
+	}
+	return int(cred.Uid), nil
+}