Utilities: Add fusermount

This only contains the bare minimum amount of functionality required
by libfuse.

Userland/Utilities/fusermount.cpp

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2024, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/ByteString.h>
+#include <AK/Error.h>
+#include <Kernel/API/FileSystem/MountSpecificFlags.h>
+#include <Kernel/API/Ioctl.h>
+#include <LibCore/ArgsParser.h>
+#include <LibCore/System.h>
+#include <LibMain/Main.h>
+
+static ErrorOr<void> set_mount_flag(ByteString key, u64 value, int mount_fd)
+{
+    MountSpecificFlag flag;
+    flag.key_string_length = key.bytes().size();
+    flag.key_string_addr = key.bytes().data();
+    flag.value_type = MountSpecificFlag::ValueType::UnsignedInteger;
+    flag.value_length = 8;
+    flag.value_addr = &value;
+
+    return Core::System::ioctl(mount_fd, MOUNT_IOCTL_SET_MOUNT_SPECIFIC_FLAG, &flag);
+}
+
+ErrorOr<int> serenity_main(Main::Arguments arguments)
+{
+    StringView fd_string;
+    StringView target;
+
+    Core::ArgsParser args_parser;
+    args_parser.set_general_help("Mount a FUSE-based filesystem");
+    args_parser.add_positional_argument(fd_string, "File descriptor to mount", "fd");
+    args_parser.add_positional_argument(target, "Path to mount location", "target");
+    args_parser.parse(arguments);
+
+    if (fd_string.is_empty())
+        return Error::from_string_literal("No file descriptor passed");
+
+    if (target.is_empty())
+        return Error::from_string_literal("No target passed");
+
+    auto maybe_fd = fd_string.to_number<int>();
+    if (!maybe_fd.has_value())
+        return Error::from_string_literal("Invalid file descriptor passed");
+
+    int fd = maybe_fd.release_value();
+
+    int mount_fd = TRY(Core::System::fsopen("FUSE"sv, 0));
+
+    TRY(set_mount_flag("fd", fd, mount_fd));
+    TRY(set_mount_flag("rootmode", 40000, mount_fd));
+
+    TRY(Core::System::fsmount(mount_fd, -1, target));
+
+    return 0;
+}