Kernel/Devices: Introduce the LoopDevice device

This device is a block device that allows a user to effectively treat an
Inode as a block device.

The static construction method is given an OpenFileDescription reference
but validates that:
- The description has a valid custody (so it's not some arbitrary file).
  Failing this requirement will yield EINVAL.
- The description custody points to an Inode which is a regular file, as
  we only support (seekable) regular files. Failing this requirement
  will yield ENOTSUP.

LoopDevice can be used to mount a regular file on the filesystem like
other supported types of (physical) block devices.

Kernel/FileSystem/Ext2FS/FileSystem.h

@@ -47,6 +47,7 @@ public:
     virtual unsigned free_inode_count() const override;
 
     virtual bool supports_watchers() const override { return true; }
+    virtual bool supports_backing_loop_devices() const override { return true; }
 
     virtual u8 internal_file_type_to_directory_entry_type(DirectoryEntryView const& entry) const override;
 

Kernel/FileSystem/File.h

@@ -115,6 +115,7 @@ public:
     virtual bool is_socket() const { return false; }
     virtual bool is_inode_watcher() const { return false; }
     virtual bool is_mount_file() const { return false; }
+    virtual bool is_loop_device() const { return false; }
 
     virtual bool is_regular_file() const { return false; }
 

Kernel/FileSystem/FileSystem.h

@@ -34,6 +34,11 @@ public:
     virtual Inode& root_inode() = 0;
     virtual bool supports_watchers() const { return false; }
 
+    // FIXME: We should aim to provide more concise mechanism to ensure
+    // that backing Inodes from the FileSystem are kept intact so we can
+    // attach them to a loop device.
+    virtual bool supports_backing_loop_devices() const { return false; }
+
     bool is_readonly() const { return m_readonly; }
 
     virtual unsigned total_block_count() const { return 0; }

Kernel/FileSystem/RAMFS/FileSystem.h

@@ -24,6 +24,7 @@ public:
     virtual StringView class_name() const override { return "RAMFS"sv; }
 
     virtual bool supports_watchers() const override { return true; }
+    virtual bool supports_backing_loop_devices() const override { return true; }
 
     virtual Inode& root_inode() override;
 

Kernel/FileSystem/SysFS/Subsystems/Kernel/DiskUsage.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/JsonObjectSerializer.h>
+#include <Kernel/Devices/Loop/LoopDevice.h>
 #include <Kernel/FileSystem/FileBackedFileSystem.h>
 #include <Kernel/FileSystem/SysFS/Subsystems/Kernel/DiskUsage.h>
 #include <Kernel/FileSystem/VirtualFileSystem.h>
@@ -40,8 +41,15 @@ ErrorOr<void> SysFSDiskUsage::try_generate(KBufferBuilder& builder)
         TRY(fs_object.add("mount_flags"sv, mount.flags()));
 
         if (fs.is_file_backed()) {
-            auto pseudo_path = TRY(static_cast<const FileBackedFileSystem&>(fs).file_description().pseudo_path());
-            TRY(fs_object.add("source"sv, pseudo_path->view()));
+            auto& file = static_cast<const FileBackedFileSystem&>(fs).file();
+            if (file.is_loop_device()) {
+                auto& device = static_cast<LoopDevice const&>(file);
+                auto path = TRY(device.custody().try_serialize_absolute_path());
+                TRY(fs_object.add("source"sv, path->view()));
+            } else {
+                auto pseudo_path = TRY(static_cast<const FileBackedFileSystem&>(fs).file_description().pseudo_path());
+                TRY(fs_object.add("source"sv, pseudo_path->view()));
+            }
         } else {
             TRY(fs_object.add("source"sv, "none"));
         }

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -14,6 +14,7 @@
 #include <Kernel/Debug.h>
 #include <Kernel/Devices/BlockDevice.h>
 #include <Kernel/Devices/DeviceManagement.h>
+#include <Kernel/Devices/Loop/LoopDevice.h>
 #include <Kernel/FileSystem/Custody.h>
 #include <Kernel/FileSystem/FileBackedFileSystem.h>
 #include <Kernel/FileSystem/FileSystem.h>
@@ -213,6 +214,12 @@ ErrorOr<void> VirtualFileSystem::mount(MountFile& mount_file, OpenFileDescriptio
             }));
             TRY(fs->initialize());
         }
+        if (source_description->file().is_loop_device()) {
+            auto& device = static_cast<LoopDevice&>(source_description->file());
+            auto path = TRY(device.custody().try_serialize_absolute_path());
+            dbgln("VirtualFileSystem: mounting from loop device {}, originated from {}", device.index(), path->view());
+        }
+
         TRY(add_file_system_to_mount_table(*fs, mount_point, flags));
         list.append(static_cast<FileBackedFileSystem&>(*fs));
         return {};