OvmfPkg/VirtioFsDxe: implement EFI_SIMPLE_FILE_SYSTEM_PROTOCOL.OpenVolume()

With the help of the VirtioFsFuseOpenDir() and
VirtioFsFuseReleaseFileOrDir() functions introduced previously, we can now
open and close the root directory. So let's implement
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL.OpenVolume().

OpenVolume() creates a new EFI_FILE_PROTOCOL object -- a reference to the
root directory of the filesystem. Thus, we have to start tracking
references to EFI_SIMPLE_FILE_SYSTEM_PROTOCOL, lest we unbind the
virtio-fs device while files are open.

There are two methods that release an EFI_FILE_PROTOCOL object: the
Close() and the Delete() member functions. In particular, they are not
allowed to fail with regard to resource management -- they must release
resources unconditionally. Thus, for rolling back the resource accounting
that we do in EFI_SIMPLE_FILE_SYSTEM_PROTOCOL.OpenVolume(), we have to
implement the first versions of EFI_FILE_PROTOCOL.Close() and
EFI_FILE_PROTOCOL.Delete() in this patch as well.

With this patch applied, the UEFI shell can enter the root directory of
the Virtio Filesystem (such as with the "FS3:" shell command), and the
"DIR" shell command exercises FUSE_OPENDIR and FUSE_RELEASEDIR, according
to the virtiofsd log. The "DIR" command reports the root directory as if
it were empty; probably because at this time, we only allow the shell to
open and to close the root directory, but not to read it.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3097
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201216211125.19496-12-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

OvmfPkg/VirtioFsDxe/SimpleFsClose.c

@@ -0,0 +1,48 @@
+/** @file
+  EFI_FILE_PROTOCOL.Close() member function for the Virtio Filesystem driver.
+
+  Copyright (C) 2020, Red Hat, Inc.
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#include <Library/BaseLib.h>             // RemoveEntryList()
+#include <Library/MemoryAllocationLib.h> // FreePool()
+
+#include "VirtioFsDxe.h"
+
+EFI_STATUS
+EFIAPI
+VirtioFsSimpleFileClose (
+  IN EFI_FILE_PROTOCOL *This
+  )
+{
+  VIRTIO_FS_FILE *VirtioFsFile;
+  VIRTIO_FS      *VirtioFs;
+
+  VirtioFsFile = VIRTIO_FS_FILE_FROM_SIMPLE_FILE (This);
+  VirtioFs     = VirtioFsFile->OwnerFs;
+
+  //
+  // At this point, the implementation is only suitable for closing the
+  // VIRTIO_FS_FILE that was created by VirtioFsOpenVolume().
+  //
+  ASSERT (VirtioFsFile->IsDirectory);
+  ASSERT (VirtioFsFile->NodeId == VIRTIO_FS_FUSE_ROOT_DIR_NODE_ID);
+  //
+  // Close the root directory.
+  //
+  // Ignore any errors, because EFI_FILE_PROTOCOL.Close() is required to
+  // release the EFI_FILE_PROTOCOL object unconditionally.
+  //
+  VirtioFsFuseReleaseFileOrDir (VirtioFs, VirtioFsFile->NodeId,
+    VirtioFsFile->FuseHandle, VirtioFsFile->IsDirectory);
+
+  //
+  // One fewer file left open for the owner filesystem.
+  //
+  RemoveEntryList (&VirtioFsFile->OpenFilesEntry);
+
+  FreePool (VirtioFsFile);
+  return EFI_SUCCESS;
+}