vfs: Remove vfs.DefaultDirectoryFD from embedding vfs.DefaultFD.

This fixes the implementation ambiguity issues when a filesystem
implementation embeds vfs.DefaultDirectoryFD to its directory FD along
with an internal common fileDescription utility.

For similar reasons also removes FileDescriptionDefaultImpl from
DynamicBytesFileDescriptionImpl.

PiperOrigin-RevId: 263795513

2 files changed, 20 insertions, 8 deletions

diff --git a/pkg/sentry/vfs/file_description_impl_util.go b/pkg/sentry/vfs/file_description_impl_util.go
index 4d1d447ed..ba230da72 100644
--- a/pkg/sentry/vfs/file_description_impl_util.go
+++ b/pkg/sentry/vfs/file_description_impl_util.go
@@ -28,6 +28,16 @@ import (
 	"gvisor.dev/gvisor/pkg/waiter"
 )
 
+// The following design pattern is strongly recommended for filesystem
+// implementations to adapt:
+//   - Have a local fileDescription struct (containing FileDescription) which
+//     embeds FileDescriptionDefaultImpl and overrides the default methods
+//     which are common to all fd implementations for that for that filesystem
+//     like StatusFlags, SetStatusFlags, Stat, SetStat, StatFS, etc.
+//   - This should be embedded in all file description implementations as the
+//     first field by value.
+//   - Directory FDs would also embed DirectoryFileDescriptionDefaultImpl.
+
 // FileDescriptionDefaultImpl may be embedded by implementations of
 // FileDescriptionImpl to obtain implementations of many FileDescriptionImpl
 // methods with default behavior analogous to Linux's.
@@ -119,11 +129,8 @@ func (FileDescriptionDefaultImpl) Ioctl(ctx context.Context, uio usermem.IO, arg
 
 // DirectoryFileDescriptionDefaultImpl may be embedded by implementations of
 // FileDescriptionImpl that always represent directories to obtain
-// implementations of non-directory I/O methods that return EISDIR, and
-// implementations of other methods consistent with FileDescriptionDefaultImpl.
-type DirectoryFileDescriptionDefaultImpl struct {
-	FileDescriptionDefaultImpl
-}
+// implementations of non-directory I/O methods that return EISDIR.
+type DirectoryFileDescriptionDefaultImpl struct{}
 
 // PRead implements FileDescriptionImpl.PRead.
 func (DirectoryFileDescriptionDefaultImpl) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts ReadOptions) (int64, error) {
@@ -153,8 +160,6 @@ func (DirectoryFileDescriptionDefaultImpl) Write(ctx context.Context, src userme
 // DynamicBytesFileDescriptionImpl.SetDataSource() must be called before first
 // use.
 type DynamicBytesFileDescriptionImpl struct {
-	FileDescriptionDefaultImpl
-
 	data     DynamicBytesSource // immutable
 	mu       sync.Mutex         // protects the following fields
 	buf      bytes.Buffer
diff --git a/pkg/sentry/vfs/file_description_impl_util_test.go b/pkg/sentry/vfs/file_description_impl_util_test.go
index cef7e3ce5..511b829fc 100644
--- a/pkg/sentry/vfs/file_description_impl_util_test.go
+++ b/pkg/sentry/vfs/file_description_impl_util_test.go
@@ -29,11 +29,18 @@ import (
 	"gvisor.dev/gvisor/pkg/syserror"
 )
 
+// fileDescription is the common fd struct which a filesystem implementation
+// embeds in all of its file description implementations as required.
+type fileDescription struct {
+	vfsfd FileDescription
+	FileDescriptionDefaultImpl
+}
+
 // genCountFD is a read-only FileDescriptionImpl representing a regular file
 // that contains the number of times its DynamicBytesSource.Generate()
 // implementation has been called.
 type genCountFD struct {
-	vfsfd FileDescription
+	fileDescription
 	DynamicBytesFileDescriptionImpl
 
 	count uint64 // accessed using atomic memory ops