Remove vfs.FileDescriptionOptions.InvalidWrite.

Compare:
https://elixir.bootlin.com/linux/v5.6/source/fs/timerfd.c#L431
PiperOrigin-RevId: 310246908

pkg/sentry/syscalls/linux/vfs2/timerfd.go

@@ -46,7 +46,10 @@ func TimerfdCreate(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel
 	default:
 		return 0, nil, syserror.EINVAL
 	}
-	file, err := t.Kernel().VFS().NewTimerFD(clock, fileFlags)
+	// Timerfds aren't writable per se (their implementation of Write just
+	// returns EINVAL), but they are "opened for writing", which is necessary
+	// to actually reach said implementation of Write.
+	file, err := t.Kernel().VFS().NewTimerFD(clock, linux.O_RDWR|fileFlags)
 	if err != nil {
 		return 0, nil, err
 	}

pkg/sentry/vfs/file_description.go

@@ -91,10 +91,6 @@ type FileDescriptionOptions struct {
 	// ESPIPE.
 	DenyPWrite bool
 
-	// if InvalidWrite is true, calls to FileDescription.Write() return
-	// EINVAL.
-	InvalidWrite bool
-
 	// If UseDentryMetadata is true, calls to FileDescription methods that
 	// interact with file and filesystem metadata (Stat, SetStat, StatFS,
 	// Listxattr, Getxattr, Setxattr, Removexattr) are implemented by calling
@@ -570,9 +566,6 @@ func (fd *FileDescription) PWrite(ctx context.Context, src usermem.IOSequence, o
 
 // Write is similar to PWrite, but does not specify an offset.
 func (fd *FileDescription) Write(ctx context.Context, src usermem.IOSequence, opts WriteOptions) (int64, error) {
-	if fd.opts.InvalidWrite {
-		return 0, syserror.EINVAL
-	}
 	if !fd.writable {
 		return 0, syserror.EBADF
 	}

pkg/sentry/vfs/timerfd.go

@@ -53,7 +53,6 @@ func (vfs *VirtualFilesystem) NewTimerFD(clock ktime.Clock, flags uint32) (*File
 		UseDentryMetadata: true,
 		DenyPRead:         true,
 		DenyPWrite:        true,
-		InvalidWrite:      true,
 	}); err != nil {
 		return nil, err
 	}