fuse: fix the ->direct_IO() treatment of iov_iter

the callers rely upon having any iov_iter_truncate() done inside
->direct_IO() countered by iov_iter_reexpand().

Reported-by: Qian Cai <cai@redhat.com>
Tested-by: Qian Cai <cai@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/fuse/file.c

@@ -3091,29 +3091,20 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	ssize_t ret = 0;
 	struct file *file = iocb->ki_filp;
 	struct fuse_file *ff = file->private_data;
-	bool async_dio = ff->fc->async_dio;
 	loff_t pos = 0;
 	struct inode *inode;
 	loff_t i_size;
-	size_t count = iov_iter_count(iter);
+	size_t count = iov_iter_count(iter), shortened = 0;
 	loff_t offset = iocb->ki_pos;
 	struct fuse_io_priv *io;
 
 	pos = offset;
 	inode = file->f_mapping->host;
 	i_size = i_size_read(inode);
 
-	if ((iov_iter_rw(iter) == READ) && (offset > i_size))
+	if ((iov_iter_rw(iter) == READ) && (offset >= i_size))
 		return 0;
 
-	/* optimization for short read */
-	if (async_dio && iov_iter_rw(iter) != WRITE && offset + count > i_size) {
-		if (offset >= i_size)
-			return 0;
-		iov_iter_truncate(iter, fuse_round_up(ff->fc, i_size - offset));
-		count = iov_iter_count(iter);
-	}
-
 	io = kmalloc(sizeof(struct fuse_io_priv), GFP_KERNEL);
 	if (!io)
 		return -ENOMEM;
@@ -3129,15 +3120,22 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	 * By default, we want to optimize all I/Os with async request
 	 * submission to the client filesystem if supported.
 	 */
-	io->async = async_dio;
+	io->async = ff->fc->async_dio;
 	io->iocb = iocb;
 	io->blocking = is_sync_kiocb(iocb);
 
+	/* optimization for short read */
+	if (io->async && !io->write && offset + count > i_size) {
+		iov_iter_truncate(iter, fuse_round_up(ff->fc, i_size - offset));
+		shortened = count - iov_iter_count(iter);
+		count -= shortened;
+	}
+
 	/*
 	 * We cannot asynchronously extend the size of a file.
 	 * In such case the aio will behave exactly like sync io.
 	 */
-	if ((offset + count > i_size) && iov_iter_rw(iter) == WRITE)
+	if ((offset + count > i_size) && io->write)
 		io->blocking = true;
 
 	if (io->async && io->blocking) {
@@ -3155,6 +3153,7 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	} else {
 		ret = __fuse_direct_read(io, iter, &pos);
 	}
+	iov_iter_reexpand(iter, iov_iter_count(iter) + shortened);
 
 	if (io->async) {
 		bool blocking = io->blocking;