fuse: return patrial success from fuse_direct_io()

If a user calls writev/readv in direct io mode with partially valid data
in the iovec array such that any vector other than the first one in the
array contains invalid data, we currently return the error for the invalid
iovec.

Instead, we should return the number of bytes already written/read and not
the error as we do in the non direct_io case.
Reported-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: Ashish Samant <ashish.samant@oracle.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -1247,6 +1247,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 			       size_t *nbytesp, int write)
 {
 	size_t nbytes = 0;  /* # bytes already packed in req */
+	ssize_t ret = 0;
 
 	/* Special case for kernel I/O: can copy directly into the buffer */
 	if (ii->type & ITER_KVEC) {
@@ -1266,13 +1267,12 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 	while (nbytes < *nbytesp && req->num_pages < req->max_pages) {
 		unsigned npages;
 		size_t start;
-		ssize_t ret = iov_iter_get_pages(ii,
-					&req->pages[req->num_pages],
+		ret = iov_iter_get_pages(ii, &req->pages[req->num_pages],
 					*nbytesp - nbytes,
 					req->max_pages - req->num_pages,
 					&start);
 		if (ret < 0)
-			return ret;
+			break;
 
 		iov_iter_advance(ii, ret);
 		nbytes += ret;
@@ -1295,7 +1295,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 
 	*nbytesp = nbytes;
 
-	return 0;
+	return ret;
 }
 
 static inline int fuse_iter_npages(const struct iov_iter *ii_p)
@@ -1319,6 +1319,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 	pgoff_t idx_to = (pos + count - 1) >> PAGE_CACHE_SHIFT;
 	ssize_t res = 0;
 	struct fuse_req *req;
+	int err = 0;
 
 	if (io->async)
 		req = fuse_get_req_for_background(fc, fuse_iter_npages(iter));
@@ -1339,11 +1340,9 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 		size_t nres;
 		fl_owner_t owner = current->files;
 		size_t nbytes = min(count, nmax);
-		int err = fuse_get_user_pages(req, iter, &nbytes, write);
-		if (err) {
-			res = err;
+		err = fuse_get_user_pages(req, iter, &nbytes, write);
+		if (err && !nbytes)
 			break;
-		}
 
 		if (write)
 			nres = fuse_send_write(req, io, pos, nbytes, owner);
@@ -1353,11 +1352,11 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 		if (!io->async)
 			fuse_release_user_pages(req, !write);
 		if (req->out.h.error) {
-			if (!res)
-				res = req->out.h.error;
+			err = req->out.h.error;
 			break;
 		} else if (nres > nbytes) {
-			res = -EIO;
+			res = 0;
+			err = -EIO;
 			break;
 		}
 		count -= nres;
@@ -1381,7 +1380,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 	if (res > 0)
 		*ppos = pos;
 
-	return res;
+	return res > 0 ? res : err;
 }
 EXPORT_SYMBOL_GPL(fuse_direct_io);