io_uring: zero iocb->ki_pos for stream file types

io_uring supports using offset == -1 for using the current file position,
and we read that in as part of read/write command setup. For the non-iter
read/write types we pass in NULL for the position pointer, but for the
iter types we should not be passing any anything but 0 for the position
for a stream.

Clear kiocb->ki_pos if the file is a stream, don't leave it as -1. If we
do, then the request will error with -ESPIPE.

Fixes: ba04291e ("io_uring: allow use of offset == -1 to mean file position")
Link: https://github.com/axboe/liburing/discussions/501Reported-by: Samuel Williams <samuel.williams@oriontransfer.co.nz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2891,9 +2891,13 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		req->flags |= io_file_get_flags(file) << REQ_F_SUPPORT_NOWAIT_BIT;
 
 	kiocb->ki_pos = READ_ONCE(sqe->off);
-	if (kiocb->ki_pos == -1 && !(file->f_mode & FMODE_STREAM)) {
+	if (kiocb->ki_pos == -1) {
+		if (!(file->f_mode & FMODE_STREAM)) {
 			req->flags |= REQ_F_CUR_POS;
 			kiocb->ki_pos = file->f_pos;
+		} else {
+			kiocb->ki_pos = 0;
+		}
 	}
 	kiocb->ki_flags = iocb_flags(file);
 	ret = kiocb_set_rw_flags(kiocb, READ_ONCE(sqe->rw_flags));