io_uring: do not clamp read length for multishot read

When doing a multishot read, the code path reuses the old read
paths. However this breaks an assumption built into those paths,
namely that struct io_rw::len is available for reuse by __io_import_iovec.

For multishot this results in len being set for the first receive
call, and then subsequent calls are clamped to that buffer length
incorrectly.

Instead keep len as zero after recycling buffers, to reuse the full
buffer size of the next selected buffer.

Fixes: fc68fcda ("io_uring/rw: add support for IORING_OP_READ_MULTISHOT")
Signed-off-by: Dylan Yudaken <dyudaken@gmail.com>
Link: https://lore.kernel.org/r/20231106203909.197089-4-dyudaken@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rw.c

@@ -912,6 +912,7 @@ int io_read(struct io_kiocb *req, unsigned int issue_flags)
 
 int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
 {
+	struct io_rw *rw = io_kiocb_to_cmd(req, struct io_rw);
 	unsigned int cflags = 0;
 	int ret;
 
@@ -928,7 +929,12 @@ int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
 	 * handling arm it.
 	 */
 	if (ret == -EAGAIN) {
-		io_kbuf_recycle(req, issue_flags);
+		/*
+		 * Reset rw->len to 0 again to avoid clamping future mshot
+		 * reads, in case the buffer size varies.
+		 */
+		if (io_kbuf_recycle(req, issue_flags))
+			rw->len = 0;
 		return -EAGAIN;
 	}
 
@@ -941,6 +947,7 @@ int io_read_mshot(struct io_kiocb *req, unsigned int issue_flags)
 		 * jump to the termination path. This request is then done.
 		 */
 		cflags = io_put_kbuf(req, issue_flags);
+		rw->len = 0; /* similarly to above, reset len to 0 */
 
 		if (io_fill_cqe_req_aux(req,
 					issue_flags & IO_URING_F_COMPLETE_DEFER,