io_uring: fix using under-expanded iters

[ upstream commit cd65869 ]

The issue was first described and addressed in
89c2b3b ("io_uring: reexpand under-reexpanded iters"), but
shortly after reimplemented as.
cd65869 ("io_uring: use iov_iter state save/restore helpers").

Here we follow the approach from the second patch but without in-callback
resubmissions, fixups for not yet supported in 5.10 short read retries
and replacing iov_iter_state with iter copies to not pull even more
dependencies, and because it's just much simpler.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/io_uring.c

@@ -3389,6 +3389,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
 	struct kiocb *kiocb = &req->rw.kiocb;
 	struct iov_iter __iter, *iter = &__iter;
+	struct iov_iter iter_cp;
 	struct io_async_rw *rw = req->async_data;
 	ssize_t io_size, ret, ret2;
 	bool no_async;
@@ -3399,6 +3400,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 	ret = io_import_iovec(READ, req, &iovec, iter, !force_nonblock);
 	if (ret < 0)
 		return ret;
+	iter_cp = *iter;
 	io_size = iov_iter_count(iter);
 	req->result = io_size;
 	ret = 0;
@@ -3434,7 +3436,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
 		if (req->file->f_flags & O_NONBLOCK)
 			goto done;
 		/* some cases will consume bytes even on error returns */
-		iov_iter_revert(iter, io_size - iov_iter_count(iter));
+		*iter = iter_cp;
 		ret = 0;
 		goto copy_iov;
 	} else if (ret < 0) {
@@ -3517,6 +3519,7 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
 	struct kiocb *kiocb = &req->rw.kiocb;
 	struct iov_iter __iter, *iter = &__iter;
+	struct iov_iter iter_cp;
 	struct io_async_rw *rw = req->async_data;
 	ssize_t ret, ret2, io_size;
 
@@ -3526,6 +3529,7 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
 	ret = io_import_iovec(WRITE, req, &iovec, iter, !force_nonblock);
 	if (ret < 0)
 		return ret;
+	iter_cp = *iter;
 	io_size = iov_iter_count(iter);
 	req->result = io_size;
 
@@ -3587,7 +3591,7 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
 	} else {
 copy_iov:
 		/* some cases will consume bytes even on error returns */
-		iov_iter_revert(iter, io_size - iov_iter_count(iter));
+		*iter = iter_cp;
 		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
 		if (!ret)
 			return -EAGAIN;