Re: [PATCH 3/4] io_uring/net: move send zc fixed buffer import to issue path

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/22/24 14:32, Jens Axboe wrote:
> Let's keep it close with the actual import, there's no reason to do this
> on the prep side. With that, we can drop one of the branches checking
> for whether or not IORING_RECVSEND_FIXED_BUF is set.
> 
> As a side-effect, get rid of req->imu usage.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>   io_uring/net.c | 29 ++++++++++++++++-------------
>   1 file changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/io_uring/net.c b/io_uring/net.c
> index 18507658a921..a5b875a40bbf 100644
> --- a/io_uring/net.c
> +++ b/io_uring/net.c
> @@ -76,6 +76,7 @@ struct io_sr_msg {
>   	/* initialised and used only by !msg send variants */
>   	u16				addr_len;
>   	u16				buf_group;
> +	u16				buf_index;

There is req->buf_index we can use

>   	void __user			*addr;
>   	void __user			*msg_control;
>   	/* used only for send zerocopy */
> @@ -1254,16 +1255,6 @@ int io_send_zc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>   		}
>   	}
>   
> -	if (zc->flags & IORING_RECVSEND_FIXED_BUF) {
> -		unsigned idx = READ_ONCE(sqe->buf_index);
> -
> -		if (unlikely(idx >= ctx->nr_user_bufs))
> -			return -EFAULT;
> -		idx = array_index_nospec(idx, ctx->nr_user_bufs);
> -		req->imu = READ_ONCE(ctx->user_bufs[idx]);
> -		io_req_set_rsrc_node(notif, ctx, 0);
> -	}
> -
>   	if (req->opcode == IORING_OP_SEND_ZC) {
>   		if (READ_ONCE(sqe->__pad3[0]))
>   			return -EINVAL;
> @@ -1279,6 +1270,7 @@ int io_send_zc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>   	zc->buf = u64_to_user_ptr(READ_ONCE(sqe->addr));
>   	zc->len = READ_ONCE(sqe->len);
>   	zc->msg_flags = READ_ONCE(sqe->msg_flags) | MSG_NOSIGNAL | MSG_ZEROCOPY;
> +	zc->buf_index = READ_ONCE(sqe->buf_index);
>   	if (zc->msg_flags & MSG_DONTWAIT)
>   		req->flags |= REQ_F_NOWAIT;
>   
> @@ -1339,13 +1331,24 @@ static int io_sg_from_iter(struct sk_buff *skb,
>   	return ret;
>   }
>   
> -static int io_send_zc_import(struct io_kiocb *req, struct io_async_msghdr *kmsg)
> +static int io_send_zc_import(struct io_kiocb *req, unsigned int issue_flags)
>   {
>   	struct io_sr_msg *sr = io_kiocb_to_cmd(req, struct io_sr_msg);
> +	struct io_async_msghdr *kmsg = req->async_data;
>   	int ret;
>   
>   	if (sr->flags & IORING_RECVSEND_FIXED_BUF) {
> -		ret = io_import_fixed(ITER_SOURCE, &kmsg->msg.msg_iter, req->imu,
> +		struct io_ring_ctx *ctx = req->ctx;
> +		struct io_mapped_ubuf *imu;
> +		int idx;
> +
> +		if (unlikely(sr->buf_index >= ctx->nr_user_bufs))
> +			return -EFAULT;
> +		idx = array_index_nospec(sr->buf_index, ctx->nr_user_bufs);
> +		imu = READ_ONCE(ctx->user_bufs[idx]);
> +		io_req_set_rsrc_node(sr->notif, ctx, issue_flags);

This entire section should be under uring_lock. First, we're looking
at a imu that can be freed at any moment because io_req_set_rsrc_node()
is done after. And even if change the order nothing guarantees that the
CPU sees the imu content right.

FWIW, seems nobody was passing non-zero flags to io_req_set_rsrc_node()
before this series, we should just kill the parameter.

> +
> +		ret = io_import_fixed(ITER_SOURCE, &kmsg->msg.msg_iter, imu,
>   					(u64)(uintptr_t)sr->buf, sr->len);
>   		if (unlikely(ret))
>   			return ret;
> @@ -1382,7 +1385,7 @@ int io_send_zc(struct io_kiocb *req, unsigned int issue_flags)
>   		return -EAGAIN;
>   
>   	if (!zc->done_io) {
> -		ret = io_send_zc_import(req, kmsg);
> +		ret = io_send_zc_import(req, issue_flags);
>   		if (unlikely(ret))
>   			return ret;
>   	}

-- 
Pavel Begunkov