Re: [PATCH] io_uring/net: disable partial retries for recvmsg with cmsg

[Stefan Metzmacher <metze@samba.org>]

Hi Jens,

> We cannot sanely handle partial retries for recvmsg if we have cmsg
> attached. If we don't, then we'd just be overwriting the initial cmsg
> header on retries. Alternatively we could increment and handle this
> appropriately, but it doesn't seem worth the complication.
> 
> Link: https://lore.kernel.org/io-uring/0b0d4411-c8fd-4272-770b-e030af6919a0@kernel.dk/
> Cc: stable@vger.kernel.org # 5.10+
> Reported-by: Stefan Metzmacher <metze@samba.org>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> diff --git a/io_uring/net.c b/io_uring/net.c
> index fe1c478c7dec..6674a0759390 100644
> --- a/io_uring/net.c
> +++ b/io_uring/net.c
> @@ -788,7 +788,8 @@ int io_recvmsg(struct io_kiocb *req, unsigned int issue_flags)
>   	flags = sr->msg_flags;
>   	if (force_nonblock)
>   		flags |= MSG_DONTWAIT;
> -	if (flags & MSG_WAITALL)
> +	/* disable partial retry for recvmsg with cmsg attached */
> +	if (flags & MSG_WAITALL && !kmsg->controllen)
>   		min_ret = iov_iter_count(&kmsg->msg.msg_iter);

Isn't kmsg->controllen only used for REQ_F_APOLL_MULTISHOT?

I guess the correct value would be kmsg->msg.msg_controllen?

Maybe the safest change would be something like this (completely untested!):

diff --git a/io_uring/net.c b/io_uring/net.c
index 89e839013837..1dd5322fb732 100644
--- a/io_uring/net.c
+++ b/io_uring/net.c
@@ -781,14 +781,14 @@ int io_recvmsg(struct io_kiocb *req, unsigned int issue_flags)
  	flags = sr->msg_flags;
  	if (force_nonblock)
  		flags |= MSG_DONTWAIT;
-	if (flags & MSG_WAITALL)
-		min_ret = iov_iter_count(&kmsg->msg.msg_iter);

  	kmsg->msg.msg_get_inq = 1;
  	if (req->flags & REQ_F_APOLL_MULTISHOT)
  		ret = io_recvmsg_multishot(sock, sr, kmsg, flags,
  					   &mshot_finished);
  	else
+		if (flags & MSG_WAITALL && !kmsg->msg.msg_controllen)
+			min_ret = iov_iter_count(&kmsg->msg.msg_iter);
  		ret = __sys_recvmsg_sock(sock, &kmsg->msg, sr->umsg,
  					 kmsg->uaddr, flags);


metze