Subject: Re: [PATCH net-next v2 2/3] netfilter: nfnetlink: Handle ACK flags for batch messages

From: Pablo Neira Ayuso <[email-masked]>
Date: Thu, 11 Apr 2024 01:13:13 +0200

On Wed, Apr 10, 2024 at 11:11:07PM +0100, Donald Hunter wrote:
> The NLM_F_ACK flag is not processed for nfnetlink batch messages.

Let me clarify: It is not processed for the begin and end marker
netlink message, but it is processed for command messages.

> This is a problem for ynl which wants to receive an ack for every
> message it sends. Add processing for ACK and provide responses when
> requested.

NLM_F_ACK is regarded for the specific command messages that are
contained in the batch, that is:

batch begin
command
command
...
command
batch end

Thus, NLM_F_ACK can be set on for the command messages and it is not
ignore in that case.

May I ask why do you need this? Is it to make your userspace tool happy?

> I have checked that iproute2, pyroute2 and systemd are unaffected by
> this change since none of them use NLM_F_ACK for batch begin/end.
> I also ran a search on github and did not spot any usage that would
> break.
> 
> Signed-off-by: Donald Hunter <[email-masked]>
> ---
>  net/netfilter/nfnetlink.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
> index c9fbe0f707b5..37762941c288 100644
> --- a/net/netfilter/nfnetlink.c
> +++ b/net/netfilter/nfnetlink.c
> @@ -427,6 +427,9 @@ static void nfnetlink_rcv_batch(struct sk_buff *skb, struct nlmsghdr *nlh,
>  
>  	nfnl_unlock(subsys_id);
>  
> +	if (nlh->nlmsg_flags & NLM_F_ACK)
> +		nfnl_err_add(&err_list, nlh, 0, &extack);
> +
>  	while (skb->len >= nlmsg_total_size(0)) {
>  		int msglen, type;
>  
> @@ -463,6 +466,8 @@ static void nfnetlink_rcv_batch(struct sk_buff *skb, struct nlmsghdr *nlh,
>  			goto done;
>  		} else if (type == NFNL_MSG_BATCH_END) {
>  			status |= NFNL_BATCH_DONE;
> +			if (nlh->nlmsg_flags & NLM_F_ACK)
> +				nfnl_err_add(&err_list, nlh, 0, &extack);

if (status == NFNL_BATCH_DONE) should probably be a better place for
this. I would like to have userspace that uses this, I don't have a
usecase at this moment for this new code.

Thanks.

>  			goto done;
>  		} else if (type < NLMSG_MIN_TYPE) {
>  			err = -EINVAL;
> -- 
> 2.43.0
>