Re: [PATCH net-next] net: mctp: use nlmsg_payload() for netlink message data extraction

[Simon Horman <horms@kernel.org>]

On Tue, May 20, 2025 at 03:02:10PM +0800, Jeremy Kerr wrote:
> Jakub suggests:
> 
> > I have a different request :) Matt, once this ends up in net-next
> > (end of this week) could you refactor it to use nlmsg_payload() ?
> > It doesn't exist in net but this is exactly why it was added.
> 
> This refactors the additions to both mctp_dump_addrinfo(), and
> mctp_rtm_getneigh() - two cases where we're calling nlh_data() on an
> an incoming netlink message, without a prior nlmsg_parse().
> 
> Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
> ---
>  net/mctp/device.c | 4 ++--
>  net/mctp/neigh.c  | 5 ++++-
>  2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/net/mctp/device.c b/net/mctp/device.c
> index 7c0dcf3df3196207af6e1a1c002f388265c49fa1..4d404edd7446e187dd3aa18ee2086c4e2e3da3ee 100644
> --- a/net/mctp/device.c
> +++ b/net/mctp/device.c
> @@ -120,8 +120,8 @@ static int mctp_dump_addrinfo(struct sk_buff *skb, struct netlink_callback *cb)
>  	int ifindex = 0, rc;
>  
>  	/* Filter by ifindex if a header is provided */
> -	if (cb->nlh->nlmsg_len >= nlmsg_msg_size(sizeof(*hdr))) {
> -		hdr = nlmsg_data(cb->nlh);
> +	hdr = nlmsg_payload(cb->nlh, sizeof(*hdr));
> +	if (hdr) {
>  		ifindex = hdr->ifa_index;
>  	} else {
>  		if (cb->strict_check) {

Hi Jeremy,

This looks like a refactor, as per the commit message.
All good.

> diff --git a/net/mctp/neigh.c b/net/mctp/neigh.c
> index 590f642413e4ef113a1a9fa96cb548b98cb55621..05b899f22d902b275ca1e300542a8d546d59ea15 100644
> --- a/net/mctp/neigh.c
> +++ b/net/mctp/neigh.c
> @@ -250,7 +250,10 @@ static int mctp_rtm_getneigh(struct sk_buff *skb, struct netlink_callback *cb)
>  		int idx;
>  	} *cbctx = (void *)cb->ctx;
>  
> -	ndmsg = nlmsg_data(cb->nlh);
> +	ndmsg = nlmsg_payload(cb->nlh, sizeof(*ndmsg));
> +	if (!ndmsg)
> +		return -EINVAL;
> +

But is this one a bug fix?

>  	req_ifindex = ndmsg->ndm_ifindex;
>  
>  	idx = 0;
> 
> ---
> base-commit: f685204c57e87d2a88b159c7525426d70ee745c9
> change-id: 20250520-mctp-nlmsg-payload-0711973470bf
> 
> Best regards,
> -- 
> Jeremy Kerr <jk@codeconstruct.com.au>
>