netdev - Re: [PATCH net-next v2] netlink: split up copies in the ack construction

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Date:   Fri, 18 Nov 2022 08:59:51 -0700
From:   David Ahern <dsahern@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>, Kees Cook <keescook@...omium.org>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
        pabeni@...hat.com, linux-hardening@...r.kernel.org
Subject: Re: [PATCH net-next v2] netlink: split up copies in the ack
 construction

On 11/17/22 5:28 PM, Jakub Kicinski wrote:
> On Thu, 17 Nov 2022 14:35:32 -0800 Kees Cook wrote:
>> As for testing, I can do that if you want -- the goal was to make sure
>> the final result doesn't trip FORTIFY when built with -fstrict-flex-arrays
>> (not yet in a released compiler version, but present in both GCC and Clang
>> truck builds) and with __builtin_dynamic_object_size() enabled (which
>> is not yet in -next, as it is waiting on the last of ksize() clean-ups).
> 
> I got distracted, sorry. Does this work?
> 
> -->8--------------
> 
> From: Jakub Kicinski <kuba@...nel.org>
> Subject: netlink: remove the flex array from struct nlmsghdr
> 
> I've added a flex array to struct nlmsghdr to allow accessing
> the data easily. But it leads to warnings with clang, when user
> space wraps this structure into another struct and the flex
> array is not at the end of the container.
> 
> Link: https://lore.kernel.org/all/20221114023927.GA685@u2004-local/
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
>  include/uapi/linux/netlink.h | 2 --
>  net/netlink/af_netlink.c     | 2 +-
>  2 files changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> index 5da0da59bf01..e2ae82e3f9f7 100644
> --- a/include/uapi/linux/netlink.h
> +++ b/include/uapi/linux/netlink.h
> @@ -48,7 +48,6 @@ struct sockaddr_nl {
>   * @nlmsg_flags: Additional flags
>   * @nlmsg_seq:   Sequence number
>   * @nlmsg_pid:   Sending process port ID
> - * @nlmsg_data:  Message payload
>   */
>  struct nlmsghdr {
>  	__u32		nlmsg_len;
> @@ -56,7 +55,6 @@ struct nlmsghdr {
>  	__u16		nlmsg_flags;
>  	__u32		nlmsg_seq;
>  	__u32		nlmsg_pid;
> -	__u8		nlmsg_data[];
>  };
>  
>  /* Flags values */
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index 9ebdf3262015..d73091f6bb0f 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -2514,7 +2514,7 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
>  		if (!nlmsg_append(skb, nlmsg_len(nlh)))
>  			goto err_bad_put;
>  
> -		memcpy(errmsg->msg.nlmsg_data, nlh->nlmsg_data,
> +		memcpy(nlmsg_data(&errmsg->msg), nlmsg_data(nlh),
>  		       nlmsg_len(nlh));
>  	}
>  

LGTM and removing the flex array fixes the iproute2 compile.
Reviewed-by: David Ahern <dsahern@...nel.org>