netdev - Re: [PATCH net-next] docs: netlink: clarify the historical baggage of Netlink flags

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Date:   Wed, 28 Sep 2022 17:46:28 +0300
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     nicolas.dichtel@...nd.com,
        Florent Fourcot <florent.fourcot@...irst.fr>,
        netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com, Johannes Berg <johannes@...solutions.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Guillaume Nault <gnault@...hat.com>,
        Hangbin Liu <liuhangbin@...il.com>
Subject: Re: [PATCH net-next] docs: netlink: clarify the historical baggage of
 Netlink flags

On 28/09/2022 17:37, Jakub Kicinski wrote:
> On Wed, 28 Sep 2022 12:21:57 +0300 Nikolay Aleksandrov wrote:
>> On 28/09/2022 11:55, Nicolas Dichtel wrote:
>>> Le 28/09/2022 à 10:04, Florent Fourcot a écrit :  
>>>> About NLM_F_EXCL, I'm not sure that my comment is relevant for your intro.rst
>>>> document, but it has another usage in ipset submodule. For IPSET_CMD_DEL,
>>>> setting NLM_F_EXCL means "raise an error if entry does not exist before the
>>>> delete".  
> 
> Interesting.
> 
>>> So NLM_F_EXCL could be used with DEL command for netfilter netlink but cannot be
>>> used (it overlaps with NLM_F_BULK, see [1]) with DEL command for rtnetlink.
>>> Sigh :(
>>>
>>> [1] https://lore.kernel.org/netdev/0198618f-7b52-3023-5e9f-b38c49af1677@6wind.com/
>>
>> One could argue that's abuse of the api, but since it's part of a different family
>> I guess it's ok. NLM_F_EXCL is a modifier of NEW cmd as the comment above it says
>> and has never had rtnetlink DEL users.
> 
> It's fine in the sense that it works, but it's rather pointless to call
> the flags common if they have different semantics depending on the
> corner of the kernel they are used in, right?
> 

Right, and their comments and docs become kind of meaningless because of that.

> I was very tempted to send a patch which would validate the top
> byte of flags in genetlink for new commands, this way we may some day
> find a truly common (as in enforced by the code) use for the bits. 
> 
> WDYT?
> 

I like it, can't check right now if we can get into the same issue as with BULK where
someone is passing unused/wrong flags with the command and we break him though.
But I'd bite the bullet and maybe issue an extack msg as well.

> diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
> index 7c136de117eb..0fbaed49e23f 100644
> --- a/net/netlink/genetlink.c
> +++ b/net/netlink/genetlink.c
> @@ -739,6 +739,22 @@ static int genl_family_rcv_msg_doit(const struct genl_family *family,
>  	return err;
>  }
>  
> +static int genl_header_check(struct nlmsghdr *nlh, struct genlmsghdr *hdr)
> +{
> +	if (hdr->reserved)
> +		return -EINVAL;
> +
> +	/* Flags - lower byte check */
> +	if (nlh->nlmsg_flags & 0xff & ~(NLM_F_REQUEST | NLM_F_ACK | NLM_F_ECHO))
> +		return -EINVAL;
> +	/* Flags - higher byte check */
> +	if (nlh->nlmsg_flags & 0xff00 &&
> +	    nlh->nlmsg_flags & 0xff00 != NLM_F_DUMP)
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
>  static int genl_family_rcv_msg(const struct genl_family *family,
>  			       struct sk_buff *skb,
>  			       struct nlmsghdr *nlh,
> @@ -757,7 +773,7 @@ static int genl_family_rcv_msg(const struct genl_family *family,
>  	if (nlh->nlmsg_len < nlmsg_msg_size(hdrlen))
>  		return -EINVAL;
>  
> -	if (hdr->cmd >= family->resv_start_op && hdr->reserved)
> +	if (hdr->cmd >= family->resv_start_op && genl_header_check(nlh, hdr))
>  		return -EINVAL;
>  
>  	if (genl_get_cmd(hdr->cmd, family, &op))