Re: [PATCH net-next v2] genetlink: do not parse attributes for families with zero maxattr

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Thu, 10 Oct 2019 12:34:02 +0200 (CEST), Michal Kubecek wrote:
> Commit c10e6cf85e7d ("net: genetlink: push attrbuf allocation and parsing
> to a separate function") moved attribute buffer allocation and attribute
> parsing from genl_family_rcv_msg_doit() into a separate function
> genl_family_rcv_msg_attrs_parse() which, unlike the previous code, calls
> __nlmsg_parse() even if family->maxattr is 0 (i.e. the family does its own
> parsing). The parser error is ignored and does not propagate out of
> genl_family_rcv_msg_attrs_parse() but an error message ("Unknown attribute
> type") is set in extack and if further processing generates no error or
> warning, it stays there and is interpreted as a warning by userspace.
> 
> Dumpit requests are not affected as genl_family_rcv_msg_dumpit() bypasses
> the call of genl_family_rcv_msg_doit() if family->maxattr is zero. Do the
> same also in genl_family_rcv_msg_doit().
> 
> Fixes: c10e6cf85e7d ("net: genetlink: push attrbuf allocation and parsing to a separate function")
> Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
> ---
>  net/netlink/genetlink.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
> index ecc2bd3e73e4..1f14e55ad3ad 100644
> --- a/net/netlink/genetlink.c
> +++ b/net/netlink/genetlink.c
> @@ -639,21 +639,23 @@ static int genl_family_rcv_msg_doit(const struct genl_family *family,
>  				    const struct genl_ops *ops,
>  				    int hdrlen, struct net *net)
>  {
> -	struct nlattr **attrbuf;
> +	struct nlattr **attrbuf = NULL;
>  	struct genl_info info;
>  	int err;
>  
>  	if (!ops->doit)
>  		return -EOPNOTSUPP;
>  
> +	if (!family->maxattr)
> +		goto no_attrs;
>  	attrbuf = genl_family_rcv_msg_attrs_parse(family, nlh, extack,
>  						  ops, hdrlen,
>  						  GENL_DONT_VALIDATE_STRICT,
> -						  family->maxattr &&
>  						  family->parallel_ops);
>  	if (IS_ERR(attrbuf))
>  		return PTR_ERR(attrbuf);
>  
> +no_attrs:

The use of a goto statement as a replacement for an if is making me
uncomfortable. 

Looks like both callers of genl_family_rcv_msg_attrs_parse() jump
around it if !family->maxattr and then check the result with IS_ERR().

Would it not make more sense to have genl_family_rcv_msg_attrs_parse()
return NULL if !family->maxattr?

Just wondering, if you guys prefer this version I can apply..

>  	info.snd_seq = nlh->nlmsg_seq;
>  	info.snd_portid = NETLINK_CB(skb).portid;
>  	info.nlhdr = nlh;
> @@ -676,8 +678,7 @@ static int genl_family_rcv_msg_doit(const struct genl_family *family,
>  		family->post_doit(ops, skb, &info);
>  
>  out:
> -	genl_family_rcv_msg_attrs_free(family, attrbuf,
> -				       family->maxattr && family->parallel_ops);
> +	genl_family_rcv_msg_attrs_free(family, attrbuf, family->parallel_ops);
>  
>  	return err;
>  }