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

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Date:   Thu, 10 Oct 2019 11:13:47 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...lanox.com>,
        Johannes Berg <johannes@...solutions.net>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] genetlink: do not parse attributes for families
 with zero maxattr

On Thu, Oct 10, 2019 at 10:39:58AM +0200, Jiri Pirko wrote:
> Wed, Oct 09, 2019 at 06:44:32PM CEST, mkubecek@...e.cz 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().
> 
> This is the original code before the changes:
> 
>         if (ops->doit == NULL)
>                 return -EOPNOTSUPP;
> 
>         if (family->maxattr && family->parallel_ops) {
>                 attrbuf = kmalloc_array(family->maxattr + 1,
>                                         sizeof(struct nlattr *),
>                                         GFP_KERNEL);
>                 if (attrbuf == NULL)
>                         return -ENOMEM;
>         } else
>                 attrbuf = family->attrbuf;
> 
>         if (attrbuf) {
>                 enum netlink_validation validate = NL_VALIDATE_STRICT;
> 
>                 if (ops->validate & GENL_DONT_VALIDATE_STRICT)
>                         validate = NL_VALIDATE_LIBERAL;
> 
>                 err = __nlmsg_parse(nlh, hdrlen, attrbuf, family->maxattr,
>                                     family->policy, validate, extack);
>                 if (err < 0)
>                         goto out;
>         }
> 
> Looks like the __nlmsg_parse() is called no matter if maxattr if 0 or
> not. It is only considered for allocation of attrbuf. This is in-sync
> with the current code.

If family->maxattr is 0, genl_register_family() sets family->attrbuf to
NULL so that attrbuf is also NULL and the whole "if (attrbuf) { ... }"
block is not executed.

Michal