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SubjectRe: [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@suse.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

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