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

[Jiri Pirko]

Thu, Oct 10, 2019 at 11:13:47AM CEST, mkubecek@xxxxxxx wrote:
>On Thu, Oct 10, 2019 at 10:39:58AM +0200, Jiri Pirko wrote:
>> Wed, Oct 09, 2019 at 06:44:32PM CEST, mkubecek@xxxxxxx 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.

Ah, I missed that. Thanks!

>
>Michal