libnl 3 with kernel < 2.6.38

[Thierry Reding]

* Thomas Graf wrote:
> On Wed, Mar 23, 2011 at 01:03:29PM +0100, Thierry Reding wrote:
> 
> > -- Original message: type=0x13 length=36 flags=<REQUEST,ACK> sequence-nr=1009839803 pid=1980
> > -- Debug: Sent Message:
> > --------------------------   BEGIN NETLINK MESSAGE ---------------------------
> >   [HEADER] 16 octets
> >     .nlmsg_len = 36
> >     .nlmsg_type = 19 <route/link::set>
> >     .nlmsg_flags = 5 <REQUEST,ACK>
> >     .nlmsg_seq = 1009839804
> >     .nlmsg_pid = 1980
> >   [PAYLOAD] 16 octets
> >     00 00 00 00 04 00 00 00 03 10 00 00 00 00 00 00       ................
> >   [ATTR 26] 0 octets
> >     ---------------------------  END NETLINK MESSAGE   ---------------------------
> 
> Problem is this empty nested atribtue (IFLA_AF_SPEC)
> 
> Fixed with the following commit:
> 
> commit a0fe7a1c9abe3ab5bf8c9253fab50e114b02b87b
> Author: Thomas Graf <tgraf at suug.ch>
> Date:   Wed Mar 23 13:39:18 2011 +0100
> 
>     Omit empty nested attributes
>     
>     Check for empty nested attributes in nla_nest_end() and omit the
>     attribute alltogether if is is the case.
> 
> diff --git a/lib/attr.c b/lib/attr.c
> index cccd50d..a045351 100644
> --- a/lib/attr.c
> +++ b/lib/attr.c
> @@ -1151,10 +1151,22 @@ struct nlattr *nla_nest_start(struct nl_msg *msg, int attrtype)
>   */
>  int nla_nest_end(struct nl_msg *msg, struct nlattr *start)
>  {
> -       size_t pad;
> +       size_t pad, len;
>  
> -       start->nla_len = (unsigned char *) nlmsg_tail(msg->nm_nlh) -
> -                               (unsigned char *) start;
> +       len = (void *) nlmsg_tail(msg->nm_nlh) - (void *) start;
> +
> +       if (len == NLA_HDRLEN) {
> +               /*
> +                * Kernel can't handle empty nested attributes, trim the
> +                * attribute header again
> +                */
> +               msg->nm_nlh->nlmsg_len -= NLA_HDRLEN;
> +               memset(nlmsg_tail(msg->nm_nlh), 0, NLA_HDRLEN);
> +
> +               return 0;
> +       }
> +
> +       start->nla_len = len;
>  
>         pad = NLMSG_ALIGN(msg->nm_nlh->nlmsg_len) - msg->nm_nlh->nlmsg_len;
>         if (pad > 0) {

I can confirm that this patch works.

Cheers,
Thierry
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