Re: [PATCH net] netlink: don't send unknown nsid

[Flavio Leitner <fbl@sysclose.org>]

On Thu, Jun 01, 2017 at 10:00:07AM +0200, Nicolas Dichtel wrote:
> The NETLINK_F_LISTEN_ALL_NSID otion enables to listen all netns that have a
> nsid assigned into the netns where the netlink socket is opened.
> The nsid is sent as metadata to userland, but the existence of this nsid is
> checked only for netns that are different from the socket netns. Thus, if
> no nsid is assigned to the socket netns, NETNSA_NSID_NOT_ASSIGNED is
> reported to the userland. This value is confusing and useless.
> After this patch, only valid nsid are sent to userland.
> 
> Reported-by: Flavio Leitner <fbl@sysclose.org>
> Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
> ---
>  net/netlink/af_netlink.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index ee841f00a6ec..7586d446d7dc 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -62,6 +62,7 @@
>  #include <asm/cacheflush.h>
>  #include <linux/hash.h>
>  #include <linux/genetlink.h>
> +#include <linux/net_namespace.h>
>  
>  #include <net/net_namespace.h>
>  #include <net/sock.h>
> @@ -1415,7 +1416,8 @@ static void do_one_broadcast(struct sock *sk,
>  		goto out;
>  	}
>  	NETLINK_CB(p->skb2).nsid = peernet2id(sock_net(sk), p->net);
> -	NETLINK_CB(p->skb2).nsid_is_set = true;
> +	if (NETLINK_CB(p->skb2).nsid != NETNSA_NSID_NOT_ASSIGNED)
> +		NETLINK_CB(p->skb2).nsid_is_set = true;
>  	val = netlink_broadcast_deliver(sk, p->skb2);
>  	if (val < 0) {
>  		netlink_overrun(sk);

If the assumption is that nsid allocation can never fail or that if it
does, we can't report to userspace, then the patch is good, but it
doesn't sound like a good long term solution.

Let's consider that the allocation of an id fails for whatever reason.
I think that should be reported to userspace to allow it to retry, or
do something else to handle this situation properly.  Not sending
anything means that it's in the same netns as the old kernels did,
which is incorrect.

On the other hand, with the original patch, if the socket and the
device are in the same netns, we don't need to report any ID.  Previous
kernels did that, so we are not breaking anything.  When the netns
differs, then we either should report the real ID or an error.

-- 
Flavio