Re: [PATCH] net: neigh: decrement the family specific qlen

[Paolo Abeni <pabeni@redhat.com>]

Hello,

On Sat, 2022-11-12 at 11:48 +0100, Thomas Zeitlhofer wrote:
> Commit 0ff4eb3d5ebb ("neighbour: make proxy_queue.qlen limit
> per-device") introduced the length counter qlen in struct neigh_parms.
> There are separate neigh_parms instances for IPv4/ARP and IPv6/ND, and
> while the family specific qlen is incremented in pneigh_enqueue(), the
> mentioned commit decrements always the IPv4/ARP specific qlen,
> regardless of the currently processed family, in pneigh_queue_purge()
> and neigh_proxy_process().
> 
> As a result, with IPv6/ND, the family specific qlen is only incremented
> (and never decremented) until it exceeds PROXY_QLEN, and then, according
> to the check in pneigh_enqueue(), neighbor solicitations are not
> answered anymore. As an example, this is noted when using the
> subnet-router anycast address to access a Linux router. After a certain
> amount of time (in the observed case, qlen exceeded PROXY_QLEN after two
> days), the Linux router stops answering neighbor solicitations for its
> subnet-router anycast address and effectively becomes unreachable.
> 
> Another result with IPv6/ND is that the IPv4/ARP specific qlen is
> decremented more often than incremented. This leads to negative qlen
> values, as a signed integer has been used for the length counter qlen,
> and potentially to an integer overflow.
> 
> Fix this by introducing the helper function neigh_parms_qlen_dec(),
> which decrements the family specific qlen. Thereby, make use of the
> existing helper function neigh_get_dev_parms_rcu(), whose definition
> therefore needs to be placed earlier in neighbour.c. Take the family
> member from struct neigh_table to determine the currently processed
> family and appropriately call neigh_parms_qlen_dec() from
> pneigh_queue_purge() and neigh_proxy_process().
> 
> Additionally, use an unsigned integer for the length counter qlen.
> 
> Fixes: 0ff4eb3d5ebb ("neighbour: make proxy_queue.qlen limit per-device")
> Signed-off-by: Thomas Zeitlhofer <thomas.zeitlhofer+lkml@ze-it.at>
> ---
>  include/net/neighbour.h |  2 +-
>  net/core/neighbour.c    | 58 +++++++++++++++++++++--------------------
>  2 files changed, 31 insertions(+), 29 deletions(-)
> 
> diff --git a/include/net/neighbour.h b/include/net/neighbour.h
> index 20745cf7ae1a..cc0b65b7c829 100644
> --- a/include/net/neighbour.h
> +++ b/include/net/neighbour.h
> @@ -83,7 +83,7 @@ struct neigh_parms {
>  	struct rcu_head rcu_head;
>  
>  	int	reachable_time;
> -	int	qlen;
> +	__u32	qlen;
>  	int	data[NEIGH_VAR_DATA_MAX];
>  	DECLARE_BITMAP(data_state, NEIGH_VAR_DATA_MAX);
>  };

The patch LGTM, but why did you use __u32 above? this is not part of
uAPI, plain u32 should be fine. 

Thanks!

Paolo