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

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