Date:   Thu, 1 Jun 2017 22:34:09 +0300 (EEST)
From:   Julian Anastasov <ja@....bg>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
cc:     netdev@...r.kernel.org, davem@...emloft.net,
        stephen@...workplumber.org
Subject: Re: [PATCH V4] neigh: Really delete an arp/neigh entry on "ip neigh
 delete" or "arp -d"


	Hello,

On Wed, 31 May 2017, Sowmini Varadhan wrote:

> The command
>   # arp -s 62.2.0.1 a:b:c:d:e:f dev eth2
> adds an entry like the following (listed by "arp -an")
>   ? (62.2.0.1) at 0a:0b:0c:0d:0e:0f [ether] PERM on eth2
> but the symmetric deletion command
>   # arp -i eth2 -d 62.2.0.1
> does not remove the PERM entry from the table, and instead leaves behind
>   ? (62.2.0.1) at <incomplete> on eth2
> 
> The reason is that there is a refcnt of 1 for the arp_tbl itself
> (neigh_alloc starts off the entry with a refcnt of 1), thus
> the neigh_release() call from arp_invalidate() will (at best) just
> decrement the ref to 1, but will never actually free it from the
> table.
> 
> To fix this, we need to do something like neigh_forced_gc: if
> the refcnt is 1 (i.e., on the table's ref), remove the entry from
> the table and free it. This patch refactors and shares common code
> between neigh_forced_gc and the newly added neigh_remove_one.
> 
> A similar issue exists for IPv6 Neighbor Cache entries, and is fixed
> in a similar manner by this patch.
> 
> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>

	Change looks ok to me but with some non-fatal
warnings, see below.

Reviewed-by: Julian Anastasov <ja@....bg>

> ---
> v2: kbuild-test-robot compile error
> v3: do not export_symbol neigh_remove_one (David Miller comment)
> v4: rearrange locking for tbl->lock (Julian Anastasov comment)
> 
>  include/net/neighbour.h |    1 +
>  net/core/neighbour.c    |   61 ++++++++++++++++++++++++++++++++++++++--------
>  net/ipv4/arp.c          |    4 +++
>  3 files changed, 55 insertions(+), 11 deletions(-)
> 
> diff --git a/include/net/neighbour.h b/include/net/neighbour.h
> index e4dd3a2..639b675 100644
> --- a/include/net/neighbour.h
> +++ b/include/net/neighbour.h
> @@ -317,6 +317,7 @@ struct neighbour *__neigh_create(struct neigh_table *tbl, const void *pkey,
>  int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new, u32 flags,
>  		 u32 nlmsg_pid);
>  void __neigh_set_probe_once(struct neighbour *neigh);
> +bool neigh_remove_one(struct neighbour *ndel, struct neigh_table *tbl);
>  void neigh_changeaddr(struct neigh_table *tbl, struct net_device *dev);
>  int neigh_ifdown(struct neigh_table *tbl, struct net_device *dev);
>  int neigh_resolve_output(struct neighbour *neigh, struct sk_buff *skb);
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index d274f81..b473f9f 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c

...

> +bool neigh_remove_one(struct neighbour *ndel, struct neigh_table *tbl)
> +{
> +	struct neigh_hash_table *nht;
> +	void *pkey = ndel->primary_key;
> +	u32 hash_val;
> +	struct neighbour *n;
> +	struct neighbour __rcu **np;
> +	bool retval = false;
> +
> +	nht = rcu_dereference_protected(tbl->nht,
> +					lockdep_is_held(&tbl->lock));
> +	hash_val = tbl->hash(pkey, ndel->dev, nht->hash_rnd);
> +	hash_val = hash_val >> (32 - nht->hash_shift);
> +
> +	np = &nht->hash_buckets[hash_val];
> +	while ((n = rcu_dereference_protected(*np,
> +				lockdep_is_held(&tbl->lock))) != NULL) {

	checkpatch shows some warnings:

scripts/checkpatch.pl --strict /tmp/file.patch

> +		if (n == ndel) {
> +			retval = neigh_del(n, 0, np, tbl);
> +			return retval;

	In case there is another patch version,
the retval can be removed:

		if (n == ndel)
			return neigh_del(n, 0, np, tbl);

> +		}
> +		np = &n->next;
> +	}
> +	return retval;

	return false;

> +}
> +
>  static int neigh_forced_gc(struct neigh_table *tbl)
>  {
>  	int shrunk = 0;
> @@ -140,19 +185,10 @@ static int neigh_forced_gc(struct neigh_table *tbl)
>  			 * - nobody refers to it.
>  			 * - it is not permanent
>  			 */
> -			write_lock(&n->lock);
> -			if (atomic_read(&n->refcnt) == 1 &&
> -			    !(n->nud_state & NUD_PERMANENT)) {
> -				rcu_assign_pointer(*np,
> -					rcu_dereference_protected(n->next,
> -						  lockdep_is_held(&tbl->lock)));
> -				n->dead = 1;
> -				shrunk	= 1;
> -				write_unlock(&n->lock);
> -				neigh_cleanup_and_release(n);
> +			if (neigh_del(n, NUD_PERMANENT, np, tbl)) {
> +				shrunk = 1;
>  				continue;
>  			}
> -			write_unlock(&n->lock);
>  			np = &n->next;
>  		}
>  	}
> @@ -1649,7 +1685,10 @@ static int neigh_delete(struct sk_buff *skb, struct nlmsghdr *nlh,
>  			   NEIGH_UPDATE_F_OVERRIDE |
>  			   NEIGH_UPDATE_F_ADMIN,
>  			   NETLINK_CB(skb).portid);
> +	write_lock_bh(&tbl->lock);
>  	neigh_release(neigh);
> +	neigh_remove_one(neigh, tbl);

	Looks like we can also call neigh_remove_one only when !err.
But this is some corner case when n->dead is set by GC and entry
was unlinked, neigh_remove_one simply will not find it in the list,
so it is not fatal to call neigh_remove_one unconditionally.

> +	write_unlock_bh(&tbl->lock);
>  
>  out:
>  	return err;
> diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
> index e9f3386..a651c53 100644
> --- a/net/ipv4/arp.c
> +++ b/net/ipv4/arp.c
> @@ -1113,13 +1113,17 @@ static int arp_invalidate(struct net_device *dev, __be32 ip)
>  {
>  	struct neighbour *neigh = neigh_lookup(&arp_tbl, &ip, dev);
>  	int err = -ENXIO;
> +	struct neigh_table *tbl = &arp_tbl;
>  
>  	if (neigh) {
>  		if (neigh->nud_state & ~NUD_NOARP)
>  			err = neigh_update(neigh, NULL, NUD_FAILED,
>  					   NEIGH_UPDATE_F_OVERRIDE|
>  					   NEIGH_UPDATE_F_ADMIN, 0);
> +		write_lock_bh(&tbl->lock);
>  		neigh_release(neigh);
> +		neigh_remove_one(neigh, tbl);

	Here the same race with GC already assigned
neigh->dead to 1 is possible but it is more tricky to catch
that exactly neigh_update() has failed. So, may be better to
call neigh_remove_one like now.

> +		write_unlock_bh(&tbl->lock);
>  	}
>  
>  	return err;
> -- 
> 1.7.1

Regards

--
Julian Anastasov <ja@....bg>

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