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Date:	Sun, 17 Nov 2013 17:14:57 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	netdev@...r.kernel.org, Johannes Berg <johannes.berg@...el.com>
Subject: Re: [RFC 2/8] drop_monitor/genetlink: use proper genetlink multicast
 APIs

On Sat, Nov 16, 2013 at 06:03:43PM +0100, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> The drop monitor code is abusing the genetlink API and is
> statically using the generic netlink multicast group 1, even
> if that group belongs to somebody else (which it invariably
> will, since it's not reserved.)
> 
> Make the drop monitor code use the proper APIs to reserve a
> group ID, but also reserve the group id 1 in generic netlink
> code to preserve the userspace API. Since drop monitor can
> be a module, don't clear the bit for it on unregistration.
> 
> Cc: Neil Horman <nhorman@...driver.com>
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> ---
>  net/core/drop_monitor.c | 13 ++++++++++++-
>  net/netlink/genetlink.c | 13 ++++++++++---
>  2 files changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
> index 0efc502..46ee488 100644
> --- a/net/core/drop_monitor.c
> +++ b/net/core/drop_monitor.c
> @@ -106,6 +106,10 @@ static struct sk_buff *reset_per_cpu_data(struct per_cpu_dm_data *data)
>  	return skb;
>  }
>  
> +static struct genl_multicast_group dm_mcgrp = {
> +	.name = "events",
> +};
> +
>  static void send_dm_alert(struct work_struct *work)
>  {
>  	struct sk_buff *skb;
> @@ -116,7 +120,7 @@ static void send_dm_alert(struct work_struct *work)
>  	skb = reset_per_cpu_data(data);
>  
>  	if (skb)
> -		genlmsg_multicast(skb, 0, NET_DM_GRP_ALERT, GFP_KERNEL);
> +		genlmsg_multicast(skb, 0, dm_mcgrp.id, GFP_KERNEL);
>  }
>  
>  /*
> @@ -371,6 +375,13 @@ static int __init init_net_drop_monitor(void)
>  		return rc;
>  	}
>  
> +	rc = genl_register_mc_group(&net_drop_monitor_family, &dm_mcgrp);
> +	if (rc) {
> +		pr_err("Failed to register drop monitor mcast group\n");
> +		goto out_unreg;
> +	}
> +	WARN_ON(dm_mcgrp.id != NET_DM_GRP_ALERT);
> +
>  	rc = register_netdevice_notifier(&dropmon_net_notifier);
>  	if (rc < 0) {
>  		pr_crit("Failed to register netdevice notifier\n");
> diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
> index 2b57aee..8a2ed2c 100644
> --- a/net/netlink/genetlink.c
> +++ b/net/netlink/genetlink.c
> @@ -65,8 +65,12 @@ static struct list_head family_ht[GENL_FAM_TAB_SIZE];
>   * To avoid an allocation at boot of just one unsigned long,
>   * declare it global instead.
>   * Bit 0 is marked as already used since group 0 is invalid.
> + * Bit 1 is marked as already used since the drop-monitor code
> + * abuses the API and thinks it can statically use group 1.
> + * That group will typically conflict with other groups that
> + * any proper users use.
>   */
> -static unsigned long mc_group_start = 0x1;
> +static unsigned long mc_group_start = 0x3;
>  static unsigned long *mc_groups = &mc_group_start;
>  static unsigned long mc_groups_longs = 1;
>  
> @@ -160,9 +164,11 @@ int genl_register_mc_group(struct genl_family *family,
>  
>  	genl_lock_all();
>  
> -	/* special-case our own group */
> +	/* special-case our own group and hacks */
>  	if (grp == &notify_grp)
>  		id = GENL_ID_CTRL;
> +	else if (strcmp(family->name, "NET_DM") == 0)
> +		id = 1;
>  	else
>  		id = find_first_zero_bit(mc_groups,
>  					 mc_groups_longs * BITS_PER_LONG);
> @@ -245,7 +251,8 @@ static void __genl_unregister_mc_group(struct genl_family *family,
>  	rcu_read_unlock();
>  	netlink_table_ungrab();
>  
> -	clear_bit(grp->id, mc_groups);
> +	if (grp->id != 1)
> +		clear_bit(grp->id, mc_groups);
>  	list_del(&grp->list);
>  	genl_ctrl_event(CTRL_CMD_DELMCAST_GRP, grp);
>  	grp->id = 0;
> -- 
> 1.8.4.rc3
> 
> 

Acked-by: Neil Horman <nhorman@...driver.com>

Thanks Johannes!
Neil

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