Subject: Re: [PATCH net-next 1/4] net: mpls: Convert number of nexthops to u8

From: [email-masked] (Eric W. Biederman)
Date: Sun, 26 Mar 2017 22:11:51 -0500

David Ahern <[email-masked]> writes:

> Number of nexthops and number of alive nexthops are tracked using an
> unsigned int. A route should never have more than 255 nexthops so
> convert both to u8. Update all references and intermediate variables
> to consistently use u8 as well.
>
> Shrinks the size of mpls_route from 32 bytes to 24 bytes with a 2-byte
> hole before the nexthops.
>
> Also, the ACCESS_ONCE is changed to READ_ONCE per checkpatch message.

I don't like this.  Byte writes don't exist on all architectures.

So while I think always writing to rtn_nhn_alive under the
rtn_lock ensures that we don't have wrong values written
it is quite subtle.  And I don't know how this will interact with other
fields that you are introducing.

AKA this might be ok, but I expect this formulation of the code
will easily bit-rot and break.

Eric

> Signed-off-by: David Ahern <[email-masked]>
> ---
>  net/mpls/af_mpls.c  | 29 ++++++++++++++++++-----------
>  net/mpls/internal.h |  4 ++--
>  2 files changed, 20 insertions(+), 13 deletions(-)
>
> diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
> index cd8be8d5e4ad..d3dc6c43a1d1 100644
> --- a/net/mpls/af_mpls.c
> +++ b/net/mpls/af_mpls.c
> @@ -192,7 +192,7 @@ static u32 mpls_multipath_hash(struct mpls_route *rt, struct sk_buff *skb)
>  static struct mpls_nh *mpls_select_multipath(struct mpls_route *rt,
>  					     struct sk_buff *skb)
>  {
> -	int alive = ACCESS_ONCE(rt->rt_nhn_alive);
> +	u8 alive = READ_ONCE(rt->rt_nhn_alive);
>  	u32 hash = 0;
>  	int nh_index = 0;
>  	int n = 0;
> @@ -458,7 +458,7 @@ struct mpls_route_config {
>  	int			rc_mp_len;
>  };
>  
> -static struct mpls_route *mpls_rt_alloc(int num_nh, u8 max_alen)
> +static struct mpls_route *mpls_rt_alloc(u8 num_nh, u8 max_alen)
>  {
>  	u8 max_alen_aligned = ALIGN(max_alen, VIA_ALEN_ALIGN);
>  	struct mpls_route *rt;
> @@ -736,11 +736,11 @@ static int mpls_nh_build(struct net *net, struct mpls_route *rt,
>  	return err;
>  }
>  
> -static int mpls_count_nexthops(struct rtnexthop *rtnh, int len,
> -			       u8 cfg_via_alen, u8 *max_via_alen)
> +static u8 mpls_count_nexthops(struct rtnexthop *rtnh, int len,
> +			      u8 cfg_via_alen, u8 *max_via_alen)
>  {
> -	int nhs = 0;
>  	int remaining = len;
> +	u8 nhs = 0;
>  
>  	if (!rtnh) {
>  		*max_via_alen = cfg_via_alen;
> @@ -765,7 +765,14 @@ static int mpls_count_nexthops(struct rtnexthop *rtnh, int len,
>  						      via_alen);
>  		}
>  
> +		/* number of nexthops is tracked by a u8.
> +		 * Check for overflow.
> +		 */
> +		if (nhs == 255)
> +			return 0;
> +
>  		nhs++;
> +
>  		rtnh = rtnh_next(rtnh, &remaining);
>  	}
>  
> @@ -779,8 +786,8 @@ static int mpls_nh_build_multi(struct mpls_route_config *cfg,
>  	struct rtnexthop *rtnh = cfg->rc_mp;
>  	struct nlattr *nla_via, *nla_newdst;
>  	int remaining = cfg->rc_mp_len;
> -	int nhs = 0;
>  	int err = 0;
> +	u8 nhs = 0;
>  
>  	change_nexthops(rt) {
>  		int attrlen;
> @@ -834,7 +841,7 @@ static int mpls_route_add(struct mpls_route_config *cfg)
>  	int err = -EINVAL;
>  	u8 max_via_alen;
>  	unsigned index;
> -	int nhs;
> +	u8 nhs;
>  
>  	index = cfg->rc_label;
>  
> @@ -1299,8 +1306,8 @@ static void mpls_ifdown(struct net_device *dev, int event)
>  	struct mpls_route __rcu **platform_label;
>  	struct net *net = dev_net(dev);
>  	unsigned int nh_flags = RTNH_F_DEAD | RTNH_F_LINKDOWN;
> -	unsigned int alive;
>  	unsigned index;
> +	u8 alive;
>  
>  	platform_label = rtnl_dereference(net->mpls.platform_label);
>  	for (index = 0; index < net->mpls.platform_labels; index++) {
> @@ -1339,7 +1346,7 @@ static void mpls_ifup(struct net_device *dev, unsigned int nh_flags)
>  	struct mpls_route __rcu **platform_label;
>  	struct net *net = dev_net(dev);
>  	unsigned index;
> -	int alive;
> +	u8 alive;
>  
>  	platform_label = rtnl_dereference(net->mpls.platform_label);
>  	for (index = 0; index < net->mpls.platform_labels; index++) {
> @@ -1761,8 +1768,8 @@ static int mpls_dump_route(struct sk_buff *skb, u32 portid, u32 seq, int event,
>  	} else {
>  		struct rtnexthop *rtnh;
>  		struct nlattr *mp;
> -		int dead = 0;
> -		int linkdown = 0;
> +		u8 linkdown = 0;
> +		u8 dead = 0;
>  
>  		mp = nla_nest_start(skb, RTA_MULTIPATH);
>  		if (!mp)
> diff --git a/net/mpls/internal.h b/net/mpls/internal.h
> index 62928d8fabd1..66f388ba2d49 100644
> --- a/net/mpls/internal.h
> +++ b/net/mpls/internal.h
> @@ -123,8 +123,8 @@ struct mpls_route { /* next hop label forwarding entry */
>  	u8			rt_payload_type;
>  	u8			rt_max_alen;
>  	u8			rt_ttl_propagate;
> -	unsigned int		rt_nhn;
> -	unsigned int		rt_nhn_alive;
> +	u8			rt_nhn;
> +	u8			rt_nhn_alive;
>  	struct mpls_nh		rt_nh[0];
>  };