[iproute,1/2] ipaddress: Simplify vf_info parsing

Commit 7b8179c780a1a ("iproute2: Add new command to ip link to
enable/disable VF spoof check") tried to add support for
IFLA_VF_SPOOFCHK in a backwards-compatible manner, but aparently overdid
it: parse_rtattr_nested() handles missing attributes perfectly fine in
that it will leave the relevant field unassigned so calling code can
just compare against NULL. There is no need to layback from the previous
(IFLA_VF_TX_RATE) attribute to the next to check if IFLA_VF_SPOOFCHK is
present or not. To the contrary, it establishes a potentially incorrect
assumption of these two attributes directly following each other which
may not be the case (although up to now, kernel aligns them this way).

This patch cleans up the code to adhere to the common way of checking
for attribute existence. It has been tested to return correct results
regardless of whether the kernel exports IFLA_VF_SPOOFCHK or not.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 ip/ipaddress.c | 44 ++++++++++----------------------------------
 1 file changed, 10 insertions(+), 34 deletions(-)

On Tue, 2016-11-08 at 22:29 +0100, Phil Sutter wrote:
> Commit 7b8179c780a1a ("iproute2: Add new command to ip link to
> enable/disable VF spoof check") tried to add support for
> IFLA_VF_SPOOFCHK in a backwards-compatible manner, but aparently overdid
> it: parse_rtattr_nested() handles missing attributes perfectly fine in
> that it will leave the relevant field unassigned so calling code can
> just compare against NULL. There is no need to layback from the previous
> (IFLA_VF_TX_RATE) attribute to the next to check if IFLA_VF_SPOOFCHK is
> present or not. To the contrary, it establishes a potentially incorrect
> assumption of these two attributes directly following each other which
> may not be the case (although up to now, kernel aligns them this way).
> 
> This patch cleans up the code to adhere to the common way of checking
> for attribute existence. It has been tested to return correct results
> regardless of whether the kernel exports IFLA_VF_SPOOFCHK or not.
> 
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
>  ip/ipaddress.c | 44 ++++++++++----------------------------------
>  1 file changed, 10 insertions(+), 34 deletions(-)
> 
> diff --git a/ip/ipaddress.c b/ip/ipaddress.c
> index 7f05258f43453..df0f1b9c94c58 100644
> --- a/ip/ipaddress.c
> +++ b/ip/ipaddress.c
> @@ -322,10 +322,7 @@ static void print_vfinfo(FILE *fp, struct rtattr *vfinfo)
>  {
>  	struct ifla_vf_mac *vf_mac;
>  	struct ifla_vf_tx_rate *vf_tx_rate;
> -	struct ifla_vf_spoofchk *vf_spoofchk;
> -	struct ifla_vf_link_state *vf_linkstate;
>  	struct rtattr *vf[IFLA_VF_MAX + 1] = {};
> -	struct rtattr *tmp;
>  
>  	SPRINT_BUF(b1);
>  
> @@ -339,31 +336,6 @@ static void print_vfinfo(FILE *fp, struct rtattr *vfinfo)
>  	vf_mac = RTA_DATA(vf[IFLA_VF_MAC]);
>  	vf_tx_rate = RTA_DATA(vf[IFLA_VF_TX_RATE]);
>  
> -	/* Check if the spoof checking vf info type is supported by
> -	 * this kernel.
> -	 */
> -	tmp = (struct rtattr *)((char *)vf[IFLA_VF_TX_RATE] +
> -				vf[IFLA_VF_TX_RATE]->rta_len);
> -
> -	if (tmp->rta_type != IFLA_VF_SPOOFCHK)
> -		vf_spoofchk = NULL;
> -	else
> -		vf_spoofchk = RTA_DATA(vf[IFLA_VF_SPOOFCHK]);
> -
> -	if (vf_spoofchk) {
> -		/* Check if the link state vf info type is supported by
> -		 * this kernel.
> -		 */
> -		tmp = (struct rtattr *)((char *)vf[IFLA_VF_SPOOFCHK] +
> -					vf[IFLA_VF_SPOOFCHK]->rta_len);
> -
> -		if (tmp->rta_type != IFLA_VF_LINK_STATE)
> -			vf_linkstate = NULL;
> -		else
> -			vf_linkstate = RTA_DATA(vf[IFLA_VF_LINK_STATE]);
> -	} else
> -		vf_linkstate = NULL;
> -
>  	fprintf(fp, "%s    vf %d MAC %s", _SL_, vf_mac->vf,
>  		ll_addr_n2a((unsigned char *)&vf_mac->mac,
>  			    ETH_ALEN, 0, b1, sizeof(b1)));
> @@ -407,14 +379,18 @@ static void print_vfinfo(FILE *fp, struct rtattr *vfinfo)
>  		if (vf_rate->min_tx_rate)
>  			fprintf(fp, ", min_tx_rate %dMbps", vf_rate->min_tx_rate);
>  	}
> +	if (vf[IFLA_VF_SPOOFCHK]) {
> +		struct ifla_vf_spoofchk *vf_spoofchk =
> +			RTA_DATA(vf[IFLA_VF_SPOOFCHK]);
>  
> -	if (vf_spoofchk && vf_spoofchk->setting != -1) {
> -		if (vf_spoofchk->setting)
> -			fprintf(fp, ", spoof checking on");
> -		else
> -			fprintf(fp, ", spoof checking off");
> +		if (vf_spoofchk->setting != -1)
> +			fprintf(fp, ", spoof checking %s",
> +			        vf_spoofchk->setting ? "on" : "off");

I wrote some of this code at a time when I was pretty new to Linux
kernel net programming and I really just didn't understand it.  It
appears you're doing it more correctly than I.

Thanks for cleaning it up.

Reviewed-by: Greg Rose <grose@lightfleet.com>

- Greg

>  	}
> -	if (vf_linkstate) {
> +	if (vf[IFLA_VF_LINK_STATE]) {
> +		struct ifla_vf_link_state *vf_linkstate =
> +			RTA_DATA(vf[IFLA_VF_LINK_STATE]);
> +
>  		if (vf_linkstate->link_state == IFLA_VF_LINK_STATE_AUTO)
>  			fprintf(fp, ", link-state auto");
>  		else if (vf_linkstate->link_state == IFLA_VF_LINK_STATE_ENABLE)