Re: [PATCH 1/2] netlink: Fix iterations over nexthop objects

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Apr 23, 2024 at 10:41:24PM +0200, Stefano Brivio wrote:
> Somewhat confusingly, RTNH_NEXT(), as defined by <linux/rtnetlink.h>,
> doesn't take an attribute length parameter like RTA_NEXT() does, and
> I just modelled loops over nexthops after RTA loops, forgetting to
> decrease the remaining length we pass to RTNH_OK().
> 
> In practice, this didn't cause issue in any of the combinations I
> checked, at least without the next patch.
> 
> We seem to be the only user of RTNH_OK(): even iproute2 has an
> open-coded version of it in print_rta_multipath() (ip/iproute.c).
> 
> Introduce RTNH_NEXT_AND_DEC(), similar to RTA_NEXT(), and use it.
> 
> Fixes: 6c7623d07bbd ("netlink: Add support to fetch default gateway from multipath routes")
> Fixes: f4e38b5cd232 ("netlink: Adjust interface index inside copied nexthop objects too")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  netlink.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/netlink.c b/netlink.c
> index 89c0641..a5a4870 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -36,6 +36,10 @@
>  #include "ip.h"
>  #include "netlink.h"
>  
> +/* Same as RTA_NEXT() but for nexthops: RTNH_NEXT() doesn't take 'attrlen' */
> +#define RTNH_NEXT_AND_DEC(rtnh, attrlen)				\
> +	((attrlen) -= RTNH_ALIGN((rtnh)->rtnh_len), RTNH_NEXT(rtnh))
> +
>  /* Netlink expects a buffer of at least 8kiB or the system page size,
>   * whichever is larger.  32kiB is recommended for more efficient.
>   * Since the largest page size on any remotely common Linux setup is
> @@ -349,12 +353,13 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
>   */
>  bool nl_route_get_def_multipath(struct rtattr *rta, void *gw)
>  {
> +	size_t nh_len = RTA_PAYLOAD(rta);
>  	struct rtnexthop *rtnh;
>  	bool found = false;
>  	int hops = -1;
>  
>  	for (rtnh = (struct rtnexthop *)RTA_DATA(rta);
> -	     RTNH_OK(rtnh, RTA_PAYLOAD(rta)); rtnh = RTNH_NEXT(rtnh)) {
> +	     RTNH_OK(rtnh, nh_len); rtnh = RTNH_NEXT_AND_DEC(rtnh, nh_len)) {
>  		size_t len = rtnh->rtnh_len - sizeof(*rtnh);
>  		struct rtattr *rta_inner;
>  
> @@ -566,11 +571,12 @@ int nl_route_dup(int s_src, unsigned int ifi_src,
>  			if (rta->rta_type == RTA_OIF) {
>  				*(unsigned int *)RTA_DATA(rta) = ifi_dst;
>  			} else if (rta->rta_type == RTA_MULTIPATH) {
> +				size_t nh_len = RTA_PAYLOAD(rta);
>  				struct rtnexthop *rtnh;
>  
>  				for (rtnh = (struct rtnexthop *)RTA_DATA(rta);
> -				     RTNH_OK(rtnh, RTA_PAYLOAD(rta));
> -				     rtnh = RTNH_NEXT(rtnh))
> +				     RTNH_OK(rtnh, nh_len);
> +				     rtnh = RTNH_NEXT_AND_DEC(rtnh, nh_len))
>  					rtnh->rtnh_ifindex = ifi_dst;
>  			} else if (rta->rta_type == RTA_PREFSRC) {
>  				/* Host routes might include a preferred source

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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