Re: [PATCH net 1/2] net: ioam6: use "new" dst entry with skb_cow_head

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2024-07-02 at 19:44 +0200, Justin Iurman wrote:
> In ioam6_output(), we call skb_cow_head() with LL_RESERVED_SPACE(). The
> latter uses "dst", which is potentially not the good one anymore. As a
> consequence, there might not be enough headroom, depending on the "new"
> dev (e.g., needed_headroom > 0). Therefore, we first need to get the
> "new" dst entry (from the cache or by calling ip6_route_output()), just
> like seg6 and rpl both do, and only then call skb_cow_head() with
> LL_RESERVED_SPACE() on the "new" dst entry.
> 
> Fixes: 8cb3bf8bff3c ("ipv6: ioam: Add support for the ip6ip6 encapsulation")
> Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
> ---
>  net/ipv6/ioam6_iptunnel.c | 65 +++++++++++++++++++--------------------
>  1 file changed, 32 insertions(+), 33 deletions(-)
> 
> diff --git a/net/ipv6/ioam6_iptunnel.c b/net/ipv6/ioam6_iptunnel.c
> index bf7120ecea1e..b08c13550144 100644
> --- a/net/ipv6/ioam6_iptunnel.c
> +++ b/net/ipv6/ioam6_iptunnel.c
> @@ -295,7 +295,7 @@ static int ioam6_do_encap(struct net *net, struct sk_buff *skb,
>  
>  static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
>  {
> -	struct dst_entry *dst = skb_dst(skb);
> +	struct dst_entry *dst, *orig_dst = skb_dst(skb);
>  	struct in6_addr orig_daddr;
>  	struct ioam6_lwt *ilwt;
>  	int err = -EINVAL;
> @@ -304,7 +304,7 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
>  	if (skb->protocol != htons(ETH_P_IPV6))
>  		goto drop;
>  
> -	ilwt = ioam6_lwt_state(dst->lwtstate);
> +	ilwt = ioam6_lwt_state(orig_dst->lwtstate);
>  
>  	/* Check for insertion frequency (i.e., "k over n" insertions) */
>  	pkt_cnt = atomic_fetch_inc(&ilwt->pkt_cnt);
> @@ -346,45 +346,44 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
>  		goto drop;
>  	}
>  
> -	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
> -	if (unlikely(err))
> -		goto drop;
> +	local_bh_disable();
> +	dst = dst_cache_get(&ilwt->cache);
> +	local_bh_enable();

This makes the dst lookup and cache update unconditional, at best
slowing down ioam6_output(), possibly introducing unintended side
effects.

> +
> +	if (unlikely(!dst)) {
> +		struct ipv6hdr *hdr = ipv6_hdr(skb);
> +		struct flowi6 fl6;
> +
> +		memset(&fl6, 0, sizeof(fl6));
> +		fl6.daddr = hdr->daddr;
> +		fl6.saddr = hdr->saddr;
> +		fl6.flowlabel = ip6_flowinfo(hdr);
> +		fl6.flowi6_mark = skb->mark;
> +		fl6.flowi6_proto = hdr->nexthdr;
> +
> +		dst = ip6_route_output(net, NULL, &fl6);
> +		if (dst->error) {
> +			err = dst->error;
> +			dst_release(dst);
> +			goto drop;
> +		}
>  
> -	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr)) {
>  		local_bh_disable();
> -		dst = dst_cache_get(&ilwt->cache);
> +		dst_cache_set_ip6(&ilwt->cache, dst, &fl6.saddr);
>  		local_bh_enable();
> +	}
>  
> -		if (unlikely(!dst)) {
> -			struct ipv6hdr *hdr = ipv6_hdr(skb);
> -			struct flowi6 fl6;
> -
> -			memset(&fl6, 0, sizeof(fl6));
> -			fl6.daddr = hdr->daddr;
> -			fl6.saddr = hdr->saddr;
> -			fl6.flowlabel = ip6_flowinfo(hdr);
> -			fl6.flowi6_mark = skb->mark;
> -			fl6.flowi6_proto = hdr->nexthdr;
> -
> -			dst = ip6_route_output(net, NULL, &fl6);
> -			if (dst->error) {
> -				err = dst->error;
> -				dst_release(dst);
> -				goto drop;
> -			}
> -
> -			local_bh_disable();
> -			dst_cache_set_ip6(&ilwt->cache, dst, &fl6.saddr);
> -			local_bh_enable();
> -		}
> +	skb_dst_drop(skb);
> +	skb_dst_set(skb, dst);
>  
> -		skb_dst_drop(skb);
> -		skb_dst_set(skb, dst);
> +	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
> +	if (unlikely(err))
> +		goto drop;

This is quite a bit of code churn, but you just need to postpone the
cow operation after the dst lookup right?

I suggest to simply move the cow there, something alike (completely
untested): 
---
diff --git a/net/ipv6/ioam6_iptunnel.c b/net/ipv6/ioam6_iptunnel.c
index bf7120ecea1e..636480bded0e 100644
--- a/net/ipv6/ioam6_iptunnel.c
+++ b/net/ipv6/ioam6_iptunnel.c
@@ -346,10 +346,6 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		goto drop;
 	}
 
-	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
-	if (unlikely(err))
-		goto drop;
-
 	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr)) {
 		local_bh_disable();
 		dst = dst_cache_get(&ilwt->cache);
@@ -381,9 +377,17 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		skb_dst_drop(skb);
 		skb_dst_set(skb, dst);
 
+		err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
+		if (unlikely(err))
+			goto drop;
+
 		return dst_output(net, sk, skb);
 	}
 out:
+	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
+	if (unlikely(err))
+		goto drop;
+
 	return dst->lwtstate->orig_output(net, sk, skb);
 drop:
 	kfree_skb(skb);
---
Thanks,

Paolo