Re: [PATCH nf-next] netfilter: ingress: translate 0 nf_hook_slow retval to -EINPROGRESS

[Pablo Neira Ayuso]

On Mon, Nov 28, 2016 at 11:40:05AM +0100, Florian Westphal wrote:
> The caller assumes that < 0 means that skb was stolen (or free'd).
> 
> All other return values continue skb processing.
> 
> nf_hook_slow returns 3 different return value types:
> 
> A) a (negative) errno value: the skb was dropped (NF_DROP, e.g.
> by iptables '-j DROP' rule).
> 
> B) 0. The skb was stolen by the hook or queued to userspace.
> 
> C) 1. all hooks returned NF_ACCEPT so the caller should invoke
>the okfn so packet processing can continue.
> 
> nft ingress facility currently doesn't have the 'okfn' that
> the NF_HOOK() macros use; there is no nfqueue support either.
> 
> So 1 means that nf_hook_ingress() caller should go on processing the skb.
> 
> In order to allow use of NF_STOLEN from ingress we need to translate
> this to an errno number, else we'd crash because we continue with
> already-free'd (or about to be free-d) skb.
> 
> Random dice roll has chosen EINPROGRESS.  The errno value isn't checked,
> its just important that its less than 0.

Not really comments to block anything, so take them lightly.

Probably we can just set this to -1, I know this maps to some errno
value, but at least from code reading it will be make think that we
meant to return EINPROGRESS. If you agree, I can just mangle the
patch.

One more question below.

> Signed-off-by: Florian Westphal 
> ---
>  include/linux/netfilter_ingress.h | 7 ++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/netfilter_ingress.h 
> b/include/linux/netfilter_ingress.h
> index 2dc3b49b804a..d5027e470a24 100644
> --- a/include/linux/netfilter_ingress.h
> +++ b/include/linux/netfilter_ingress.h
> @@ -19,6 +19,7 @@ static inline int nf_hook_ingress(struct sk_buff *skb)
>  {
>   struct nf_hook_entry *e = rcu_dereference(skb->dev->nf_hooks_ingress);
>   struct nf_hook_state state;
> + int ret;
>  
>   /* Must recheck the ingress hook head, in the event it became NULL
>* after the check in nf_hook_ingress_active evaluated to true.
> @@ -29,7 +30,11 @@ static inline int nf_hook_ingress(struct sk_buff *skb)
>   nf_hook_state_init(, NF_NETDEV_INGRESS,
>  NFPROTO_NETDEV, skb->dev, NULL, NULL,
>  dev_net(skb->dev), NULL);
> - return nf_hook_slow(skb, , e);
> + ret = nf_hook_slow(skb, , e);
> + if (unlikely(ret == 0))
> + return -EINPROGRESS;

Do you prefer to keep this branch as unlikely? I understand most
people are not using fwd much so far, but we're targeting to enrich
ingress so I would expect users will be using this more and more.
Anyway, we can revisit this later on.
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[PATCH nf-next] netfilter: ingress: translate 0 nf_hook_slow retval to -EINPROGRESS

[Florian Westphal]

The caller assumes that < 0 means that skb was stolen (or free'd).

All other return values continue skb processing.

nf_hook_slow returns 3 different return value types:

A) a (negative) errno value: the skb was dropped (NF_DROP, e.g.
by iptables '-j DROP' rule).

B) 0. The skb was stolen by the hook or queued to userspace.

C) 1. all hooks returned NF_ACCEPT so the caller should invoke
   the okfn so packet processing can continue.

nft ingress facility currently doesn't have the 'okfn' that
the NF_HOOK() macros use; there is no nfqueue support either.

So 1 means that nf_hook_ingress() caller should go on processing the skb.

In order to allow use of NF_STOLEN from ingress we need to translate
this to an errno number, else we'd crash because we continue with
already-free'd (or about to be free-d) skb.

Random dice roll has chosen EINPROGRESS.  The errno value isn't checked,
its just important that its less than 0.

Signed-off-by: Florian Westphal 
---
 include/linux/netfilter_ingress.h | 7 ++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/netfilter_ingress.h 
b/include/linux/netfilter_ingress.h
index 2dc3b49b804a..d5027e470a24 100644
--- a/include/linux/netfilter_ingress.h
+++ b/include/linux/netfilter_ingress.h
@@ -19,6 +19,7 @@ static inline int nf_hook_ingress(struct sk_buff *skb)
 {
struct nf_hook_entry *e = rcu_dereference(skb->dev->nf_hooks_ingress);
struct nf_hook_state state;
+   int ret;
 
/* Must recheck the ingress hook head, in the event it became NULL
 * after the check in nf_hook_ingress_active evaluated to true.
@@ -29,7 +30,11 @@ static inline int nf_hook_ingress(struct sk_buff *skb)
nf_hook_state_init(, NF_NETDEV_INGRESS,
   NFPROTO_NETDEV, skb->dev, NULL, NULL,
   dev_net(skb->dev), NULL);
-   return nf_hook_slow(skb, , e);
+   ret = nf_hook_slow(skb, , e);
+   if (unlikely(ret == 0))
+   return -EINPROGRESS;
+
+   return ret;
 }
 
 static inline void nf_hook_ingress_init(struct net_device *dev)
-- 
2.7.3

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