Re: [PATCH net-next v4 03/10] gre: make gre_parse_header() report a drop reason

[Ido Schimmel <idosch@nvidia.com>]

On Wed, Sep 23, 2026 at 01:15:00AM +0300, Anton Danilov wrote:
> diff --git a/net/ipv4/gre_demux.c b/net/ipv4/gre_demux.c
> index 96fd7dc6d82d..e117056525f0 100644
> --- a/net/ipv4/gre_demux.c
> +++ b/net/ipv4/gre_demux.c
> @@ -56,28 +56,35 @@ int gre_del_protocol(const struct gre_protocol *proto, u8 version)
>  }
>  EXPORT_SYMBOL_GPL(gre_del_protocol);
>  
> -/* Fills in tpi and returns header length to be pulled.
> +/* Fills in tpi, including the header length to be pulled in tpi->hdr_len,
> + * and returns SKB_NOT_DROPPED_YET, or the reason to drop the packet if the
> + * header is rejected.
>   * Note that caller must use pskb_may_pull() before pulling GRE header.
> + *
> + * @icmp_err is set by the ICMP error handlers, which only get a part of
> + * the original packet: a checksum failure does not reject the header then,
> + * the checksum is still computed and the rest of the header is parsed.
>   */
> -int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
> -		     bool *csum_err, __be16 proto, int nhs)
> +enum skb_drop_reason
> +gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
> +		 bool icmp_err, __be16 proto, int nhs)
>  {
>  	const struct gre_base_hdr *greh;
>  	__be32 *options;
>  	int hdr_len;
>  
>  	if (unlikely(!pskb_may_pull(skb, nhs + sizeof(struct gre_base_hdr))))
> -		return -EINVAL;
> +		return SKB_DROP_REASON_HDR_TRUNC;

Please use pskb_may_pull_reason() as is done throughout the kernel.
AFAICT, there's only one user of HDR_TRUNC (in tun) and it should also
be converted to pskb_may_pull_reason() so that we could remove
HDR_TRUNC. Can you send a patch?

>  
>  	greh = (struct gre_base_hdr *)(skb->data + nhs);
>  	if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING)))
> -		return -EINVAL;
> +		return SKB_DROP_REASON_GRE_INVALID_HDR;

[...]

>  static int gre_rcv(struct sk_buff *skb)
>  {
> +	enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
>  	const struct gre_protocol *proto;
>  	u8 ver;
>  	int ret;
>  
> -	if (!pskb_may_pull(skb, 12))
> +	reason = pskb_may_pull_reason(skb, 12);
> +	if (reason)
>  		goto drop;
>  
>  	ver = skb->data[1]&0x7f;
> -	if (ver >= GREPROTO_MAX)
> +	if (ver >= GREPROTO_MAX) {
> +		reason = SKB_DROP_REASON_UNHANDLED_PROTO;

This should be TUNNEL_INVALID_HDR to be consistent with the hunk above.

>  		goto drop;
> +	}
>  
>  	rcu_read_lock();
>  	proto = rcu_dereference(gre_proto[ver]);
> @@ -167,11 +177,11 @@ static int gre_rcv(struct sk_buff *skb)
>  drop_nohandler:
>  	rcu_read_unlock();
>  	dev_core_stats_rx_nohandler_inc(skb->dev);
> -	kfree_skb(skb);
> +	kfree_skb_reason(skb, SKB_DROP_REASON_UNHANDLED_PROTO);

This looks correct.

>  	return NET_RX_DROP;
>  drop:
>  	dev_core_stats_rx_dropped_inc(skb->dev);
> -	kfree_skb(skb);
> +	kfree_skb_reason(skb, reason);
>  	return NET_RX_DROP;
>  }