Re: [PATCH v2 5/5] bpf: clear decap tunnel GSO state in skb_adjust_room

[Willem de Bruijn]

Nick Hudson wrote:
> On shrink in bpf_skb_adjust_room(), clear tunnel-specific GSO flags
> according to the decapsulation flags:
> 
> - BPF_F_ADJ_ROOM_DECAP_L4_UDP clears SKB_GSO_UDP_TUNNEL{,_CSUM}
> - BPF_F_ADJ_ROOM_DECAP_L4_GRE clears SKB_GSO_GRE{,_CSUM}
> - BPF_F_ADJ_ROOM_DECAP_IPXIP4 clears SKB_GSO_IPXIP4
> - BPF_F_ADJ_ROOM_DECAP_IPXIP6 clears SKB_GSO_IPXIP6
> 
> When all tunnel-related GSO bits are cleared, also clear
> skb->encapsulation.
> 
> Co-developed-by: Max Tottenham <mtottenh@xxxxxxxxxx>
> Signed-off-by: Max Tottenham <mtottenh@xxxxxxxxxx>
> Co-developed-by: Anna Glasgall <aglasgal@xxxxxxxxxx>
> Signed-off-by: Anna Glasgall <aglasgal@xxxxxxxxxx>
> Signed-off-by: Nick Hudson <nhudson@xxxxxxxxxx>
> ---
>  net/core/filter.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 47aec44a9cd3..35af1199ab97 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3665,6 +3665,37 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
>  		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
>  			skb_increase_gso_size(shinfo, len_diff);
>  
> +		/* Selective GSO flag clearing based on decap type.
> +		 * Only clear the flags for the tunnel layer being removed.
> +		 */
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP) &&
> +		    (shinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
> +					 SKB_GSO_UDP_TUNNEL_CSUM)))
> +			shinfo->gso_type &= ~(SKB_GSO_UDP_TUNNEL |
> +					      SKB_GSO_UDP_TUNNEL_CSUM);
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_GRE) &&
> +		    (shinfo->gso_type & (SKB_GSO_GRE | SKB_GSO_GRE_CSUM)))
> +			shinfo->gso_type &= ~(SKB_GSO_GRE |
> +					      SKB_GSO_GRE_CSUM);
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP4) &&
> +		    (shinfo->gso_type & SKB_GSO_IPXIP4))
> +			shinfo->gso_type &= ~SKB_GSO_IPXIP4;
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP6) &&
> +		    (shinfo->gso_type & SKB_GSO_IPXIP6))
> +			shinfo->gso_type &= ~SKB_GSO_IPXIP6;
> +
> +		/* Clear encapsulation flag only when no tunnel GSO flags remain */
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_MASK) {
> +			if (!(shinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
> +						  SKB_GSO_UDP_TUNNEL_CSUM |
> +						  SKB_GSO_GRE |
> +						  SKB_GSO_GRE_CSUM |
> +						  SKB_GSO_IPXIP4 |
> +						  SKB_GSO_IPXIP6)))
> +				if (skb->encapsulation)
> +					skb->encapsulation = 0;

Is there any chance that this might clear it while some other tunnel
is still active? From a quick grep on skb->encapsulation the only
possible hit I see is SKB_GSO_ESP.

> +		}
> +
>  		/* Header must be checked, and gso_segs recomputed. */
>  		shinfo->gso_type |= SKB_GSO_DODGY;
>  		shinfo->gso_segs = 0;
> -- 
> 2.34.1
>