Re: [PATCH v2 4/5] bpf: allow new DECAP flags and add guard rails

[Martin KaFai Lau <martin.lau@linux.dev>]

On 3/18/26 6:42 AM, Nick Hudson wrote:
> Add checks to require shrink-only decap, reject conflicting decap flag
> combinations, and verify removed length is sufficient for claimed header
> decapsulation.
> 
> Co-developed-by: Max Tottenham <mtottenh@akamai.com>
> Signed-off-by: Max Tottenham <mtottenh@akamai.com>
> Co-developed-by: Anna Glasgall <aglasgal@akamai.com>
> Signed-off-by: Anna Glasgall <aglasgal@akamai.com>
> Signed-off-by: Nick Hudson <nhudson@akamai.com>
> ---
>   net/core/filter.c | 47 +++++++++++++++++++++++++++++++++++++----------
>   1 file changed, 37 insertions(+), 10 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7c2871b40fe4..47aec44a9cd3 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -56,6 +56,7 @@
>   #include <net/sock_reuseport.h>
>   #include <net/busy_poll.h>
>   #include <net/tcp.h>
> +#include <net/gre.h>
>   #include <net/xfrm.h>
>   #include <net/udp.h>
>   #include <linux/bpf_trace.h>
> @@ -3496,7 +3497,9 @@ static u32 bpf_skb_net_base_len(const struct sk_buff *skb)
>   					 BPF_F_ADJ_ROOM_ENCAP_L2( \
>   					  BPF_ADJ_ROOM_ENCAP_L2_MASK))
>   
> -#define BPF_F_ADJ_ROOM_DECAP_MASK	(BPF_F_ADJ_ROOM_DECAP_L3_MASK)
> +#define BPF_F_ADJ_ROOM_DECAP_MASK	(BPF_F_ADJ_ROOM_DECAP_L3_MASK | \
> +					 BPF_F_ADJ_ROOM_DECAP_L4_MASK | \
> +					 BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)
>   
>   #define BPF_F_ADJ_ROOM_MASK		(BPF_F_ADJ_ROOM_FIXED_GSO | \
>   					 BPF_F_ADJ_ROOM_ENCAP_MASK | \
> @@ -3743,20 +3746,44 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
>   		return -ENOTSUPP;
>   	}
>   
> -	if (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
> +	if (flags & BPF_F_ADJ_ROOM_DECAP_MASK) {

This change should be done together with the macro refactoring patch 
mentioned in patch 3.

> +		u32 len_decap_min = 0;
> +
>   		if (!shrink)
>   			return -EINVAL;
>   
> -		switch (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
> -		case BPF_F_ADJ_ROOM_DECAP_L3_IPV4:
> +		/* Reject mutually exclusive decap flag pairs. */
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) ==
> +		    BPF_F_ADJ_ROOM_DECAP_L3_MASK)

iiuc, this 'if' and the len_min assignment changes below replace the 
existing switch case. Please separate this no-op change from the new 
flag validation logic. It is small enough to be done together in the 
macro refactoring patch also.

> +			return -EINVAL;
> +
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) ==
> +		    BPF_F_ADJ_ROOM_DECAP_L4_MASK)
> +			return -EINVAL;
> +
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK) ==
> +		    BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)
> +			return -EINVAL;
> +
> +		/* Reject mutually exclusive decap tunnel type flags. */
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) &&
> +		    (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
> +			return -EINVAL;
> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP)
> +			len_decap_min += sizeof(struct udphdr);
> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L4_GRE)
> +			len_decap_min += sizeof(struct gre_base_hdr);
> +
> +		if (len_diff_abs < len_decap_min)
> +			return -EINVAL;
> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L3_IPV4)
>   			len_min = sizeof(struct iphdr);
> -			break;
> -		case BPF_F_ADJ_ROOM_DECAP_L3_IPV6:
> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L3_IPV6)
>   			len_min = sizeof(struct ipv6hdr);
> -			break;
> -		default:
> -			return -EINVAL;
> -		}
>   	}
>   
>   	len_cur = skb->len - skb_network_offset(skb);