[PATCH v1 3/5] bpf: add helper masks for ADJ_ROOM decap flags

[PATCH v1 3/5] bpf: add helper masks for ADJ_ROOM decap flags

[Nick Hudson]

Introduce helper masks for bpf_skb_adjust_room() decapsulation flags
to simplify validation and keep flag handling readable:

- BPF_F_ADJ_ROOM_DECAP_L4_MASK
- BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK
- BPF_F_ADJ_ROOM_ENCAP_MASK
- BPF_F_ADJ_ROOM_DECAP_MASK

Also fold these masks into BPF_F_ADJ_ROOM_MASK and per-path flag
validation checks.

This is a refactoring/plumbing change; functional decapsulation and
GSO behavior updates are handled in later patches.

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 | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 0d5d5a17acb2..ac7e1068fe4c 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3483,14 +3483,27 @@ static u32 bpf_skb_net_base_len(const struct sk_buff *skb)
 #define BPF_F_ADJ_ROOM_DECAP_L3_MASK	(BPF_F_ADJ_ROOM_DECAP_L3_IPV4 | \
 					 BPF_F_ADJ_ROOM_DECAP_L3_IPV6)
 
-#define BPF_F_ADJ_ROOM_MASK		(BPF_F_ADJ_ROOM_FIXED_GSO | \
-					 BPF_F_ADJ_ROOM_ENCAP_L3_MASK | \
+#define BPF_F_ADJ_ROOM_DECAP_L4_MASK	(BPF_F_ADJ_ROOM_DECAP_L4_UDP | \
+					 BPF_F_ADJ_ROOM_DECAP_L4_GRE)
+
+#define BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK	(BPF_F_ADJ_ROOM_DECAP_IPXIP4 | \
+					 BPF_F_ADJ_ROOM_DECAP_IPXIP6)
+
+#define BPF_F_ADJ_ROOM_ENCAP_MASK	(BPF_F_ADJ_ROOM_ENCAP_L3_MASK | \
 					 BPF_F_ADJ_ROOM_ENCAP_L4_GRE | \
 					 BPF_F_ADJ_ROOM_ENCAP_L4_UDP | \
 					 BPF_F_ADJ_ROOM_ENCAP_L2_ETH | \
 					 BPF_F_ADJ_ROOM_ENCAP_L2( \
-					  BPF_ADJ_ROOM_ENCAP_L2_MASK) | \
-					 BPF_F_ADJ_ROOM_DECAP_L3_MASK)
+					  BPF_ADJ_ROOM_ENCAP_L2_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 | \
+					 BPF_F_ADJ_ROOM_DECAP_MASK | \
+					 BPF_F_ADJ_ROOM_NO_CSUM_RESET)
 
 static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
 			    u64 flags)
@@ -3502,6 +3515,11 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
 	unsigned int gso_type = SKB_GSO_DODGY;
 	int ret;
 
+	if (unlikely(flags & ~(BPF_F_ADJ_ROOM_ENCAP_MASK |
+			       BPF_F_ADJ_ROOM_NO_CSUM_RESET |
+			       BPF_F_ADJ_ROOM_FIXED_GSO)))
+		return -EINVAL;
+
 	if (skb_is_gso(skb) && !skb_is_gso_tcp(skb)) {
 		/* udp gso_size delineates datagrams, only allow if fixed */
 		if (!(skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) ||
@@ -3611,8 +3629,8 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
 {
 	int ret;
 
-	if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO |
-			       BPF_F_ADJ_ROOM_DECAP_L3_MASK |
+	if (unlikely(flags & ~(BPF_F_ADJ_ROOM_DECAP_MASK |
+			       BPF_F_ADJ_ROOM_FIXED_GSO |
 			       BPF_F_ADJ_ROOM_NO_CSUM_RESET)))
 		return -EINVAL;
 
@@ -3708,8 +3726,7 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 	u32 off;
 	int ret;
 
-	if (unlikely(flags & ~(BPF_F_ADJ_ROOM_MASK |
-			       BPF_F_ADJ_ROOM_NO_CSUM_RESET)))
+	if (unlikely(flags & ~BPF_F_ADJ_ROOM_MASK))
 		return -EINVAL;
 	if (unlikely(len_diff_abs > 0xfffU))
 		return -EFAULT;
-- 
2.34.1

Re: [PATCH v1 3/5] bpf: add helper masks for ADJ_ROOM decap flags

[Willem de Bruijn]

Nick Hudson wrote:
> Introduce helper masks for bpf_skb_adjust_room() decapsulation flags
> to simplify validation and keep flag handling readable:
> 
> - BPF_F_ADJ_ROOM_DECAP_L4_MASK
> - BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK
> - BPF_F_ADJ_ROOM_ENCAP_MASK
> - BPF_F_ADJ_ROOM_DECAP_MASK
> 
> Also fold these masks into BPF_F_ADJ_ROOM_MASK and per-path flag
> validation checks.
> 
> This is a refactoring/plumbing change; functional decapsulation and
> GSO behavior updates are handled in later patches.
> 
> 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>

Only patches 3 through 5 were sent to the netdev list.

Please resubmit the entire series (after the customary 24 hr min) to
the full list.

Re: [PATCH v1 3/5] bpf: add helper masks for ADJ_ROOM decap flags

[Hudson, Nick]

[-- Attachment #1: Type: text/plain, Size: 1374 bytes --]



> On 17 Mar 2026, at 13:27, Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:
> 
> !-------------------------------------------------------------------|
>  This Message Is From an External Sender
>  This message came from outside your organization.
> |-------------------------------------------------------------------!
> 
> Nick Hudson wrote:
>> Introduce helper masks for bpf_skb_adjust_room() decapsulation flags
>> to simplify validation and keep flag handling readable:
>> 
>> - BPF_F_ADJ_ROOM_DECAP_L4_MASK
>> - BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK
>> - BPF_F_ADJ_ROOM_ENCAP_MASK
>> - BPF_F_ADJ_ROOM_DECAP_MASK
>> 
>> Also fold these masks into BPF_F_ADJ_ROOM_MASK and per-path flag
>> validation checks.
>> 
>> This is a refactoring/plumbing change; functional decapsulation and
>> GSO behavior updates are handled in later patches.
>> 
>> 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>
> 
> Only patches 3 through 5 were sent to the netdev list.

oops.

> 
> Please resubmit the entire series (after the customary 24 hr min) to
> the full list.

Will do… as v2 with patch re-order you mentioned?


[-- Attachment #2: smime.p7s --]
[-- Type: application/pkcs7-signature, Size: 3067 bytes --]

Re: [PATCH v1 3/5] bpf: add helper masks for ADJ_ROOM decap flags

[Willem de Bruijn]

Hudson, Nick wrote:
> 
> 
> > On 17 Mar 2026, at 13:27, Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:
> > 
> > !-------------------------------------------------------------------|
> >  This Message Is From an External Sender
> >  This message came from outside your organization.
> > |-------------------------------------------------------------------!
> > 
> > Nick Hudson wrote:
> >> Introduce helper masks for bpf_skb_adjust_room() decapsulation flags
> >> to simplify validation and keep flag handling readable:
> >> 
> >> - BPF_F_ADJ_ROOM_DECAP_L4_MASK
> >> - BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK
> >> - BPF_F_ADJ_ROOM_ENCAP_MASK
> >> - BPF_F_ADJ_ROOM_DECAP_MASK
> >> 
> >> Also fold these masks into BPF_F_ADJ_ROOM_MASK and per-path flag
> >> validation checks.
> >> 
> >> This is a refactoring/plumbing change; functional decapsulation and
> >> GSO behavior updates are handled in later patches.
> >> 
> >> 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>
> > 
> > Only patches 3 through 5 were sent to the netdev list.
> 
> oops.
> 
> > 
> > Please resubmit the entire series (after the customary 24 hr min) to
> > the full list.
> 
> Will do… as v2 with patch re-order you mentioned?
 
Yes

[PATCH v1 4/5] bpf: add guard rails for new DECAP flags

[Nick Hudson]

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 | 45 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 36 insertions(+), 9 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index ac7e1068fe4c..437e0da34f84 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>
@@ -3745,20 +3746,46 @@ 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) {
+		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:
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) ==
+		    BPF_F_ADJ_ROOM_DECAP_L3_MASK)
+			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;
+
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) &&
+		    (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
+			return -EINVAL;
+
+		if (mode == BPF_ADJ_ROOM_MAC)
+			len_decap_min += proto == htons(ETH_P_IP) ?
+					 sizeof(struct iphdr) : sizeof(struct ipv6hdr);
+
+		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);
-- 
2.34.1

Re: [PATCH v1 4/5] bpf: add guard rails for new DECAP flags

[Willem de Bruijn]

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>


This patch probably should come before 3, as 3 enables the features
without the guard rails in place.

> ---
>  net/core/filter.c | 45 ++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 36 insertions(+), 9 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index ac7e1068fe4c..437e0da34f84 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>
> @@ -3745,20 +3746,46 @@ 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) {
> +		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:
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) ==
> +		    BPF_F_ADJ_ROOM_DECAP_L3_MASK)
> +			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;
> +

Are these equality tests shorthand based on knowledge that each only
have two options, so equality implies more than one option set? That
is not obvious/self documenting. Please add a brief comment.

> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) &&
> +		    (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
> +			return -EINVAL;
> +
> +		if (mode == BPF_ADJ_ROOM_MAC)
> +			len_decap_min += proto == htons(ETH_P_IP) ?
> +					 sizeof(struct iphdr) : sizeof(struct ipv6hdr);

MAC is not a GSO related decap, can be used for insertion/deletion of
L2.5 headers. This should be dropped.

> +
> +		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);
> -- 
> 2.34.1
> 

[PATCH v1 5/5] bpf: clear decap tunnel GSO state in skb_adjust_room

[Nick Hudson]

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@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 | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 437e0da34f84..8a7c06a10dd4 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;
+		}
+
 		/* Header must be checked, and gso_segs recomputed. */
 		shinfo->gso_type |= SKB_GSO_DODGY;
 		shinfo->gso_segs = 0;
-- 
2.34.1

Re: [PATCH v1 5/5] bpf: clear decap tunnel GSO state in skb_adjust_room

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 2457 bytes --]

> diff --git a/net/core/filter.c b/net/core/filter.c
> --- 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);

Should SKB_GSO_TUNNEL_REMCSUM also be cleared here when decapping
a UDP tunnel?

VXLAN and Geneve set SKB_GSO_TUNNEL_REMCSUM alongside
SKB_GSO_UDP_TUNNEL during encapsulation:

net/ipv4/udp_offload.c:udp_gro_complete():
    skb_shinfo(skb)->gso_type |= SKB_GSO_TUNNEL_REMCSUM;

drivers/net/vxlan/vxlan_core.c:vxlan_build_skb():
    type |= SKB_GSO_TUNNEL_REMCSUM;

After clearing SKB_GSO_UDP_TUNNEL{,_CSUM} but not TUNNEL_REMCSUM,
the gso_type has an orphaned tunnel flag with no associated tunnel
type.

> +
> +		/* 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)))

This check does not include SKB_GSO_TUNNEL_REMCSUM. If a packet
had both SKB_GSO_UDP_TUNNEL and SKB_GSO_TUNNEL_REMCSUM set, the
L4_UDP clearing above removes UDP_TUNNEL but leaves REMCSUM. This
check then concludes no tunnel GSO flags remain and clears
skb->encapsulation, even though a tunnel-related GSO bit is still
present.

The kernel's own UDP tunnel segmentation in
__skb_udp_tunnel_segment() explicitly handles TUNNEL_REMCSUM as
part of tunnel processing:

    remcsum = !!(skb_shinfo(skb)->gso_type &
                 SKB_GSO_TUNNEL_REMCSUM);

Would it make sense to include TUNNEL_REMCSUM in both the L4_UDP
clearing and this encapsulation check?

> +				if (skb->encapsulation)
> +					skb->encapsulation = 0;
> +		}


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/23194329334