Re: [PATCH v6 6/6] selftests/bpf: tc_tunnel - validate decap GSO and encapsulation state

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> On May 5, 2026, at 5:14=E2=80=AFAM, [email protected] wrote:
>=20
> !-------------------------------------------------------------------|
>  This Message Is =46rom an External Sender
>  This message came from outside your organization.
> |-------------------------------------------------------------------!
>=20
> Thank you for your contribution! Sashiko AI review found 2 potential =
issue(s) to consider:
> - [High] Pointer arithmetic `kskb->head + kskb->end` causes build =
failures and runtime logic errors on 32-bit kernels.
> - [Medium] Missing kernel feature causes test to fail instead of skip, =
breaking CI on older kernels.
> --
>=20
> commit f73fb4725baa7b601a1b82c32e5095c6b4294f5c
> Author: Nick Hudson <[email protected]>
>=20
> selftests/bpf: tc_tunnel - validate decap GSO and encapsulation state
>=20
> This commit enhances the tc_tunnel selftest to validate post-decap =
tunnel
> state for both GSO and non-GSO packets, and checks IPXIP decap for =
IPIP/SIT
> paths.
>=20
>> diff --git a/tools/testing/selftests/bpf/progs/test_tc_tunnel.c =
b/tools/testing/selftests/bpf/progs/test_tc_tunnel.c
>> index 7376df405a6b..853bca962910 100644
>> --- a/tools/testing/selftests/bpf/progs/test_tc_tunnel.c
>> +++ b/tools/testing/selftests/bpf/progs/test_tc_tunnel.c
> [ ... ]
>> @@ -621,6 +645,11 @@ static int decap_internal(struct __sk_buff *skb, =
int off, int len, char proto)
>> break;
>> case IPPROTO_GRE:
>> olen +=3D sizeof(struct gre_hdr);
>> + if (!bpf_core_enum_value_exists(enum bpf_adj_room_flags,
>> + BPF_F_ADJ_ROOM_DECAP_L4_GRE))
>> + return TC_ACT_SHOT;
>=20
> If the kernel is missing support for these new decapsulation flags, =
will
> returning TC_ACT_SHOT cause the test harness to time out and fail the =
test?
>=20
> Since selftests often run on older kernels, should missing features =
gracefully
> skip the test instead of failing it to prevent false positives and CI
> breakages on stable or LTS branches?

The current TC_ACT_SHOT fallback prevents silent passes, but it does =
cause affected subtests to fail on kernels without the new decap flags. =
We should move feature gating to prog_tests and skip unsupported =
subtests there, so older kernels report SKIP instead of FAIL while =
preserving strict checks when the feature exists.

>=20
> [ ... ]
>> @@ -655,6 +688,40 @@ static int decap_internal(struct __sk_buff *skb, =
int off, int len, char proto)
>> if (bpf_skb_adjust_room(skb, -olen, BPF_ADJ_ROOM_MAC, flags))
>> return TC_ACT_SHOT;
>>=20
>> + kskb =3D bpf_cast_to_kern_ctx(skb);
>> + shinfo =3D bpf_core_cast(kskb->head + kskb->end, struct =
skb_shared_info);
>=20
> Will this arithmetic work correctly on 32-bit kernels?
>=20
> On 64-bit architectures, sk_buff_data_t is an integer offset, so
> kskb->head + kskb->end works. However, on 32-bit platforms, =
sk_buff_data_t is
> a pointer (unsigned char *), making the addition of two pointers =
invalid C and
> causing build failures.
>=20
> Furthermore, even if cross-compiled with a 64-bit vmlinux.h, adding
> kskb->head to an absolute kskb->end pointer would yield an invalid =
address at
> runtime.

Martin previous replied

"Other tests have been using it to get shinfo already. I would leave it =
as is.
If it really would be needed to run and compile on 32-bit in the future,
it could use bpf_core_field_size(kskb->end) and then do things =
differently.=E2=80=9D

To this.



>=20
> --=20
> Sashiko AI review =C2=B7 =
https://urldefense.com/v3/__https://sashiko.dev/*/patchset/20260504101759.=
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mLQFQ7MMBQ5-R9rsHjqekoHUFNBl0WsEAX6ynWPASieLSi7E1DjAA-Rf64aLeCR$


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