Re: [PATCH net v3 8/8] selftests: net: tuntap: write UDP tunnel offsets in little endian

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] selftests: net: tun: UDP tunnel GSO tests still fail on
  big-endian hosts
--

commit 19b0a422177cb62b0357c401d17b939feb10dca3
Author: Eric Dumazet <edumazet@kernel.org>

selftests: net: tuntap: write UDP tunnel offsets in little endian

This commit converts the outer_th_offset and inner_nh_offset fields of
struct virtio_net_hdr_v1_hash_tunnel to little-endian in the tuntap
selftests while leaving the base virtio_net_hdr_v1 fields in host byte
order.

> diff --git a/tools/testing/selftests/net/tuntap_helpers.h b/tools/testing/selftests/net/tuntap_helpers.h
> index ecad83fae1582..dc85cd9bc0aec 100644
> --- a/tools/testing/selftests/net/tuntap_helpers.h
> +++ b/tools/testing/selftests/net/tuntap_helpers.h

[ ... ]

> @@ -375,11 +377,14 @@ static inline int build_virtio_net_hdr_v1_hash_tunnel(uint8_t *buf, bool is_tap,
>  	inner_iphlen = (inner_family == AF_INET) ? sizeof(struct iphdr) :
>  						   sizeof(struct ipv6hdr);
>  
> -	vh_tunnel->outer_th_offset = eth_hlen + outer_iphlen;
> -	vh_tunnel->inner_nh_offset = vh_tunnel->outer_th_offset + ETH_HLEN +
> -				     GENEVE_HLEN + sizeof(struct udphdr);
> +	outer_th = eth_hlen + outer_iphlen;
> +	inner_nh = outer_th + ETH_HLEN + GENEVE_HLEN + sizeof(struct udphdr);
>  
> -	vh->csum_start = vh_tunnel->inner_nh_offset + inner_iphlen;
> +	/* The tunnel offsets are always little endian. */
> +	vh_tunnel->outer_th_offset = htole16(outer_th);
> +	vh_tunnel->inner_nh_offset = htole16(inner_nh);
> +
> +	vh->csum_start = inner_nh + inner_iphlen;
>  	vh->csum_offset = __builtin_offsetof(struct udphdr, check);
>  	vh->flags = VIRTIO_NET_HDR_F_NEEDS_CSUM;
>  	vh->hdr_len = hdr_len;

[Severity: Medium]
Will the UDP tunnel GSO tests still fail on big-endian hosts because the
kernel hardcodes little_endian = true when delegating the base header fields
in virtio_net_hdr_tnl_to_skb() and virtio_net_hdr_tnl_from_skb()?

In tools/testing/selftests/net/tun.c, tun_open() does not set TUNSETVNETLE,
so tun_vnet_is_little_endian() is false and __tun_vnet_hdr_get() reads
hdr_len, csum_start, and csum_offset in host order. However, when
tun_vnet_hdr_tnl_to_skb() calls virtio_net_hdr_tnl_to_skb() with
little_endian = false:

include/linux/virtio_net.h:virtio_net_hdr_tnl_to_skb() {
	...
	inner_th = __virtio16_to_cpu(little_endian, hdr->csum_start);
	inner_nh = le16_to_cpu(vhdr->inner_nh_offset);
	outer_th = le16_to_cpu(vhdr->outer_th_offset);
	if (outer_th < outer_l3min ||
	    inner_nh < outer_th + sizeof(struct udphdr) ||
	    inner_th < inner_nh + inner_l3min)
		return -EINVAL;

	/* Let the basic parsing deal with plain GSO features. */
	ret = __virtio_net_hdr_to_skb(skb, hdr, true,
				      hdr->gso_type & ~gso_tunnel_type);
	if (ret)
		return ret;

	if (!virtio_net_tnl_offsets_ok(skb, has_eth, outer_isv6, outer_th))
		return -EINVAL;
	...
}

Because __virtio_net_hdr_to_skb() is called with little_endian hardcoded to
true instead of passing little_endian, it byte-swaps the host-order
csum_start, csum_offset, and gso_size on big-endian hosts. That causes
__virtio_net_hdr_to_skb() to return -EINVAL (either in pskb_may_pull() or the
skb->csum_offset check) before virtio_net_tnl_offsets_ok() is reached in
tun_vnet_udptnl_proto, tun_vnet_udptnl_proto_l3, and tun_vnet_udptnl.

Similarly, on the receive path in virtio_net_hdr_tnl_from_skb():

include/linux/virtio_net.h:virtio_net_hdr_tnl_from_skb() {
	...
	/* Let the basic parsing deal with plain GSO features. */
	skb_shinfo(skb)->gso_type &= ~tnl_gso_type;
	ret = virtio_net_hdr_from_skb(skb, hdr, true, false, vlan_hlen);
	skb_shinfo(skb)->gso_type |= tnl_gso_type;
	...
}

virtio_net_hdr_from_skb() is also called with little_endian hardcoded to
true, writing hdr->gso_size in little-endian while recv_gso_packet in
tools/testing/selftests/net/tun.c compares vh->gso_size against host-order
variant->gso_size.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261105115715.1466924-1-edumazet@kernel.org?part=8