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SubjectRe: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
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On 2023/10/09 18:54, Willem de Bruijn wrote:
> On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>
>> On 2023/10/09 17:13, Willem de Bruijn wrote:
>>> On Sun, Oct 8, 2023 at 12:22 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>>>
>>>> virtio-net have two usage of hashes: one is RSS and another is hash
>>>> reporting. Conventionally the hash calculation was done by the VMM.
>>>> However, computing the hash after the queue was chosen defeats the
>>>> purpose of RSS.
>>>>
>>>> Another approach is to use eBPF steering program. This approach has
>>>> another downside: it cannot report the calculated hash due to the
>>>> restrictive nature of eBPF.
>>>>
>>>> Introduce the code to compute hashes to the kernel in order to overcome
>>>> thse challenges. An alternative solution is to extend the eBPF steering
>>>> program so that it will be able to report to the userspace, but it makes
>>>> little sense to allow to implement different hashing algorithms with
>>>> eBPF since the hash value reported by virtio-net is strictly defined by
>>>> the specification.
>>>>
>>>> The hash value already stored in sk_buff is not used and computed
>>>> independently since it may have been computed in a way not conformant
>>>> with the specification.
>>>>
>>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>>>
>>>> @@ -2116,31 +2172,49 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>>>> }
>>>>
>>>> if (vnet_hdr_sz) {
>>>> - struct virtio_net_hdr gso;
>>>> + union {
>>>> + struct virtio_net_hdr hdr;
>>>> + struct virtio_net_hdr_v1_hash v1_hash_hdr;
>>>> + } hdr;
>>>> + int ret;
>>>>
>>>> if (iov_iter_count(iter) < vnet_hdr_sz)
>>>> return -EINVAL;
>>>>
>>>> - if (virtio_net_hdr_from_skb(skb, &gso,
>>>> - tun_is_little_endian(tun), true,
>>>> - vlan_hlen)) {
>>>> + if ((READ_ONCE(tun->vnet_hash.flags) & TUN_VNET_HASH_REPORT) &&
>>>> + vnet_hdr_sz >= sizeof(hdr.v1_hash_hdr) &&
>>>> + skb->tun_vnet_hash) {
>>>
>>> Isn't vnet_hdr_sz guaranteed to be >= hdr.v1_hash_hdr, by virtue of
>>> the set hash ioctl failing otherwise?
>>>
>>> Such checks should be limited to control path where possible
>>
>> There is a potential race since tun->vnet_hash.flags and vnet_hdr_sz are
>> not read at once.
>
> It should not be possible to downgrade the hdr_sz once v1 is selected.

I see nothing that prevents shrinking the header size.

tun->vnet_hash.flags is read after vnet_hdr_sz so the race can happen
even for the case the header size grows though this can be fixed by
reordering the two reads.

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