Re: [PATCH] virtio-net: Copy all for dhclient workaround

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2025/04/07 17:29, Antoine Damhet wrote:
> On Sat, Apr 05, 2025 at 05:04:28PM +0900, Akihiko Odaki wrote:
>> The goal of commit 7987d2be5a8b ("virtio-net: Copy received header to
>> buffer") was to remove the need to patch the (const) input buffer with a
>> recomputed UDP checksum by copying headers to a RW region and inject the
>> checksum there. The patch computed the checksum only from the header
>> fields (missing the rest of the payload) producing an invalid one
>> and making guests fail to acquire a DHCP lease.
>>
>> Fix the issue by copying the entire packet instead of only copying the
>> headers.
>>
>> Fixes: 7987d2be5a8b ("virtio-net: Copy received header to buffer")
>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2727
>> Cc: qemu-stable@nongnu.org
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> 
> Tested-By: Antoine Damhet <adamhet@scaleway.com>
> 
>> ---
>> This patch aims to resolves the issue the following one also does:
>> https://lore.kernel.org/qemu-devel/20250404151835.328368-1-adamhet@scaleway.com
>>
>> The difference from the mentioned patch is that this patch also
>> preserves that the original intent of regressing change, which is to
>> remove the need to patch the (const) input buffer with a recomputed UDP
>> checksum.
>>
>> To Antoine Damhet:
>> I confirmed that DHCP is currently not working and this patch fixes the
>> issue, but I would appreciate if you also confirm the fix as I already
>> have done testing badly for the regressing patch.
> 
> Thanks for the swift response, ideally I'd like a non-regression test in
> the testsuite but a quick test showed me that I couldn't easily
> reproduce with user networking so unless someone has a great idea it
> would be a pain.
> 
>> ---
>>   hw/net/virtio-net.c | 35 ++++++++++++++++-------------------
>>   1 file changed, 16 insertions(+), 19 deletions(-)
>>
>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>> index de87cfadffe1..a920358a89c5 100644
>> --- a/hw/net/virtio-net.c
>> +++ b/hw/net/virtio-net.c
>> @@ -1687,6 +1687,11 @@ static void virtio_net_hdr_swap(VirtIODevice *vdev, struct virtio_net_hdr *hdr)
>>       virtio_tswap16s(vdev, &hdr->csum_offset);
>>   }
>>   
>> +typedef struct Header {
>> +    struct virtio_net_hdr_v1_hash virtio_net;
>> +    uint8_t payload[1500];
>> +} Header;
>> +
>>   /* dhclient uses AF_PACKET but doesn't pass auxdata to the kernel so
>>    * it never finds out that the packets don't have valid checksums.  This
>>    * causes dhclient to get upset.  Fedora's carried a patch for ages to
>> @@ -1701,7 +1706,7 @@ static void virtio_net_hdr_swap(VirtIODevice *vdev, struct virtio_net_hdr *hdr)
>>    * we should provide a mechanism to disable it to avoid polluting the host
>>    * cache.
>>    */
>> -static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
>> +static void work_around_broken_dhclient(struct Header *hdr,
>>                                           size_t *hdr_len, const uint8_t *buf,
>>                                           size_t buf_size, size_t *buf_offset)
>>   {
>> @@ -1711,20 +1716,20 @@ static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
>>       buf += *buf_offset;
>>       buf_size -= *buf_offset;
>>   
>> -    if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */
>> -        (buf_size >= csum_size && buf_size < 1500) && /* normal sized MTU */
>> +    if ((hdr->virtio_net.hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */
>> +        (buf_size >= csum_size && buf_size < sizeof(hdr->payload)) && /* normal sized MTU */
>>           (buf[12] == 0x08 && buf[13] == 0x00) && /* ethertype == IPv4 */
>>           (buf[23] == 17) && /* ip.protocol == UDP */
>>           (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */
>> -        memcpy((uint8_t *)hdr + *hdr_len, buf, csum_size);
>> -        net_checksum_calculate((uint8_t *)hdr + *hdr_len, csum_size, CSUM_UDP);
>> -        hdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
>> -        *hdr_len += csum_size;
>> -        *buf_offset += csum_size;
>> +        memcpy((uint8_t *)hdr + *hdr_len, buf, buf_size);
>> +        net_checksum_calculate((uint8_t *)hdr + *hdr_len, buf_size, CSUM_UDP);
>> +        hdr->virtio_net.hdr.flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
>> +        *hdr_len += buf_size;
>> +        *buf_offset += buf_size;
>>       }
>>   }
>>   
>> -static size_t receive_header(VirtIONet *n, struct virtio_net_hdr *hdr,
>> +static size_t receive_header(VirtIONet *n, Header *hdr,
>>                                const void *buf, size_t buf_size,
>>                                size_t *buf_offset)
> 
> `receive_header` can now "receive" the whole packet that's kinda
> misleading. I though another approach would be to only do the
> detection/flag patching from receive_header and recompute the checksum
> directly in the final `iov`, this would also eliminate the extra payload
> copy.

It is possible to avoid copying but I chose not to do that because this 
is not a hot path and the code complexity required for that does not 
look worthwhile for me.

But I agree that the names of receive_header() and Header structure are 
misleading. The reasoning I used to convince myself is that the "Header" 
is at the head of the packet at least. I'd like to hear if you have an 
idea of better naming; otherwise I would rather leave it as is.

Regards,
Akihiko Odaki

> 
> Cheers,
>