Re: [PATCH v2 2/3] tun: Pad virtio header with zero

Akihiko Odaki
2025-01-10T04:38:13Z

On 2025/01/09 21:46, Willem de Bruijn wrote:
> Akihiko Odaki wrote:
>> On 2025/01/09 16:31, Michael S. Tsirkin wrote:
>>> On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote:
>>>> tun used to simply advance iov_iter when it needs to pad virtio header,
>>>> which leaves the garbage in the buffer as is. This is especially
>>>> problematic when tun starts to allow enabling the hash reporting
>>>> feature; even if the feature is enabled, the packet may lack a hash
>>>> value and may contain a hole in the virtio header because the packet
>>>> arrived before the feature gets enabled or does not contain the
>>>> header fields to be hashed. If the hole is not filled with zero, it is
>>>> impossible to tell if the packet lacks a hash value.
>
> Zero is a valid hash value, so cannot be used as an indication that
> hashing is inactive.

Zeroing will initialize the hash_report field to
VIRTIO_NET_HASH_REPORT_NONE, which tells it does not have a hash value.

>
>>>> In theory, a user of tun can fill the buffer with zero before calling
>>>> read() to avoid such a problem, but leaving the garbage in the buffer is
>>>> awkward anyway so fill the buffer in tun.
>>>>
>>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>>>
>>> But if the user did it, you have just overwritten his value,
>>> did you not?
>>
>> Yes. but that means the user expects some part of buffer is not filled
>> after read() or recvmsg(). I'm a bit worried that not filling the buffer
>> may break assumptions others (especially the filesystem and socket
>> infrastructures in the kernel) may have.
>
> If this is user memory that is ignored by the kernel, just reflected
> back, then there is no need in general to zero it. There are many such
> instances, also in msg_control.

More specifically, is there any instance of recvmsg() implementation
which returns N and does not fill the complete N bytes of msg_iter?

>
> If not zeroing leads to ambiguity with the new feature, that would be
> a reason to add it -- it is always safe to do so.
>
>> If we are really confident that it will not cause problems, this
>> behavior can be opt-in based on a flag or we can just write some
>> documentation warning userspace programmers to initialize the buffer.