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

Akihiko Odaki
2025-01-09T09:37:00Z

On 2025/01/09 16:43, Michael S. Tsirkin wrote:
> On Thu, Jan 09, 2025 at 04:41:50PM +0900, 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.
>>>>
>>>> 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 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.
>
> It's been like this for years, I'd say we wait until we know there's a problem?

Perhaps we can just leave it as is. Let me ask filesystem and networking
people:

Is it OK to leave some part of buffer uninitialized with read_iter() or
recvmsg()?

>
>>>
>>>> ---
>>>> drivers/net/tun_vnet.c | 3 ++-
>>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c
>>>> index fe842df9e9ef..ffb2186facd3 100644
>>>> --- a/drivers/net/tun_vnet.c
>>>> +++ b/drivers/net/tun_vnet.c
>>>> @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
>>>> if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr))
>>>> return -EFAULT;
>>>> - iov_iter_advance(iter, sz - sizeof(*hdr));
>>>> + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr))
>>>> + return -EFAULT;
>>>> return 0;
>>>> }
>>>>
>>>> --
>>>> 2.47.1
>>>
>