netdev - Re: [PATCH net-next v2] tun: Pad virtio headers

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Message-ID: <d4b7f8a0-db50-4b48-b5a3-f60eab76e96b@redhat.com>
Date: Thu, 20 Feb 2025 08:58:38 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>, Jonathan Corbet
 <corbet@....net>, Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jason Wang <jasowang@...hat.com>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
 Yuri Benditovich <yuri.benditovich@...nix.com>,
 Andrew Melnychenko <andrew@...nix.com>,
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com,
 devel@...nix.com
Subject: Re: [PATCH net-next v2] tun: Pad virtio headers

Hi,

On 2/15/25 7:04 AM, Akihiko Odaki wrote:
> tun simply advances iov_iter when it needs to pad virtio header,
> which leaves the garbage in the buffer as is. This will become
> 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.

Should virtio starting sending packets only after feature negotiation?
In other words, can the above happen without another bug somewhere else?

I guess the following question is mostly for Jason and Michael: could be
possible (/would it make any sense) to use a virtio_net_hdr `flags` bit
to explicitly signal the hash fields presence? i.e. making the actual
virtio_net_hdr size 'dynamic'.

> 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 replace advancing the iterator with writing zeros.
> 
> A user might have initialized the buffer to some non-zero value,
> expecting tun to skip writing it. As this was never a documented
> feature, this seems unlikely.
> 
> The overhead of filling the hole in the header is negligible when the
> header size is specified according to the specification as doing so will
> not make another cache line dirty under a reasonable assumption. Below
> is a proof of this statement:
> 
> The first 10 bytes of the header is always written and tun also writes
> the packet itself immediately after the 
> packet unless the packet is

 ^^^^^ this possibly should be 'virtio header'. Otherwise the sentence
is hard to follow for me.

> empty. This makes a hole between these writes whose size is: sz - 10
> where sz is the specified header size.
> 
> Therefore, we will never make another cache line dirty when:
> sz < L1_CACHE_BYTES + 10
> where L1_CACHE_BYTES is the cache line size. Assuming
> L1_CACHE_BYTES >= 16, this inequation holds when: sz < 26.
> 
> sz <= 20 according to the current specification so we even have a
> margin of 5 bytes in case that the header size grows in a future version
> of the specification.

FTR, the upcoming GSO over UDP tunnel support will add other 4 bytes to
the header. but that will still fit the given boundary.

/P