netdev - Re: [PATCH RFC v3 3/8] vhost-net: allow configuring extended features

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Message-ID: <ebf8e65a-9907-4ecf-a411-7002f11b9d1f@daynix.com>
Date: Mon, 16 Jun 2025 17:57:49 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jason Wang <jasowang@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "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>, Eugenio Pérez
 <eperezma@...hat.com>, Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [PATCH RFC v3 3/8] vhost-net: allow configuring extended features

On 2025/06/14 16:27, Paolo Abeni wrote:
> On 6/8/25 8:16 AM, Akihiko Odaki wrote:
>> On 2025/06/06 20:45, Paolo Abeni wrote:
>>> +
>>> +		/* Zero the trailing space provided by user-space, if any */
>>> +		if (i < count && clear_user(argp, (count - i) * sizeof(u64)))
>>
>> I think checking i < count is a premature optimization; it doesn't
>> matter even if we spend a bit longer because of the lack of the check.
> 
> FTR, the check is not an optimization. if `i` is greater than `count`,
> `clear_user` is going to try to clear almost all the memory space (the
> 2nd argument is an unsigned one) and will likely return with error.
> 
> I think it's functionally needed.

The for loop tells i cannot be greater than count, so i < count will be 
false only when i == count. clear_user() does nothing except incurring 
small overheads in that case. i < count can be safely removed.

Regards,
Akihiko Odaki