netdev - Re: [PATCH RFC v3 4/9] tap: Pad virtio header with zero

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Message-ID: <66eaccf25bc47_29b986294eb@willemb.c.googlers.com.notmuch>
Date: Wed, 18 Sep 2024 08:52:02 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.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>, 
 Paolo Abeni <pabeni@...hat.com>, 
 "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>, 
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: Re: [PATCH RFC v3 4/9] tap: Pad virtio header with zero

Akihiko Odaki wrote:
> tap used to simply advance iov_iter when it needs to pad virtio header.
> This leaves the garbage in the buffer as is and prevents telling if the
> header is padded or contains some real data.
> 
> In theory, a user of tap 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 tap.

This description does not describe the need for this operation.

The new extension seemingly requires these bytes to be cleared?
Please make that explicit.

> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
> ---
>  drivers/net/tap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> index 77574f7a3bd4..ba044302ccc6 100644
> --- a/drivers/net/tap.c
> +++ b/drivers/net/tap.c
> @@ -813,7 +813,7 @@ static ssize_t tap_put_user(struct tap_queue *q,
>  		    sizeof(vnet_hdr))
>  			return -EFAULT;
>  
> -		iov_iter_advance(iter, vnet_hdr_len - sizeof(vnet_hdr));
> +		iov_iter_zero(vnet_hdr_len - sizeof(vnet_hdr), iter);
>  	}
>  	total = vnet_hdr_len;
>  	total += skb->len;
> 
> -- 
> 2.46.0
>