netdev - Re: [PATCH net-next v4 7/9] tap: Avoid double-tracking iov

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Message-ID: <678e321430c77_19c737294c5@willemb.c.googlers.com.notmuch>
Date: Mon, 20 Jan 2025 06:23:00 -0500
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>, 
 Stephen Hemminger <stephen@...workplumber.org>, 
 gur.stavi@...wei.com, 
 devel@...nix.com, 
 Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: Re: [PATCH net-next v4 7/9] tap: Avoid double-tracking iov_iter
 length changes

Akihiko Odaki wrote:
> tap_get_user() used to track the length of iov_iter with another
> variable. We can use iov_iter_count() to determine the current length
> to avoid such chores.
> 
> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
> ---
>  drivers/net/tap.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> index 5aa41d5f7765a6dcf185bccd3cba2299bad89398..061c2f27dfc83f5e6d0bea4da0e845cc429b1fd8 100644
> --- a/drivers/net/tap.c
> +++ b/drivers/net/tap.c
> @@ -641,7 +641,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>  	struct sk_buff *skb;
>  	struct tap_dev *tap;
>  	unsigned long total_len = iov_iter_count(from);
> -	unsigned long len = total_len;
> +	unsigned long len;
>  	int err;
>  	struct virtio_net_hdr vnet_hdr = { 0 };
>  	int vnet_hdr_len = 0;
> @@ -655,9 +655,8 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>  		vnet_hdr_len = READ_ONCE(q->vnet_hdr_sz);
>  
>  		err = -EINVAL;
> -		if (len < vnet_hdr_len)
> +		if (iov_iter_count(from) < vnet_hdr_len)
>  			goto err;
> -		len -= vnet_hdr_len;
>  
>  		err = -EFAULT;
>  		if (!copy_from_iter_full(&vnet_hdr, sizeof(vnet_hdr), from))
> @@ -671,10 +670,12 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>  				 tap16_to_cpu(q, vnet_hdr.csum_start) +
>  				 tap16_to_cpu(q, vnet_hdr.csum_offset) + 2);
>  		err = -EINVAL;
> -		if (tap16_to_cpu(q, vnet_hdr.hdr_len) > len)
> +		if (tap16_to_cpu(q, vnet_hdr.hdr_len) > iov_iter_count(from))
>  			goto err;
>  	}
>  
> +	len = iov_iter_count(from);
> +

Since len is still used in the rest of the function, might as well use
it from the start. I'd drop this patch.