Re: [PATCH net-next v4 7/9] tap: Avoid double-tracking iov_iter length changes

[Willem de Bruijn]

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@xxxxxxxxxx>
> ---
>  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.