netdev - Re: [PATCH net] vsock/virtio: fix "comparison of distinct pointer types lacks a cast" warning

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Date: Wed, 6 Dec 2023 13:28:18 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	virtualization@...ts.linux.dev,
	Arseniy Krasnov <avkrasnov@...utedevices.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Stefan Hajnoczi <stefanha@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] vsock/virtio: fix "comparison of distinct pointer
 types lacks a cast" warning

On Wed, Dec 06, 2023 at 05:41:43PM +0100, Stefano Garzarella wrote:
> After backporting commit 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY
> flag support") in CentOS Stream 9, CI reported the following error:
> 
>     In file included from ./include/linux/kernel.h:17,
>                      from ./include/linux/list.h:9,
>                      from ./include/linux/preempt.h:11,
>                      from ./include/linux/spinlock.h:56,
>                      from net/vmw_vsock/virtio_transport_common.c:9:
>     net/vmw_vsock/virtio_transport_common.c: In function ???virtio_transport_can_zcopy???:
>     ./include/linux/minmax.h:20:35: error: comparison of distinct pointer types lacks a cast [-Werror]
>        20 |         (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
>           |                                   ^~
>     ./include/linux/minmax.h:26:18: note: in expansion of macro ???__typecheck???
>        26 |                 (__typecheck(x, y) && __no_side_effects(x, y))
>           |                  ^~~~~~~~~~~
>     ./include/linux/minmax.h:36:31: note: in expansion of macro ???__safe_cmp???
>        36 |         __builtin_choose_expr(__safe_cmp(x, y), \
>           |                               ^~~~~~~~~~
>     ./include/linux/minmax.h:45:25: note: in expansion of macro ???__careful_cmp???
>        45 | #define min(x, y)       __careful_cmp(x, y, <)
>           |                         ^~~~~~~~~~~~~
>     net/vmw_vsock/virtio_transport_common.c:63:37: note: in expansion of macro ???min???
>        63 |                 int pages_to_send = min(pages_in_iov, MAX_SKB_FRAGS);
> 
> We could solve it by using min_t(), but this operation seems entirely
> unnecessary, because we also pass MAX_SKB_FRAGS to iov_iter_npages(),
> which performs almost the same check, returning at most MAX_SKB_FRAGS
> elements. So, let's eliminate this unnecessary comparison.
> 
> Fixes: 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY flag support")
> Cc: avkrasnov@...utedevices.com
> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> ---


Acked-by: Michael S. Tsirkin <mst@...hat.com>

>  net/vmw_vsock/virtio_transport_common.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index f6dc896bf44c..c8e162c9d1df 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -59,8 +59,7 @@ static bool virtio_transport_can_zcopy(const struct virtio_transport *t_ops,
>  	t_ops = virtio_transport_get_ops(info->vsk);
>  
>  	if (t_ops->can_msgzerocopy) {
> -		int pages_in_iov = iov_iter_npages(iov_iter, MAX_SKB_FRAGS);
> -		int pages_to_send = min(pages_in_iov, MAX_SKB_FRAGS);
> +		int pages_to_send = iov_iter_npages(iov_iter, MAX_SKB_FRAGS);
>  
>  		/* +1 is for packet header. */
>  		return t_ops->can_msgzerocopy(pages_to_send + 1);
> -- 
> 2.43.0