linux-kernel - Re: [PATCH] virtio-pci: Remove wrong address verification in vp_del

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Date:   Fri, 15 Apr 2022 00:51:34 -0300
From:   Murilo Opsfelder Araújo <muriloo@...ux.ibm.com>
To:     mst@...hat.com
Cc:     jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, mopsfelder@...il.com,
        dinechin@...hat.com
Subject: Re: [PATCH] virtio-pci: Remove wrong address verification in
 vp_del_vqs()

On 4/14/22 23:30, Murilo Opsfelder Araujo wrote:
> GCC 12 enhanced -Waddress when comparing array address to null [0],
> which warns:
> 
>      drivers/virtio/virtio_pci_common.c: In function ‘vp_del_vqs’:
>      drivers/virtio/virtio_pci_common.c:257:29: warning: the comparison will always evaluate as ‘true’ for the pointer operand in ‘vp_dev->msix_affinity_masks + (sizetype)((long unsigned int)i * 256)’ must not be NULL [-Waddress]
>        257 |                         if (vp_dev->msix_affinity_masks[i])
>            |                             ^~~~~~
> 
> In fact, the verification is comparing the result of a pointer
> arithmetic, the address "msix_affinity_masks + i", which will always
> evaluate to true.
> 
> Under the hood, free_cpumask_var() calls kfree(), which is safe to pass
> NULL, not requiring non-null verification.  So remove the verification
> to make compiler happy (happy compiler, happy life).
> 
> [0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102103
> 
> Signed-off-by: Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
> ---
>   drivers/virtio/virtio_pci_common.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> index d724f676608b..5046efcffb4c 100644
> --- a/drivers/virtio/virtio_pci_common.c
> +++ b/drivers/virtio/virtio_pci_common.c
> @@ -254,8 +254,7 @@ void vp_del_vqs(struct virtio_device *vdev)
>   
>   	if (vp_dev->msix_affinity_masks) {
>   		for (i = 0; i < vp_dev->msix_vectors; i++)
> -			if (vp_dev->msix_affinity_masks[i])
> -				free_cpumask_var(vp_dev->msix_affinity_masks[i]);
> +			free_cpumask_var(vp_dev->msix_affinity_masks[i]);
>   	}
>   
>   	if (vp_dev->msix_enabled) {

After I sent this message, I realized that Christophe (copied here)
had already proposed a fix:

     https://lore.kernel.org/lkml/20220414150855.2407137-4-dinechin@redhat.com/

Christophe,

Since free_cpumask_var() calls kfree() and kfree() is null-safe,
can we just drop this null verification and call free_cpumask_var() right away?

-- 
Murilo