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

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Date:   Thu, 28 Apr 2022 11:51:40 +0200
From:   Christophe Marie Francois Dupont de Dinechin <cdupontd@...hat.com>
To:     muriloo@...ux.ibm.com
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, mopsfelder@...il.com,
        Christophe de Dinechin <dinechin@...hat.com>
Subject: Re: [PATCH] virtio-pci: Remove wrong address verification in
 vp_del_vqs()



> On 28 Apr 2022, at 11:46, Christophe Marie Francois Dupont de Dinechin <cdupontd@...hat.com> wrote:
> 
> 
> 
>> On 15 Apr 2022, at 05:51, Murilo Opsfelder Araújo <muriloo@...ux.ibm.com> wrote:
>> 
>> 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?
> 
> Apologies for the delay in responding, broken laptop…
> 
> In the case where CONFIG_CPUMASK_OFFSTACK is not defined, we have:
> 
> 	typedef struct cpumask cpumask_var_t[1];
> 
> So that vp_dev->msix_affinity_masks[i] is statically not null (that’s the warning)
> but also a static pointer, so not kfree-safe IMO.

… which also renders my own patch invalid :-/

Compiler warnings are good. Clearly not sufficient.