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

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Message-Id: <2F2EFFE9-5174-49A8-A71F-EE134D387E07@redhat.com>
Date:   Thu, 28 Apr 2022 11:55:31 +0200
From:   Christophe Marie Francois Dupont de Dinechin <cdupontd@...hat.com>
To:     Murilo Opsfelder Araujo <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 11:51, Christophe Marie Francois Dupont de Dinechin <cdupontd@...hat.com> wrote:
> 
> 
> 
>> 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.

Ah, I just noticed that free_cpumask_var is a noop in that case.

So yes, your fix is better :-)