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

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Message-Id: <0ADDC136-7BAA-4D66-97A7-31747284BE2B@redhat.com>
Date:   Thu, 28 Apr 2022 13:52:56 +0200
From:   Christophe Marie Francois Dupont de Dinechin <cdupontd@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Murilo Opsfelder Araujo <muriloo@...ux.ibm.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()

[Resend, still struggling with new laptop email settings]

> On 28 Apr 2022, at 13:03, Michael S. Tsirkin <mst@...hat.com> wrote:
> 
> On Thu, Apr 28, 2022 at 11:55:31AM +0200, Christophe Marie Francois Dupont de Dinechin wrote:
>> 
>> 
>>> 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 :-)
> 
> ACK then?

Yes.

Acked-by: Christophe de Dinechin <dinechin@...hat.com>

>