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Subject: Re: [PATCH] virtio-pci: Remove wrong address verification in
 vp_del_vqs()
From:   Christophe Marie Francois Dupont de Dinechin <cdupontd@redhat.com>
In-Reply-To: <20220428070345-mutt-send-email-mst@kernel.org>
Date:   Thu, 28 Apr 2022 13:52:56 +0200
Cc:     Murilo Opsfelder Araujo <muriloo@linux.ibm.com>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, mopsfelder@gmail.com,
        Christophe de Dinechin <dinechin@redhat.com>
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References: <20220415023002.49805-1-muriloo@linux.ibm.com>
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To:     "Michael S. Tsirkin" <mst@redhat.com>
Precedence: bulk

[Resend, still struggling with new laptop email settings]

> On 28 Apr 2022, at 13:03, Michael S. Tsirkin <mst@redhat.com> wrote:
>=20
> On Thu, Apr 28, 2022 at 11:55:31AM +0200, Christophe Marie Francois =
Dupont de Dinechin wrote:
>>=20
>>=20
>>> On 28 Apr 2022, at 11:51, Christophe Marie Francois Dupont de =
Dinechin <cdupontd@redhat.com> wrote:
>>>=20
>>>=20
>>>=20
>>>> On 28 Apr 2022, at 11:46, Christophe Marie Francois Dupont de =
Dinechin <cdupontd@redhat.com> wrote:
>>>>=20
>>>>=20
>>>>=20
>>>>> On 15 Apr 2022, at 05:51, Murilo Opsfelder Ara=C3=BAjo =
<muriloo@linux.ibm.com> wrote:
>>>>>=20
>>>>> 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 =E2=80=98vp_del_vqs=
=E2=80=99:
>>>>>> drivers/virtio/virtio_pci_common.c:257:29: warning: the =
comparison will always evaluate as =E2=80=98true=E2=80=99 for the =
pointer operand in =E2=80=98vp_dev->msix_affinity_masks + =
(sizetype)((long unsigned int)i * 256)=E2=80=99 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=3D102103
>>>>>> Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.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 =3D 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) {
>>>>>=20
>>>>> After I sent this message, I realized that Christophe (copied =
here)
>>>>> had already proposed a fix:
>>>>>=20
>>>>> =
https://lore.kernel.org/lkml/20220414150855.2407137-4-dinechin@redhat.com/=

>>>>>=20
>>>>> Christophe,
>>>>>=20
>>>>> 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?
>>>>=20
>>>> Apologies for the delay in responding, broken laptop=E2=80=A6
>>>>=20
>>>> In the case where CONFIG_CPUMASK_OFFSTACK is not defined, we have:
>>>>=20
>>>> 	typedef struct cpumask cpumask_var_t[1];
>>>>=20
>>>> So that vp_dev->msix_affinity_masks[i] is statically not null =
(that=E2=80=99s the warning)
>>>> but also a static pointer, so not kfree-safe IMO.
>>>=20
>>> =E2=80=A6 which also renders my own patch invalid :-/
>>>=20
>>> Compiler warnings are good. Clearly not sufficient.
>>=20
>> Ah, I just noticed that free_cpumask_var is a noop in that case.
>>=20
>> So yes, your fix is better :-)
>=20
> ACK then?

Yes.

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

>=20