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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: <A5CD1C91-F5D0-4109-9894-824184362B5C@redhat.com>
Date:   Thu, 28 Apr 2022 11:51:40 +0200
Cc:     "Michael S. Tsirkin" <mst@redhat.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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Message-Id: <B954A683-EE43-4469-B57A-EB8C6B671BEC@redhat.com>
References: <20220415023002.49805-1-muriloo@linux.ibm.com>
 <e8620e70-16bb-639c-9446-68dcd199253f@linux.ibm.com>
 <A5CD1C91-F5D0-4109-9894-824184362B5C@redhat.com>
To:     muriloo@linux.ibm.com
Precedence: bulk


> 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.

=E2=80=A6 which also renders my own patch invalid :-/

Compiler warnings are good. Clearly not sufficient.