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

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Date:   Thu, 28 Apr 2022 07:03:51 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Christophe Marie Francois Dupont de Dinechin <cdupontd@...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()

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?