linux-kernel - Re: [PATCH] virtio: Change typecasts in vring

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Date:   Sat, 31 Aug 2019 13:43:19 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Matej Genci <matej.genci@...anix.com>
Cc:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio: Change typecasts in vring_init()

On Fri, Aug 30, 2019 at 05:58:23PM +0000, Matej Genci wrote:
> On 8/30/2019 3:02 PM, Michael S. Tsirkin wrote:
> > On Tue, Aug 27, 2019 at 03:20:57PM +0000, Matej Genci wrote:
> >> Compilers such as g++ 7.3 complain about assigning void* variable to
> >> a non-void* variable (like struct pointers) and pointer arithmetics
> >> on void*.
> >>
> >> Signed-off-by: Matej Genci <matej.genci@...anix.com>
> >> ---
> >>   include/uapi/linux/virtio_ring.h | 9 +++++----
> >>   1 file changed, 5 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
> >> index 4c4e24c291a5..2c339b9e2923 100644
> >> --- a/include/uapi/linux/virtio_ring.h
> >> +++ b/include/uapi/linux/virtio_ring.h
> >> @@ -168,10 +168,11 @@ static inline void vring_init(struct vring *vr, unsigned int num, void *p,
> >>   			      unsigned long align)
> >>   {
> >>   	vr->num = num;
> >> -	vr->desc = p;
> >> -	vr->avail = p + num*sizeof(struct vring_desc);
> >> -	vr->used = (void *)(((uintptr_t)&vr->avail->ring[num] + sizeof(__virtio16)
> >> -		+ align-1) & ~(align - 1));
> >> +	vr->desc = (struct vring_desc *)p;
> >> +	vr->avail = (struct vring_avail *)((uintptr_t)p
> >> +		+ num*sizeof(struct vring_desc));
> >> +	vr->used = (struct vring_used *)(((uintptr_t)&vr->avail->ring[num]
> >> +		+ sizeof(__virtio16) + align-1) & ~(align - 1));
> >>   }
> >>   
> >>   static inline unsigned vring_size(unsigned int num, unsigned long align)
> > 
> > I'm not really interested in building with g++, sorry.
> > Centainly not if it makes code less robust by forcing
> > casts where they weren't previously necessary.
> 
> Can you elaborate on how these casts make the code less robust?

If we ever change the variable types build will still pass
because of the cast.

> They aren't necessary in C but I think being explicit can improve
> readability as argued in
> https://softwareengineering.stackexchange.com/a/275714
> 
> > 
> > However, vring_init and vring_size are legacy APIs anyway,
> > so I'd be happy to add ifndefs that will allow userspace
> > simply hide these functions if it does not need them.
> > 
> 
> I feel like my patch is a harmless way of allowing this header
> to be used in C++ projects, but I'm happy to drop it in lieu of
> the guards if you feel strongly about it.
> 
> Thanks.
> Matej

Yea let's not even start.

> > 
> >> -- 
> >> 2.22.0
> >>
>