Subject: Re: [PATCH] vringh: fix __vringh_iov() when riov and wiov are different

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 8 Oct 2020 16:28:40 -0400

On Thu, Oct 08, 2020 at 10:24:36PM +0200, Stefano Garzarella wrote:
> On Thu, Oct 08, 2020 at 04:00:51PM -0400, Michael S. Tsirkin wrote:
> > On Thu, Oct 08, 2020 at 06:13:11PM +0200, Stefano Garzarella wrote:
> > > If riov and wiov are both defined and they point to different
> > > objects, only riov is initialized. If the wiov is not initialized
> > > by the caller, the function fails returning -EINVAL and printing
> > > "Readable desc 0x... after writable" error message.
> > > 
> > > Let's replace the 'else if' clause with 'if' to initialize both
> > > riov and wiov if they are not NULL.
> > > 
> > > As checkpatch pointed out, we also avoid crashing the kernel
> > > when riov and wiov are both NULL, replacing BUG() with WARN_ON()
> > > and returning -EINVAL.
> > > 
> > > Fixes: f87d0fbb5798 ("vringh: host-side implementation of virtio rings.")
> > > Cc: [email-masked]
> > > Signed-off-by: Stefano Garzarella <[email-masked]>
> > 
> > Can you add more detail please? when does this trigger?
> 
> I'm developing vdpa_sim_blk and I'm using vringh_getdesc_iotlb()
> to get readable and writable buffers.
> 
> With virtio-blk devices a descriptors has both readable and writable
> buffers (eg. virtio_blk_outhdr in the readable buffer and status as last byte
> of writable buffer).
> So, I'm calling vringh_getdesc_iotlb() one time to get both type of buffer
> and put them in 2 iovecs:
> 
> 	ret = vringh_getdesc_iotlb(&vq->vring, &vq->riov, &vq->wiov,
> 				   &vq->head, GFP_ATOMIC);
> 
> With this patch applied it works well, without the function fails
> returning -EINVAL and printing "Readable desc 0x... after writable".
> 
> Am I using vringh_getdesc_iotlb() in the wrong way?
> 
> Thanks,
> Stefano
> 

I think it's ok, this info just needs to be in the commit log ...

> > > ---
> > >  drivers/vhost/vringh.c | 9 +++++----
> > >  1 file changed, 5 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > index e059a9a47cdf..8bd8b403f087 100644
> > > --- a/drivers/vhost/vringh.c
> > > +++ b/drivers/vhost/vringh.c
> > > @@ -284,13 +284,14 @@ __vringh_iov(struct vringh *vrh, u16 i,
> > >  	desc_max = vrh->vring.num;
> > >  	up_next = -1;
> > >  
> > > +	/* You must want something! */
> > > +	if (WARN_ON(!riov && !wiov))
> > > +		return -EINVAL;
> > > +
> > >  	if (riov)
> > >  		riov->i = riov->used = 0;
> > > -	else if (wiov)
> > > +	if (wiov)
> > >  		wiov->i = wiov->used = 0;
> > > -	else
> > > -		/* You must want something! */
> > > -		BUG();
> > >  
> > >  	for (;;) {
> > >  		void *addr;
> > > -- 
> > > 2.26.2
> >