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

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Date:   Thu, 8 Oct 2020 22:32:25 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Rusty Russell <rusty@...tcorp.com.au>, stable@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH] vringh: fix __vringh_iov() when riov and wiov are
 different

On Thu, Oct 08, 2020 at 04:28:40PM -0400, Michael S. Tsirkin wrote:
> 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: stable@...r.kernel.org
> > > > Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> > > 
> > > 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 ...

Sure, I'll send a v2 adding this info.
Sorry for not adding it earlier!

Thanks,
Stefano