Re: [syzbot] [kvm?] [net?] [virt?] WARNING in virtio_transport_send_pkt_info

[Will Deacon]

On Fri, Aug 15, 2025 at 06:44:47AM -0400, Michael S. Tsirkin wrote:
> On Fri, Aug 15, 2025 at 11:09:24AM +0100, Will Deacon wrote:
> > On Tue, Aug 12, 2025 at 06:15:46AM -0400, Michael S. Tsirkin wrote:
> > > On Tue, Aug 12, 2025 at 03:03:02AM -0700, syzbot wrote:
> > > > Hello,
> > > > 
> > > > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > > > WARNING in virtio_transport_send_pkt_info
> > > 
> > > OK so the issue triggers on
> > > commit 6693731487a8145a9b039bc983d77edc47693855
> > > Author: Will Deacon <will@xxxxxxxxxx>
> > > Date:   Thu Jul 17 10:01:16 2025 +0100
> > > 
> > >     vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers
> > >     
> > > 
> > > but does not trigger on:
> > > 
> > > commit 8ca76151d2c8219edea82f1925a2a25907ff6a9d
> > > Author: Will Deacon <will@xxxxxxxxxx>
> > > Date:   Thu Jul 17 10:01:15 2025 +0100
> > > 
> > >     vsock/virtio: Rename virtio_vsock_skb_rx_put()
> > >     
> > > 
> > > 
> > > Will, I suspect your patch merely uncovers a latent bug
> > > in zero copy handling elsewhere.

I'm still looking at this, but I'm not sure zero-copy is the right place
to focus on.

The bisected patch 6693731487a8 ("vsock/virtio: Allocate nonlinear SKBs
for handling large transmit buffers") only has two hunks. The first is
for the non-zcopy case and the latter is a no-op for zcopy, as
skb_len == VIRTIO_VSOCK_SKB_HEADROOM and so we end up with a linear SKB
regardless.

I'll keep digging...

Will