Re: upstream kernel crashes

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 15, 2022 at 01:53:30PM -0700, Andres Freund wrote:
> Hi,
> 
> On 2022-08-15 16:21:51 -0400, Michael S. Tsirkin wrote:
> > On Mon, Aug 15, 2022 at 10:46:17AM -0700, Andres Freund wrote:
> > > Hi,
> > >
> > > On 2022-08-15 12:50:52 -0400, Michael S. Tsirkin wrote:
> > > > On Mon, Aug 15, 2022 at 09:45:03AM -0700, Andres Freund wrote:
> > > > > Hi,
> > > > >
> > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote:
> > > > > > OK so this gives us a quick revert as a solution for now.
> > > > > > Next, I would appreciate it if you just try this simple hack.
> > > > > > If it crashes we either have a long standing problem in virtio
> > > > > > code or more likely a gcp bug where it can't handle smaller
> > > > > > rings than what device requestes.
> > > > > > Thanks!
> > > > >
> > > > > I applied the below and the problem persists.
> > > > >
> > > > > [...]
> > > >
> > > > Okay!
> > >
> > > Just checking - I applied and tested this atop 6.0-rc1, correct? Or did you
> > > want me to test it with the 762faee5a267 reverted? I guess what you're trying
> > > to test if a smaller queue than what's requested you'd want to do so without
> > > the problematic patch applied...
> > >
> > >
> > > Either way, I did this, and there are no issues that I could observe. No
> > > oopses, no broken networking. But:
> > >
> > > To make sure it does something I added a debugging printk - which doesn't show
> > > up. I assume this is at a point at least earlyprintk should work (which I see
> > > getting enabled via serial)?
> > >
> 
> > Sorry if I was unclear.  I wanted to know whether the change somehow
> > exposes a driver bug or a GCP bug. So what I wanted to do is to test
> > this patch on top of *5.19*, not on top of the revert.
> 
> Right, the 5.19 part was clear, just the earlier test:
> 
> > > > > On 2022-08-15 11:40:59 -0400, Michael S. Tsirkin wrote:
> > > > > > OK so this gives us a quick revert as a solution for now.
> > > > > > Next, I would appreciate it if you just try this simple hack.
> > > > > > If it crashes we either have a long standing problem in virtio
> > > > > > code or more likely a gcp bug where it can't handle smaller
> > > > > > Thanks!
> 
> I wasn't sure about.
> 
> After I didn't see any effect on 5.19 + your patch, I grew a bit suspicious
> and added the printks.
> 
> 
> > Yes I think printk should work here.
> 
> The reason the debug patch didn't change anything, and that my debug printk
> didn't show, is that gcp uses the legacy paths...

Wait a second. Eureka I think!

So I think GCP is not broken.
I think what's broken is this patch:

commit cdb44806fca2d0ad29ca644cbf1505433902ee0c
Author: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Date:   Mon Aug 1 14:38:54 2022 +0800

    virtio_pci: support the arg sizes of find_vqs()


Specifically:

diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
index 2257f1b3d8ae..d75e5c4e637f 100644
--- a/drivers/virtio/virtio_pci_legacy.c
+++ b/drivers/virtio/virtio_pci_legacy.c
@@ -112,6 +112,7 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
                                  unsigned int index,
                                  void (*callback)(struct virtqueue *vq),
                                  const char *name,
+                                 u32 size,
                                  bool ctx,
                                  u16 msix_vec)
 {
@@ -125,10 +126,13 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
        if (!num || vp_legacy_get_queue_enable(&vp_dev->ldev, index))
                return ERR_PTR(-ENOENT);
 
+       if (!size || size > num)
+               size = num;
+
        info->msix_vector = msix_vec;
 
        /* create the vring */
-       vq = vring_create_virtqueue(index, num,
+       vq = vring_create_virtqueue(index, size,
                                    VIRTIO_PCI_VRING_ALIGN, &vp_dev->vdev,
                                    true, false, ctx,
                                    vp_notify, callback, name);

   

So if you pass the size parameter for a legacy device it will
try to make the ring smaller and that is not legal with
legacy at all. But the driver treats legacy and modern
the same, it allocates a smaller queue anyway.


Lo and behold, I pass disable-modern=on to qemu and it happily
corrupts memory exactly the same as GCP does.


So the new find_vqs API is actually completely broken, it can not work for
legacy at all and for added fun there's no way to find out
that it's legacy. Maybe we should interpret the patch

So I think I will also revert

04ca0b0b16f11faf74fa92468dab51b8372586cd..fe3dc04e31aa51f91dc7f741a5f76cc4817eb5b4







> If there were a bug in the legacy path, it'd explain why the problem only
> shows on gcp, and not in other situations.
> 
> I'll queue testing the legacy path with the equivalent change.
> 
> - Andres
> 
> 
> Greetings,
> 
> Andres Freund

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