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Date: Thu, 3 Dec 2009 11:48:36 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH 06/41] virtio: Use DO_UPCAST instead of a
	cast
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To: Juan Quintela <quintela@redhat.com>
Cc: qemu-devel@nongnu.org

On Wed, Dec 02, 2009 at 08:03:22PM +0100, Juan Quintela wrote:
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > I don't understand.
> > container_of is just more generic than DO_UPCAST.
> > So why *ever* use DO_UPCAST? Let's get rid of it.
> 
> functions that use a PCIDevice and you pass FooState "require" that
> PCIDevice to be the 1st element in the struct.

If these used container_of consistently, maybe we won't get this
limitation.

> 
> Notice that it is "required", not "would be nice" or similar.  If that
> is not the way, things dont' work.
> 
>  In this particular case, it is also
> required that VirtIODevice to be the 1st element:
> 
> VirtIODevice *virtio_common_init(const char *name, uint16_t device_id,
>                                  size_t config_size, size_t struct_size)
> {
>     VirtIODevice *vdev;
> 
>     vdev = qemu_mallocz(struct_size);
> 
>     vdev->device_id = device_id;
>     vdev->status = 0;
>     vdev->isr = 0;
>     vdev->queue_sel = 0;
>     vdev->config_vector = VIRTIO_NO_VECTOR;
>     vdev->vq = qemu_mallocz(sizeof(VirtQueue) * VIRTIO_PCI_QUEUE_MAX);
> 
>     vdev->name = name;
>     vdev->config_len = config_size;
>     if (vdev->config_len)
>         vdev->config = qemu_mallocz(config_size);
>     else
>         vdev->config = NULL;
> 
>     return vdev;
> }
> 
> See how you create a device of size struct_size, but then you access it
> with vdev.  If vdev is _not_ the 1st element of the struct, you have got
> corruption.

A cleaner solution IMO would be to have callers allocate the memory
and pass VirtIODevice * to virtio_common_init.

>  DO_UPCAST() prevent you for having that error.


If we want to assert specific structure layout, this
should be a compile-time check. There's no
reason to do this check every time at a random place where
DO_UPCAST is called.

> container_of() would have leave you go around, and have a memory
> corruption not easy to fix.
> 
> DO_UPCAST() macro was created just to avoid this kind of errors.
> 
> Later, Juan.