[Qemu-devel] Re: virtio-blk.c handling of i/o which is not a 512 multiple

[Stefan Hajnoczi]

On Wed, Mar 30, 2011 at 9:15 AM, Conor Murphy
<address@hidden> wrote:
> I'm trying to write a virtio-blk driver for Solaris. I've gotten it to the 
> point
> where Solaris can see the device and create a ZFS file system on it.
>
> However when I try and create a UFS filesystem on the device, the VM crashed
> with the error
> *** glibc detected *** /usr/bin/qemu-kvm: double free or corruption (!prev):
> 0x00007f2d38000a00 ***

This is a bug in QEMU.  A guest must not be able to trigger a crash.

> I can reproduce the problem with a simple dd, i.e.
> dd if=/dev/zero of=/dev/rdsk/c2d10p0 bs=5000 count=1

I think this a raw character device, which is why you're even able to
perform non-blocksize accesses?  Have you looked at how other drivers
(like the Xen pv blkfront) handle this?

> My driver will create a virtio-blk request with two elements in the sg list, 
> one
> for the first 4096 byes and the other for the remaining 904.
>
> From stepping through with gdb, virtio_blk_handle_write will sets n_sectors 
> to 9
> (5000 / 512). Later on the code, n_sectors is used the calculate the size of 
> the
> buffer required but 9 * 512 is too small and so when the request is process it
> ends up writing past the end of the buffer and I guest this triggers the glibc
> error.

We need to validate that (qiov->size % BDRV_SECTOR_SIZE) == 0 and
reject invalid requests.

> Is there a requirement for virtio-blk guest drivers that all i/o requests are
> sized in multiples of 512 bytes?

There is no strict requirement according to the virtio specification,
but maybe there should be:

http://ozlabs.org/~rusty/virtio-spec/virtio-spec-0.8.9.pdf

Stefan