Re: [Qemu-devel] [PATCH 0/2] virtio len fixes for qemu.

[Michael S. Tsirkin]

On Wed, Mar 11, 2015 at 10:06:40PM +1030, Rusty Russell wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> > On Wed, Mar 11, 2015 at 02:47:47PM +0800, Fam Zheng wrote:
> >> On Wed, 03/11 07:19, Michael S. Tsirkin wrote:
> >> > On Wed, Mar 11, 2015 at 04:29:30PM +1030, Rusty Russell wrote:
> >> > > The virtio 'used' ring describes descriptors which have been used.  It
> >> > > also says how many bytes have been written to the ring.  For some 
> >> > > cases,
> >> > > this value is ignored by Linux guests, thus errors have not been 
> >> > > noticed.
> >> > > I was working on increasing the checking in Linux when I noticed this
> >> > > behaviour.
> >> > > 
> >> > > The first patch changes the 'len' formal parameter name to 
> >> > > 'len_written' to
> >> > > make the API clearer, and adds an assert(). The second fixes block 
> >> > > writes.
> >> > > 
> >> > > Cheers,
> >> > > Rusty.
> >> > > PS.  It's based on MST's virtio-1.0 tree, but should be easily ported.
> >> > 
> >> > Thanks, this applies to current master without issues.
> >> > However, I think  it's best to apply patch 2, then patch 1,
> >> > to avoid triggering errors when bisecting.
> >> 
> >> I'm seeing a make check failure. If this is a false alarm, the test should 
> >> be
> >> fixed too.
> >
> > Yea, I'm also now thinking we need a spec clarification on this one, and
> > some testing with non linux drivers before jumping to changing hosts and
> > guests.
> 
> The spec is very clear.  The implementation is crap; let's fix it before
> 1.0.
> 
> Quote:
> 
>         Each entry in the ring is a pair: \field{id} indicates the head
>         entry of the descriptor chain describing the buffer (this
>         matches an entry placed in the available ring by the guest
>         earlier), and \field{len} the total of bytes written into the
>         buffer. The latter is extremely useful for drivers using
>         untrusted buffers: if you do not know exactly how much has been
>         written by the device, you usually have to zero the buffer to
>         ensure no data leakage occurs.

Right so what does this "if you do not know exactly how much has been
written by the device" mean?

> I have a patch for the Linux side, too, which warns once per device
> and fixes it up.  I will make the warning conditional on v1.0.
> 
> Cheers,
> Rusty.