Re: [PATCH net-next] macvtap: advance iov iterator when needed in macvtap_put_user()

[Al Viro]

On Thu, Nov 20, 2014 at 04:34:26PM +0800, Herbert Xu wrote:
> On Thu, Nov 20, 2014 at 04:31:05PM +0800, Jason Wang wrote:
> > When mergeable buffer is used, vnet_hdr_sz is greater than sizeof struct
> > virtio_net_hdr. So we need advance the iov iterators in this case.
> > 
> > Fixes 6c36d2e26cda1ad3e2c4b90dd843825fc62fe5b4 ("macvtap: Use iovec iterators")
> > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> 
> Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

I'm not sure it's a good behaviour, actually - should we just leave an
unmodified gap in userland buffer there, with whatever garbage it might
have contained?

I do realize that this is what we used to do, but it might end up a source
of hard to debug userland bugs...  Perhaps that iov_iter_advance(iter, size)
would better be replaced with iov_iter_zero(size, iter)?
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