Re: [PATCH] net: pad virtio_net_ff_cap_data

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

On Wed, Nov 19, 2025 at 08:29:48AM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: 19 November 2025 01:54 PM
> > 
> > struct virtio_net_ff_cap_data has 4 byte fields but the size is not a multiple of
> > 4.  drivers can easily get it wrong since compilers tend to add padding to align
> > such structures.
> > 
> > Since we are always allowed to pad or truncate admin commands, let's do just
> > that here.
> > 
> > Fixes: 899bb0c ("virtio-net: Add flow filter capability")
> > Cc: "Parav Pandit" <parav@nvidia.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  device-types/net/description.tex | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/device-types/net/description.tex b/device-
> > types/net/description.tex
> > index 9833d16..0b4fcac 100644
> > --- a/device-types/net/description.tex
> > +++ b/device-types/net/description.tex
> > @@ -3055,6 +3055,7 @@ \subsubsection{Flow filter}\label{sec:Device Types
> > / Network Device / Device Ope
> >          le32 rules_per_group_limit;
> >          u8 last_rule_priority;
> >          u8 selectors_per_classifier_limit;
> > +        u8 reserved[2];
> >  };
> >  \end{lstlisting}
> > 
> Thanks for the fix.
> We should align to the 8B boundary instead of 4B.

We can but we don't have to. We need to align on the
size of the largest field in the structure. This one
only has 4B fields.


> It should be reserved[6].
> 
> > @@ -3071,6 +3072,8 @@ \subsubsection{Flow filter}\label{sec:Device Types
> > / Network Device / Device Ope  \field{selectors_per_classifier_limit} is the
> > maximum number of selectors  that a classifier can have.
> > 
> > +\field{reserved} is reserved and set to zero.
> > +
> >  \subparagraph{VIRTIO_NET_FF_SELECTOR_CAP}
> >  \label{par:Device Types / Network Device / Device Operation / Flow filter /
> > Device and driver capabilities / VIRTIO-NET-FF-SELECTOR-CAP}
> > 
> > --
> > MST