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.
> It should be reserved[6].


I prefer the 4b one because it likely is what drivers
have been doing anyway, unknowingly.

> > @@ -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