Re: [PATCH RFC net-next 1/2] hsr: Allow to send a specific port and with HSR header

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Sebastian Andrzej Siewior wrote:
> On 2026-02-04 12:30:47 [-0500], Willem de Bruijn wrote:
> > > --- a/include/linux/skbuff.h
> > > +++ b/include/linux/skbuff.h
> > > @@ -605,6 +605,7 @@ struct skb_shared_info {
> > >  	};
> > >  	unsigned int	gso_type;
> > >  	u32		tskey;
> > > +	u32		hsr_ptp;
> > 
> > skb_shared_info cannot easily be expanded.
> > 
> > This is too specific a use-case to warrant fields in every packet.
> > 
> > I'm not super familiar with High-availability Seamless Redundancy
> > (HSR). Perhaps you can use either an skb_extension. Or the skb->cb[]
> > field if this data is only needed within the HSR protocol logic, so
> > can be assured to not be overwritten by other users of the control
> > block.
> 
> skb->cb does not work because I need share it between the HSR stack
> and AF_PACKET so it gets overwritten (as it is no meant to be shared
> between layers).
> I need actually three bits: HAVE_HEADER and PORT_NUMBER which 0-2.
> Would it work if I add a :1 and :2 member to struct sk_buff into the
> header group after unreadable? In my config there is a 6bit + 1 byte
> hole. Worst case (all ifdefs enabled) it sums to 30 bits so I would have
> just 2 left. Exceeding it by one bit in this case would extend create a
> 7bit + 1 byte hole and shift shifts tc_index + alloc_cpu. However there
> is a two byte hole between alloc_cpu before csum which is removed then.
> 
> Would it be okay if I occupy three bits in sk_buff which look unused?

Have you looked into using skb_extensions?