netdev - Re: [PATCH net-next 3/5] sctp: add spp_ipv6_flowlabel and spp_dscp for sctp

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Date:   Mon, 25 Jun 2018 10:03:19 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Neil Horman <nhorman@...driver.com>
Cc:     David Miller <davem@...emloft.net>, lucien.xin@...il.com,
        netdev@...r.kernel.org, linux-sctp@...r.kernel.org
Subject: Re: [PATCH net-next 3/5] sctp: add spp_ipv6_flowlabel and spp_dscp
 for sctp_paddrparams

On Mon, Jun 25, 2018 at 07:28:47AM -0400, Neil Horman wrote:
> On Mon, Jun 25, 2018 at 04:31:26PM +0900, David Miller wrote:
> > From: Xin Long <lucien.xin@...il.com>
> > Date: Mon, 25 Jun 2018 10:14:35 +0800
> > 
> > >  struct sctp_paddrparams {
> > > @@ -773,6 +775,8 @@ struct sctp_paddrparams {
> > >  	__u32			spp_pathmtu;
> > >  	__u32			spp_sackdelay;
> > >  	__u32			spp_flags;
> > > +	__u32			spp_ipv6_flowlabel;
> > > +	__u8			spp_dscp;
> > >  } __attribute__((packed, aligned(4)));
> > 
> > I don't think you can change the size of this structure like this.
> > 
> > This check in sctp_setsockopt_peer_addr_params():
> > 
> > 	if (optlen != sizeof(struct sctp_paddrparams))
> > 		return -EINVAL;
> > 
> > is going to trigger in old kernels when executing programs
> > built against the new struct definition.

That will happen, yes, but do we really care about being future-proof
here? I mean: if we also update such check(s) to support dealing with
smaller-than-supported structs, newer kernels will be able to run
programs built against the old struct, and the new one; while building
using newer headers and running on older kernel may fool the
application in other ways too (like enabling support for something
that is available on newer kernel and that is not present in the older
one).

> > 
> I think thats also the reason its a packed aligned attribute, it can't be
> changed, or older kernels won't be able to fill it out properly.
> Neil

It's more for supporting running 32-bits apps on 64-bit kernels
(according to 20c9c825b12fc).

  Marcelo