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Date:   Fri, 27 Jul 2018 09:37:47 +0200
From:   Steffen Klassert <steffen.klassert@secunet.com>
To:     Florian Westphal <fw@strlen.de>
CC:     Dmitry Safonov <dima@arista.com>, <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        <netdev@vger.kernel.org>, "Andy Lutomirski" <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
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        John Stultz <john.stultz@linaro.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>,
        "Stephen Boyd" <sboyd@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Thomas Gleixner" <tglx@linutronix.de>, <x86@kernel.org>,
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        Fan Du <fan.du@intel.com>
Subject: Re: [PATCH 00/18] xfrm: Add compat layer
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On Thu, Jul 26, 2018 at 10:49:59AM +0200, Florian Westphal wrote:
> Dmitry Safonov <dima@arista.com> wrote:
> > So, here I add a compatible layer to xfrm.
> > As xfrm uses netlink notifications, kernel should send them in ABI
> > format that an application will parse. The proposed solution is
> > to save the ABI of bind() syscall. The realization detail is
> > to create kernel-hidden, non visible to userspace netlink groups
> > for compat applications.
> 
> Why not use exisiting netlink support?
> Just add the 32bit skb to skb64->frag_list and let
> netlink find if tasks needs 64 or 32 one.
> 
> It only needs this small fix to properly signal the end of a dump:
> https://marc.info/?l=linux-netdev&m=126625240303351&w=2
> 
> I had started a second attempt to make xfrm compat work,
> but its still in early stage.
> 
> One link that might still have some value:
> https://git.breakpoint.cc/cgit/fw/net-next.git/commit/?h=xfrm_config_compat_07&id=f64430e6d9e297f3990f485a4832e273751b9869
> (compat structure definitions with BUILD_BUG_ON checking)
> 
> My plan was to make xfrm compat work strictly as shrinker (64->32)
> and expander (32->64), i.e. no/little changes to exisiting code and
> pass all "expanded" skbs through existing xfrm rcv functions.

I agree here with Florian. The code behind this ABI
is already complicated. Please stay away from generic
code a much as possible. Generic and compat code should
be clearly separated.