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Date:   Thu, 26 Jul 2018 10:49:59 +0200
From:   Florian Westphal <fw@strlen.de>
To:     Dmitry Safonov <dima@arista.com>
Cc:     linux-kernel@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>, netdev@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        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,
        linux-efi@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Shuah Khan <shuah@kernel.org>, linux-kselftest@vger.kernel.org,
        Eric Paris <eparis@redhat.com>,
        Florian Westphal <fw@strlen.de>,
        Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>,
        Pablo Neira Ayuso <pablo@netfilter.org>,
        Paul Moore <paul@paul-moore.com>, coreteam@netfilter.org,
        linux-audit@redhat.com, netfilter-devel@vger.kernel.org,
        Fan Du <fan.du@intel.com>
Subject: Re: [PATCH 00/18] xfrm: Add compat layer
Message-ID: <20180726084959.pzjvflfjq6a76du6@breakpoint.cc>
References: <20180726023144.31066-1-dima@arista.com>
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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.

Example to illustrate idea:
https://git.breakpoint.cc/cgit/fw/net-next.git/commit/?h=xfrm_config_compat_07&id=c622f067849b02170127b69471cb3481e4bc9e49

... its supposed to take 64bit skb and create a 32bit one from it.

Just for reference; I currently don't plan to work on this again.