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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Thu, 13 Jan 2022 14:45:59 +0100
Message-ID: <CAHmME9onde38SNBBsmypzr_QDSDiQ_0opPiqJ7sU5X-iMDtncQ@mail.gmail.com>
Subject: Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation
To:     =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Erik Kline <ek@google.com>,
        Fernando Gont <fgont@si6networks.com>,
        Lorenzo Colitti <lorenzo@google.com>,
        YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
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Hi Toke,

On Thu, Jan 13, 2022 at 2:30 PM Toke H=C3=B8iland-J=C3=B8rgensen <toke@redh=
at.com> wrote:
> Right, but that implies we need to work on a transition mechanism. For
> newly deployed systems changing the hash is obviously fine, it's the
> "reboot and you have a new address" problem.
>
> We could introduce new values to the addr_gen_mode? I.e. values of 4 and
> 5 would be equivalent to 2 and 3 (respectively), but with the new
> hashing algorithm? And then document that 2 and 3 are considered
> deprecated to be removed at some point in the future...

Right, so this is exactly the flow of conversation I anticipated.
"Let's change it!" "No, we can't." "Okay, let's add a knob."

The knob I was thinking about, though, was actually a compile-time one
CONFIG_NET_OBSOLETE_INSECURE_ADDRCONF_HASH, which itself is a `depends
on CONFIG_OLD_N_CRUSTY` or something. This way we could gate the
inclusion of sha1.c/sha1.o on that at compile time, and shave down
vmlinux a bit, which would make Geert happy.

Then, at some point down the road, we can talk about removing
CONFIG_NET_OBSOLETE_INSECURE_ADDRCONF_HASH too.

Jason