Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 13 Jan 2022 at 14:46, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Toke,
>
> On Thu, Jan 13, 2022 at 2:30 PM Toke Høiland-Jørgensen <toke@redhat.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.
>

What is the point of having CONFIG_OLD_N_CRUSTY if all distros are
going to enable it indefinitely?