Re: [PATCH net-next v2 3/3] sctp: Convert cookie authentication to use HMAC-SHA256

[Jakub Kicinski <kuba@kernel.org>]

On Sat, 16 Aug 2025 13:15:12 -0400 Xin Long wrote:
> > > Ideally we'd just fail the write and remove the last mentions of md5 and
> > > sha1 from the code.  But I'm concerned there could be a case where
> > > userspace is enabling cookie authentication by setting
> > > cookie_hmac_alg=md5 or cookie_hmac_alg=sha1, and by just failing the
> > > write the system would end up with cookie authentication not enabled.
> > >
> > > It would have been nice if this sysctl had just been a boolean toggle.
> > >
> > > A deprecation warning might be a good idea.  How about the following on
> > > top of this patch:  
> >
> > No strong opinion but I find the deprecation warnings futile.
> > Chances are we'll be printing this until the end of time.
> > Either someone hard-cares and we'll need to revert, or nobody
> > does and we can deprecate today.  
> Reviewing past network sysctl changes, several commits have simply
> removed or renamed parameters:
> 
> 4a7f60094411 ("tcp: remove thin_dupack feature")
> 4396e46187ca ("tcp: remove tcp_tw_recycle")
> d8b81175e412 ("tcp: remove sk_{tr}x_skb_cache")
> 3e0b8f529c10 ("net/ipv6: Expand and rename accept_unsolicited_na to
> accept_untracked_na")
> 5027d54a9c30 ("net: change accept_ra_min_rtr_lft to affect all RA lifetimes")
> 
> It seems to me that if we deprecate something, it's okay to change the
> sysctls, so I would prefer rejecting writes with md5 or sha1, or even
> better following Eric’s suggestion and turn this into a simple boolean
> toggle.

Slight preference towards reject. bool is worse in case we need to
revert (if it takes a few releases for the regression report to appear
we may have to maintain backward compat with both string and bool
formats going forward).