Re: [RFCv3 05/15] tcp: authopt: Add crypto initialization

[Leonard Crestez]

On 8/25/21 8:55 PM, Eric Dumazet wrote:
> On Wed, Aug 25, 2021 at 9:35 AM Leonard Crestez <cdleonard@xxxxxxxxx> wrote:
> > On 25.08.2021 02:34, Eric Dumazet wrote:
> > > On 8/24/21 2:34 PM, Leonard Crestez wrote:
> > > > The crypto_shash API is used in order to compute packet signatures. The
> > > > API comes with several unfortunate limitations:
> > > >
> > > > 1) Allocating a crypto_shash can sleep and must be done in user context.
> > > > 2) Packet signatures must be computed in softirq context
> > > > 3) Packet signatures use dynamic "traffic keys" which require exclusive
> > > > access to crypto_shash for crypto_setkey.
> > > >
> > > > The solution is to allocate one crypto_shash for each possible cpu for
> > > > each algorithm at setsockopt time. The per-cpu tfm is then borrowed from
> > > > softirq context, signatures are computed and the tfm is returned.
> > >
> > > I could not see the per-cpu stuff that you mention in the changelog.
> >
> > That's a little embarrasing, I forgot to implement the actual per-cpu
> > stuff. tcp_authopt_alg_imp.tfm is meant to be an array up to NR_CPUS and
> > tcp_authopt_alg_get_tfm needs no locking other than preempt_disable
> > (which should already be the case).
>
> Well, do not use arrays of NR_CPUS and instead use normal per_cpu
> accessors (as in __tcp_alloc_md5sig_pool)
>
> > The reference counting would still only happen from very few places:
> > setsockopt, close and openreq. This would only impact request/response
> > traffic and relatively little.
>
> What I meant is that __tcp_alloc_md5sig_pool() allocates stuff one time,
> we do not care about tcp_md5sig_pool_populated going back to false.
>
> Otherwise, a single user application constantly allocating a socket,
> enabling MD5 (or authopt), then closing the socket would incur
> a big cost on hosts with a lot of cpus.

Allocating only once would definitely simply things.

I don't know if this might end up tying hardware resources forever if 
some accelerators are in play but for this feature software-only crypto 
is perfectly fine.

--
Regards,
Leonard