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

From: Eric Dumazet
Date: Wed Aug 25 2021 - 13:56:08 EST

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.

> Performance was not a major focus so far. Preventing impact on non-AO
> connections is important but typical AO usecases are long-lived
> low-traffic connections.
> --
> Regards,
> Leonard