Subject: Re: [PATCH] net: tls: fix tls with sk_redirect using a BPF verdict.

From: Jakub Kicinski <[email-masked]>
Date: Tue, 28 Jun 2022 10:34:24 -0700

On Tue, 28 Jun 2022 17:25:05 +0200 Julien Salleyron wrote:
> This patch allows to use KTLS on a socket where we apply sk_redirect using a BPF
> verdict program.
> 
> Without this patch, we see that the data received after the redirection are
> decrypted but with an incorrect offset and length. It seems to us that the
> offset and length are correct in the stream-parser data, but finally not applied
> in the skb. We have simply applied those values to the skb.
> 
> In the case of regular sockets, we saw a big performance improvement from
> applying redirect. This is not the case now with KTLS, may be related to the
> following point.

It's because kTLS does a very expensive reallocation and copy for the
non-zerocopy case (which currently means all of TLS 1.3). I have
code almost ready to fix that (just needs to be reshuffled into
upstreamable patches). Brings us up from 5.9 Gbps to 8.4 Gbps per CPU
on my test box with 16k records. Probably much more than that with
smaller records.

> It is still necessary to perform a read operation (never triggered) from user
> space despite the redirection. It makes no sense, since this read operation is
> not necessary on regular sockets without KTLS.
> 
> We do not see how to fix this problem without a change of architecture, for
> example by performing TLS decrypt directly inside the BPF verdict program.
> 
> An example program can be found at
> https://github.com/juliens/ktls-bpf_redirect-example/
> 
> Co-authored-by: Marc Vertes <[email-masked]>
> ---
>  net/tls/tls_sw.c                           | 6 ++++++
>  tools/testing/selftests/bpf/test_sockmap.c | 8 +++-----
>  2 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
> index 0513f82b8537..a409f8a251db 100644
> --- a/net/tls/tls_sw.c
> +++ b/net/tls/tls_sw.c
> @@ -1839,8 +1839,14 @@ int tls_sw_recvmsg(struct sock *sk,
>  			if (bpf_strp_enabled) {
>  				/* BPF may try to queue the skb */
>  				__skb_unlink(skb, &ctx->rx_list);
> +
>  				err = sk_psock_tls_strp_read(psock, skb);
> +
>  				if (err != __SK_PASS) {
> +                    if (err == __SK_REDIRECT) {
> +                        skb->data += rxm->offset;
> +                        skb->len = rxm->full_len;
> +                    }

IDK what this is trying to do but I certainly depends on the fact 
we run skb_cow_data() and is not "generally correct" :S