netdev - Re: [bpf-next PATCH 2/3] bpf: fix running sk

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Date:   Mon, 01 Jun 2020 08:50:00 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     John Fastabend <john.fastabend@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        alexei.starovoitov@...il.com, daniel@...earbox.net
Subject: Re: [bpf-next PATCH 2/3] bpf: fix running sk_skb program types with
 ktls

John Fastabend wrote:
> Jakub Sitnicki wrote:
> > On Fri, 29 May 2020 16:06:59 -0700
> > John Fastabend <john.fastabend@...il.com> wrote:
> > 
> > > KTLS uses a stream parser to collect TLS messages and send them to
> > > the upper layer tls receive handler. This ensures the tls receiver
> > > has a full TLS header to parse when it is run. However, when a
> > > socket has BPF_SK_SKB_STREAM_VERDICT program attached before KTLS
> > > is enabled we end up with two stream parsers running on the same
> > > socket.
> > > 
> > > The result is both try to run on the same socket. First the KTLS
> > > stream parser runs and calls read_sock() which will tcp_read_sock
> > > which in turn calls tcp_rcv_skb(). This dequeues the skb from the
> > > sk_receive_queue. When this is done KTLS code then data_ready()
> > > callback which because we stacked KTLS on top of the bpf stream
> > > verdict program has been replaced with sk_psock_start_strp(). This
> > > will in turn kick the stream parser again and eventually do the
> > > same thing KTLS did above calling into tcp_rcv_skb() and dequeuing
> > > a skb from the sk_receive_queue.
> > > 
> > > At this point the data stream is broke. Part of the stream was
> > > handled by the KTLS side some other bytes may have been handled
> > > by the BPF side. Generally this results in either missing data
> > > or more likely a "Bad Message" complaint from the kTLS receive
> > > handler as the BPF program steals some bytes meant to be in a
> > > TLS header and/or the TLS header length is no longer correct.
> > > 
> > > We've already broke the idealized model where we can stack ULPs
> > > in any order with generic callbacks on the TX side to handle this.
> > > So in this patch we do the same thing but for RX side. We add
> > > a sk_psock_strp_enabled() helper so TLS can learn a BPF verdict
> > > program is running and add a tls_sw_has_ctx_rx() helper so BPF
> > > side can learn there is a TLS ULP on the socket.
> > > 
> > > Then on BPF side we omit calling our stream parser to avoid
> > > breaking the data stream for the KTLS receiver. Then on the
> > > KTLS side we call BPF_SK_SKB_STREAM_VERDICT once the KTLS
> > > receiver is done with the packet but before it posts the
> > > msg to userspace. This gives us symmetry between the TX and
> > > RX halfs and IMO makes it usable again. On the TX side we
> > > process packets in this order BPF -> TLS -> TCP and on
> > > the receive side in the reverse order TCP -> TLS -> BPF.
> > > 
> > > Discovered while testing OpenSSL 3.0 Alpha2.0 release.
> > > 
> > > Fixes: d829e9c4112b5 ("tls: convert to generic sk_msg interface")
> > > Signed-off-by: John Fastabend <john.fastabend@...il.com>
> > > ---

[...]

> > > +static void sk_psock_tls_verdict_apply(struct sk_psock *psock,
> > > +				       struct sk_buff *skb, int verdict)
> > > +{
> > > +	switch (verdict) {
> > > +	case __SK_REDIRECT:
> > > +		sk_psock_skb_redirect(psock, skb);
> > > +		break;
> > > +	case __SK_PASS:
> > > +	case __SK_DROP:
> > 
> > The two cases above need a "fallthrough;", right?
> 
> Correct otherwise will get the "fallthrough" patch shortly after this
> lands. Thanks I'll add it.
> 

hmm actually I don't think we need 'fallthrough;' here when the
case doesn't have statements,

 switch (a) {
 case 1:
 case 2:
 default:
     break;
 }

seems OK to me. I don't have a preference though so feel free to
correct me.