netdev - Re: [PATCH bpf-next v3 1/7] bpf, sockmap: add BPF_F

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Date: Fri, 25 Aug 2023 18:32:01 -0700
From: John Fastabend <john.fastabend@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>, 
 Liu Jian <liujian56@...wei.com>
Cc: john.fastabend@...il.com, 
 ast@...nel.org, 
 daniel@...earbox.net, 
 andrii@...nel.org, 
 martin.lau@...ux.dev, 
 song@...nel.org, 
 yonghong.song@...ux.dev, 
 kpsingh@...nel.org, 
 sdf@...gle.com, 
 haoluo@...gle.com, 
 jolsa@...nel.org, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 dsahern@...nel.org, 
 netdev@...r.kernel.org, 
 bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 1/7] bpf, sockmap: add BPF_F_PERMANENT flag
 for skmsg redirect

Jakub Sitnicki wrote:
> On Thu, Aug 24, 2023 at 10:39 PM +08, Liu Jian wrote:
> > If the sockmap msg redirection function is used only to forward packets
> > and no other operation, the execution result of the BPF_SK_MSG_VERDICT
> > program is the same each time. In this case, the BPF program only needs to
> > be run once. Add BPF_F_PERMANENT flag to bpf_msg_redirect_map() and
> > bpf_msg_redirect_hash() to implement this ability.
> >
> > Then we can enable this function in the bpf program as follows:
> > bpf_msg_redirect_hash(xx, xx, xx, BPF_F_INGRESS | BPF_F_PERMANENT);
> >
> > Test results using netperf  TCP_STREAM mode:
> > for i in 1 64 128 512 1k 2k 32k 64k 100k 500k 1m;then
> > netperf -T 1,2 -t TCP_STREAM -H 127.0.0.1 -l 20 -- -m $i -s 100m,100m -S 100m,100m
> > done
> >
> > before:
> > 3.84 246.52 496.89 1885.03 3415.29 6375.03 40749.09 48764.40 51611.34 55678.26 55992.78
> > after:
> > 4.43 279.20 555.82 2080.79 3870.70 7105.44 41836.41 49709.75 51861.56 55211.00 54566.85
> >
> > Signed-off-by: Liu Jian <liujian56@...wei.com>

[...]

> >  /* BPF_FUNC_skb_set_tunnel_key and BPF_FUNC_skb_get_tunnel_key flags. */
> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > index a29508e1ff35..df1443cf5fbd 100644
> > --- a/net/core/skmsg.c
> > +++ b/net/core/skmsg.c
> > @@ -885,6 +885,11 @@ int sk_psock_msg_verdict(struct sock *sk, struct sk_psock *psock,
> >  			goto out;
> >  		}
> >  		psock->redir_ingress = sk_msg_to_ingress(msg);
> > +		if (!msg->apply_bytes && !msg->cork_bytes)
> > +			psock->redir_permanent =
> > +				msg->flags & BPF_F_PERMANENT;
> > +		else
> > +			psock->redir_permanent = false;
> 
> Above can be rewritten as:
> 
> 		psock->redir_permanent = !msg->apply_bytes &&
> 					 !msg->cork_bytes &&
> 					 (msg->flags & BPF_F_PERMANENT);
> 
> But as I wrote earlier, I don't think it's a good idea to ignore the
> flag. We can detect this conflict at the time the bpf_msg_sk_redirect_*
> helper is called and return an error.
> 
> Naturally that means that that bpf_msg_{cork,apply}_bytes helpers need
> to be adjusted to return an error if BPF_F_PERMANENT has been set.

So far we've not really done much to protect a user from doing
rather silly things. The following will all do something without
errors,

  bpf_msg_apply_bytes()
  bpf_msg_apply_bytes() <- reset apply bytes

  bpf_msg_cork_bytes()
  bpf_msg_cork_bytes() <- resets cork byte

also,

  bpf_msg_redirect(..., BPF_F_INGRESS);
  bpf_msg_redirect(..., 0); <- resets sk_redir and flags

maybe there is some valid reason to even do above if further parsing
identifies some reason to redirect to a alert socket or something.

My original thinking was in the interest of not having a bunch of
extra checks for performance reasons we shouldn't add guard rails
unless something really unexpected might happen like a kernel
panic or what not.

This does feel a bit different though because before we
didn't have calls that could impact other calls. My best idea
is to just create a precedence and follow it. I would propose,

'If BPF_F_PERMANENT is set apply_bytes and cork_bytes are
 ignored.'

The other direction (what is above?) has a bit of an inconsistency
where these two flows are different?

  bpf_apply_bytes()
  bpf_msg_redirect(..., BPF_F_PERMANENT)

and

  bpf_msg_redirect(..., BPF_F_PERMANENT)
  bpf_apply_bytes()

It would be best if order of operations doesn't change the
outcome because that starts to get really hard to reason about.

This avoids having to add checks all over the place and then
if users want we could give some mechanisms to read apply
and cork bytes so people could write macros over those if
they really want the hard error.

WDYT?

[...]

Thanks!