netdev - Re: KCM - recvmsg() mangles packets?

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Date:   Fri, 10 Aug 2018 00:06:46 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Tom Herbert <tom@...ntonium.net>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: KCM - recvmsg() mangles packets?

Tom Herbert wrote on Thu, Aug 09, 2018:
> > diff --git a/net/strparser/strparser.c b/net/strparser/strparser.c
> > index 625acb27efcc..348ff5945591 100644
> > --- a/net/strparser/strparser.c
> > +++ b/net/strparser/strparser.c
> > @@ -222,6 +222,16 @@ static int __strp_recv(read_descriptor_t *desc, struct sk_buff *orig_skb,
> >                 if (!stm->strp.full_len) {
> >                         ssize_t len;
> >
> > +                       /* Can only parse if there is no offset */
> > +                       if (unlikely(stm->strp.offset)) {
> > +                               if (!pskb_pull(skb, stm->strp.offset)) {
> > +                                       STRP_STATS_INCR(strp->stats.mem_fail);
> > +                                       strp_parser_err(strp, -ENOMEM, desc);
> > +                                       break;
> > +                               }
> > +                               stm->strp.offset = 0;
> > +                       }
> > +
> 
> Seems okay to me for a fix.

Hmm, if you say so, I'll send this as a patch for broader comments right
away.

> Looks like strp.offset is only set in one place and read in one
> place. With this pull maybe that just can go away?

Good point, when strp.offset is set the full_len is also being init'd so
we will necessarily do the pull next...

But the way tls uses strparser is also kind of weird, since they modify
the strp_msg's offset and full_len, I wouldn't want to assume we can't
have full_len == 0 *again* later with a non zero offset...
On the other hand they do handle non-zero offset in their parse function
so they'd be ok with that... Ultimately it's probably closer to a design
choice than anything else.

I'll still send a v0 of the patch as is, because I feel it's easier to
understand that we pull because the existing parse_msg functions do not
handle it properly, and will write a note that I intend to move it up a
few lines as a comment.

Thanks,
-- 
Dominique Martinet