netdev - Re: [PATCH v2 net-next] net: filter: export pkt_type

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Date:	Wed, 03 Sep 2014 18:08:13 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Denis Kirjanov <kda@...ux-powerpc.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Markos Chandras <markos.chandras@...tec.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v2 net-next] net: filter: export pkt_type_offset() helper

On Wed, 2014-09-03 at 16:14 -0700, Alexei Starovoitov wrote:
> On Wed, Sep 3, 2014 at 3:01 PM, Daniel Borkmann <dborkman@...hat.com> wrote:
> > On 09/03/2014 11:08 PM, Denis Kirjanov wrote:
> >>
> >
> >> +static int __init init_pkt_type_offset(void)
> >>   {
> >>         struct sk_buff skb_probe = { .pkt_type = ~0, };
> >>         u8 *ct = (u8 *) &skb_probe;
> >>         unsigned int off;
> >>
> >> +       pkt_type_offset = -1;
> >>         for (off = 0; off < sizeof(struct sk_buff); off++) {
> >> -               if (ct[off] == PKT_TYPE_MAX)
> >> +               if (ct[off] == PKT_TYPE_MAX) {
> >> +                       pkt_type_offset = off;
> >>                         return off;
> >> +               }
> >>         }
> >
> >
> > Why not BUG_ON() when pkt_type_offset could not be found?
> >
> > That way we would know that something is broken and you can
> > spare the checks for negative offsets everywhere ...
> 
> also such BUG_ON will be right in the face, since the kernel
> won't even boot :)
> I guess we can only hit it if compiler changes and starts
> doing crazing things with bitfields.
> Eric, you're the original author of this function, thoughts?

At the time it was written, BPF JIT would have failed and we would
revert to interpreter, thus BUG() was not needed : I used one
pr_err_once()

Note that pkt_type could be moved easily in sk_buff definition, so a
BUG() would be OK after this patch, since the check is done as a
pure_initcall() at boot time.





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