netdev - Re: [Uclinux-dist-devel] [PATCH 1/2] net: dsa: introduce STPID switch tagging handling code

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Date:	Wed, 21 Jul 2010 17:35:31 +0200
From:	Lennert Buytenhek <buytenh@...tstofly.org>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Karl Beldan <karl.beldan@...il.com>, netdev@...r.kernel.org,
	Graf Yang <graf.yang@...log.com>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Uclinux-dist-devel] [PATCH 1/2] net: dsa: introduce STPID switch tagging handling code

On Wed, Jul 21, 2010 at 11:29:30AM -0400, Mike Frysinger wrote:

> >> +     source_port = dsa_header[1] & 0x03;
> >> +     if (source_port >= DSA_MAX_PORTS || ds->ports[source_port] == NULL)
> >> +             goto out_drop;
> >> +
> >> +     if (((dsa_header[0] & ETH_P_8021QH) == ETH_P_8021QH) &&
> >
> > This is bogus -- what it does is:
> >
> >        if ((dsa_header[0] & 0x81) == 0x81)
> >
> > It doesn't look like you need to mask here at all.
> 
> where does it say dsa_header[0] will always have 0x81 set ?

Eh?

This code is checking whether the packet has a STPID tag on it or not.
A STPID tag exists if the first 12 nibbles are 0x810.

You are checking whether the first 8 nibbles of this are equal to 0x81
by doing:

	if ((byte & 0x81) == 0x81)

What if the first byte is 0x93?  Or 0xc5?
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