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

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Date:	Thu, 29 Jul 2010 13:50:45 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Lennert Buytenhek <buytenh@...tstofly.org>
Cc:	Karl Beldan <karl.beldan@...il.com>, netdev@...r.kernel.org,
	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 12:08, Mike Frysinger wrote:
> On Wed, Jul 21, 2010 at 11:35, Lennert Buytenhek wrote:
>> 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?
>
> that was my point.  should it be masking or doing a raw compare ?

and the answer is ... ?  so i can send an updated patch ;)
-mike
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