netdev - Re: [PATCH] bonding: Always assign be16 value to vlan

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Date:   Thu, 20 Apr 2023 23:23:03 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Jay Vosburgh <jay.vosburgh@...onical.com>
Cc:     Simon Horman <horms@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Andy Gospodarek <andy@...yhouse.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] bonding: Always assign be16 value to vlan_proto

On Thu, Apr 20, 2023 at 12:47:33PM -0700, Jay Vosburgh wrote:
> Simon Horman <horms@...nel.org> wrote:
> 
> >The type of the vlan_proto field is __be16.
> >And most users of the field use it as such.
> >
> >In the case of setting or testing the field for the
> >special VLAN_N_VID value, host byte order is used.
> >Which seems incorrect.
> >
> >Address this issue by converting VLAN_N_VID to __be16.
> >
> >I don't believe this is a bug because VLAN_N_VID in
> >both little-endian (and big-endian) byte order does
> >not conflict with any valid values (0 through VLAN_N_VID - 1)
> >in big-endian byte order.
> 
> 	Is that true for all cases, or am I just confused?  Doesn't VLAN
> ID 16 match VLAN_N_VID (which is 4096) if byte swapped?
> 
> 	I.e., on a little endian host, VLAN_N_VID is 0x1000 natively,
> and network byte order (big endian) of VLAN ID 16 is also 0x1000.
> 
> 	Either way, I think the change is fine; VLAN_N_VID is being used
> as a sentinel value here, so the only real requirement is that it not
> match an actual VLAN ID in network byte order.
> 
> 	-J

In a strange twist of events, VLAN_N_VID is assigned as a sentinel value
to a variable which usually holds the output of vlan_dev_vlan_proto(),
or i.o.w. values like htons(ETH_P_8021Q), htons(ETH_P_8021AD). It is
certainly a confusion of types to assign VLAN_N_VID to it, but at least
it's not a valid VLAN protocol.

To answer your question, tags->vlan_proto is never compared against a
VLAN ID.