Re: [PATCH nft 2/2] src: fix sub-bytes protocol header definition

[Florian Westphal <fw@strlen.de>]

Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> Update bitfield definitions to match according to the way they are
> expressed in RFC and IEEE specifications.

[..]

> 	len = round_up(expr->len, BITS_PER_BYTE);
> 
> Then, we substract the offset bits and the bitfield length.
> 
>         shift = len - (offset + expr->len);
> 
> From the delinearize, payload_expr_trim() needs to obtain the real
> offset through:
> 
> 	off = round_up(mask->len, BITS_PER_BYTE) -
> 		       mpz_fls(mask->value, mask->len);
> 
> For vlan id (offset 12), this gets the position of the last bit set in
> the mask (ie. 12), then we substract the length we fetch in bytes (16),
> so we obtain the real bitfield offset (4).
> 
> Then, we add that to the original payload offset that was expressed in
> bytes:
> 
> 	payload_offset += off;
> 
> Note that payload_expr_trim() now also adjusts the payload expression to
> its real length and offset.
> 
> Reported-by: Patrick McHardy <kaber@trash.net>>
> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> ---
> This aims to fix what we have, I'm willing to give another pass to this.

vlan type ip vlan id 4094 ip version 4 counter packets 567 bytes 37720
vlan type ip vlan id 4094 vlan pcp 0 vlan cfi 0 counter packets 567 bytes 37720
vlan type ip vlan id 4094 vlan pcp 1 vlan cfi 1 counter packets 0 bytes 0
vlan type ip vlan id 4094 vlan pcp 1 vlan cfi 0 counter packets 0 bytes 0
vlan type ip vlan id 4094 vlan pcp 0 vlan cfi 1 counter packets 0 bytes 0

tpcdump says:
'length 194: vlan 4094, p 0, ethertype IPv4', i.e. this looks correct.

Thanks Pablo!