Re: [PATCH net-next] net: filter: constify detection of pkt_type_offset

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Fr, 2014-09-12 at 12:55 +0200, Daniel Borkmann wrote:
> On 09/12/2014 12:22 PM, Hannes Frederic Sowa wrote:
> > Currently we have 2 pkt_type_offset functions doing the same thing and
> > spread across the architecture files. Remove those and replace them
> > with a PKT_TYPE_OFFSET macro helper which gets the constant value from a
> > zero sized sk_buff member right in front of the bitfield with offsetof.
> > This new offset marker does not change size of struct sk_buff.
> >
> > Cc: Eric Dumazet <eric.dumazet@gmail.com>
> > Cc: Markos Chandras <markos.chandras@imgtec.com>
> > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Cc: Daniel Borkmann <dborkman@redhat.com>
> > Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> > Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
> > Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
> 
> Thanks for doing this, Hannes!
> ...
> >   static u64 __skb_get_pay_offset(u64 ctx, u64 a, u64 x, u64 r4, u64 r5)
> >   {
> >   	return skb_get_poff((struct sk_buff *)(unsigned long) ctx);
> > @@ -191,7 +167,7 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
> >
> >   	case SKF_AD_OFF + SKF_AD_PKTTYPE:
> >   		*insn = BPF_LDX_MEM(BPF_B, BPF_REG_A, BPF_REG_CTX,
> > -				    pkt_type_offset());
> > +				    PKT_TYPE_OFFSET());
> >   		if (insn->off < 0)
> >   			return false;
> >   		insn++;
> 
> I think this can now be rewritten as ...
> 
> 	case SKF_AD_OFF + SKF_AD_PKTTYPE:
> 		*insn++ = BPF_LDX_MEM(BPF_B, BPF_REG_A, BPF_REG_CTX,
> 				      PKT_TYPE_OFFSET());
> 		*insn = BPF_ALU32_IMM(BPF_AND, BPF_REG_A, PKT_TYPE_MAX);
> 
> ... since we are guaranteed to have always >0 insn->off and despite the
> __s16 type in off we might be able to safely assume that sk_buff will
> never get extended so far w/o fist fights first, where this could wrap
> around. ;) Alternatively, perhaps a BUILD_BUG_ON() for really being
> paranoid?

Ok, but in a follow-up patch?

Thanks,
Hannes