[PATCHv2] networking: consolidate the IP checksum code

[Joakim Tjernlund]

>
> Baruch Siach <baruch at tkos.co.il> wrote on 2011/09/08 07:06:27:
> >
> > Hi Joakim,
>
> ...
>
> > >        if (nbytes & 1) /* Odd */
> > >          sum += *(u_char *)ptr;   /* one byte only */
> >
> > This breaks big endian systems like PowerPC. The assignment via pointer dance
> > of the original implementation is required, because we need the last byte to
> > be considered as MSB of a 16bit word, as if we had one more '\0' byte in
> > buffer.  Note that the generated checksum is the same on big and little endian
> > machines in terms of memory representation.  They differ however in their
> > numeric representation.  So, for example the checksum of
> >
> >     uint8_t buf[] = {0x12, 0x34, 0x56};
> >
> > is 0xcb97 on little endian machines, and 0x97cb on big endian machines. This
> > is OK since we use the memory representation of the network packet.
>
> Ouch, even RFC 1071 got it wrong then.

I just checked and all this ptr casting confuses gcc and generates
bad code. Using an union is better, both for ppc and x86.

This is only compile tested:

if (len & 1) {
	    union uu {
		    uint16_t val;
		    uint8_t b[2];
	    } tmp = {0};
        /* Make sure that the left-over byte is added correctly both
         * with little and big endian hosts */
        tmp.b[0] = *(uint8_t*)w;
        sum += tmp.val;
    }