Philippe De Muyter writes:
> > This patch seems a bit strange and/or incomplete. Are we trying to
> > get 2-byte alignment or 4-byte alignment of the payload? It seems
>
> Actually, we try to get a 4n+2 alignment for skb->data, to get the
> ip-addresses
> field 4bytes aligned.
> I think the only thing wrong is the old comment that said :
> /* Try to get the payload 4-byte aligned */
> and that I did not change.
Yes, the payload is the part after the protocol field, so the comment
is still correct.
> > that if the protocol field is uncompressed, we don't do anything to
> > the alignment, but if it is compressed, we do this:
> >
> > > /* protocol is compressed */
> > > - skb_push(skb, 1)[0] = 0;
> > > + if ((unsigned long)skb->data & 1)
> > > + skb_push(skb, 1)[0] = 0;
> > > + else { /* Ditto, but realign the payload to 4-byte boundary */
> > > + short len = skb->len;
> > > +
> > > + skb_put(skb, 3);
> > > + memmove(skb->data + 3, skb->data, len);
> > > + skb_pull(skb, 2)[0] = 0;
> >
> > I'm puzzled that we are not testing ((unsigned long)skb->data & 2) if
> > we are really trying to achieve 4-byte alignment. In fact, if the
> > skb->data that we get from dev_alloc_skb is 4-byte aligned to start
> > with, this will end up with the payload starting at the original
> > skb->data + 6, i.e. 2-byte aligned but not 4-byte aligned AFAICS.
> >
> > Can we assume that dev_alloc_skb will give us a 4-byte aligned
> > skb->data? If we can then I suggest we change 3 to 1 in the skb_put
>
> Are you not forgetting that the alignment of skb->data is changed (by the
> existing code ! ) :
> if (buf[0] != PPP_ALLSTATIONS)
> skb_reserve(skb, 2 + (buf[0] & 1));
No, I'm not forgetting. If we assume that skb->data starts out 4-byte
aligned, then the only case in which we will have
((unsigned long)skb->data & 1) == 0
is if we have protocol field compression (and a compressible protocol
number, i.e. less than 0x100) but not address/control compression
(which would be a weird combination, but legal). In that case, with
your patch we will move the protocol byte to the original skb->data+5
and have the payload at +6.
If there is any possibility that skb->data is not 4-byte aligned when
the skb is first allocated, I think that we should do something like
if ((unsigned long)skb->data & 3)
skb_reserve(skb, 4 - ((unsigned long)skb->data & 3));
immediately after allocating it, and then just memmove the stuff up
one byte (rather than 3) if it isn't aligned as we want.
Paul.
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