Paul Mackerras wrote :
> Jeff Garzik writes:
>
> > From: "Philippe De Muyter" <[EMAIL PROTECTED]>
> >
> > Avoid ppp-generated kernel crashes on machines where unaligned accesses are
> > forbidden (ie: 68000-based CPUs)
>
> 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.
> 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));
> and memmove above, and get rid of the if (since its condition will
> always be false).
>
> Paul.
>
Philippe
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