Re: [Fwd: Re: connector is missing in 2.6.12-rc2-mm1]

[Thomas Graf <tgraf@suug.ch>]

* Evgeniy Polyakov <20050411092228.A32699@2ka.mipt.ru> 2005-04-11 09:22
> On Sun, Apr 10, 2005 at 09:27:27PM +0200, Thomas Graf (tgraf@suug.ch) wrote:
> > +       size = NLMSG_SPACE(sizeof(*msg) + msg->len);
> > +
> > +       skb = alloc_skb(size, GFP_ATOMIC);
> > +       if (!skb) {
> > +               printk(KERN_ERR "Failed to allocate new skb with size=%u.\n", size);
> > +               return;
> > +       }
> > +
> > +       nlh = NLMSG_PUT(skb, 0, msg->seq, NLMSG_DONE, size - sizeof(*nlh));
> > 
> > This is not correct, what happens is:
> > size = NLMSG_SPACE(sizeof(*msg) + msg->len);
> >  --> align(hdr)+align(data)
> > size - sizeof(*nlh)
> >  --> (align(hdr)-hdr)+align(data)
> > NLMSG_PUT pads again to get to the end of the data block (NLMSG_LENGTH)
> >  --> align(hdr)+(align(hdr)-hdr)+align(data)
> > 
> > At the moment align(hdr) == hdr since nlmsghdr is already aligned
> > but this might change and your code will break.
> 
> As far as I remember, header is always supposed to be aligned properly
> "by design", so it even could be nonaligned here.

No, have a look at the macros:

#define NLMSG_LENGTH(len) ((len)+NLMSG_ALIGN(sizeof(struct nlmsghdr)))
#define NLMSG_SPACE(len) NLMSG_ALIGN(NLMSG_LENGTH(len))

NLMSG_LENGTH points to the end of the payload in the message, NLMSG_SPACE
represents the total size aligned properly for a possible next multipart
message.

It is unlikely that nlmsghdr will ever be unaligned but there can be no
reason to introduce code that can break with perfectly legal changes just
because of that.
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