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    SubjectRe: [Fwd: Re: connector is missing in 2.6.12-rc2-mm1]
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    On Mon, 2005-04-11 at 12:45 +0200, Thomas Graf wrote:
    > * 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.

    But NLMSG_ALIGN(sizeof(hdr)) is always equal to sizeof(hdr), that size
    was select not accidentally.

    I will change it, no problem, it is good from aesthetic point of view.

    --
    Evgeniy Polyakov

    Crash is better than data corruption -- Arthur Grabowski
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