Minor error/typo in nf_reinject. And a couple of questions.

Volodymyr G.L. voll at te.net.ua
Fri Feb 17 18:08:06 CET 2006

Hi, All.

Here is a little snippet of code from function nf_reinject (lines 207-220 in
net/netfilter/nf_queue.c in Linus' kernel tree):

------- code begins -------
/* Drop reference to owner of hook which queued us. */

list_for_each_rcu(i, &nf_hooks[info->pf][info->hook]) {
         if (i == elem)

if (elem == &nf_hooks[info->pf][info->hook]) {
         /* The module which sent it to userspace is gone. */
         NFDEBUG("%s: module disappeared, dropping packet.\n",
         verdict = NF_DROP;
------- code ends -------

I guess it is a little error/typo in the last if-statement. Apparently it was
meant to check whether end of the linked list was reached in previous
list_for_each_rcu macro, i.e. elem wasn't found on the list. But then variable
"i" not "elem" must be compared to address of the appropriate list head.

Besides I don't see why elem can be dereferenced before module_put and can't be
dereferenced after it, and therefore packet must be dropped. Yes, after
module_put (if sum of all per-cpu reference counters equals zero) unloading of
the module in question may be initiated. But in the cleanup_module function it
will invoke nf_unregister_hook which in turn will call synchronize_net which
will call synchronize_kernel, so rmmod process will be put to sleep until all
currently executing RCU read sides have completed. And almost whole nf_reinject
function is one RCU read-side critical section. So why even bother with
checking? If data referenced by elem was consistent before module_put it must be
consistent after module_put. Of course all this imho and correct only if module
can call nf_unregister_hook only from cleanup_module function.

But if modules can call nf_{un,}register_hook functions not only from
init_module and cleanup_module, provided that nf_hook_ops structure can be
freed/allocated dynamically, then a couple of other questions arise to the
entire scheme of saving (in function nf_queue) pointer to the module's
nf_hook_ops structure as elem member of nf_info structure for later use in
nf_reinject function.

I would be very appreciate if anyone can clarify this issues to me.

P.S. Sorry for my bad English.

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