From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5FD64C83000 for ; Wed, 29 Apr 2020 19:54:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4107E206D9 for ; Wed, 29 Apr 2020 19:54:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726618AbgD2TyS (ORCPT ); Wed, 29 Apr 2020 15:54:18 -0400 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:48896 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726456AbgD2TyR (ORCPT ); Wed, 29 Apr 2020 15:54:17 -0400 Received: from dimstar.local.net (n175-34-64-112.sun1.vic.optusnet.com.au [175.34.64.112]) by mail104.syd.optusnet.com.au (Postfix) with SMTP id B3003820A65 for ; Thu, 30 Apr 2020 05:54:14 +1000 (AEST) Received: (qmail 7526 invoked by uid 501); 29 Apr 2020 19:54:14 -0000 Date: Thu, 30 Apr 2020 05:54:14 +1000 From: Duncan Roe To: Netfilter Development Subject: Re: [PATCH libnetfilter_queue 0/3] pktbuff API updates Message-ID: <20200429195414.GC3833@dimstar.local.net> Mail-Followup-To: Netfilter Development References: <20200426132356.8346-1-pablo@netfilter.org> <20200427110614.GA15436@dimstar.local.net> <20200427170656.GA22296@salvia> <20200428043302.GB15436@dimstar.local.net> <20200428103407.GA1160@salvia> <20200428211452.GF15436@dimstar.local.net> <20200428225520.GA30421@salvia> <20200429132840.GA3833@dimstar.local.net> <20200429190020.GA16096@salvia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200429190020.GA16096@salvia> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=QIgWuTDL c=1 sm=1 tr=0 a=keeXcwCgVCrAuxOn72dlvA==:117 a=keeXcwCgVCrAuxOn72dlvA==:17 a=kj9zAlcOel0A:10 a=cl8xLZFz6L8A:10 a=RSmzAf-M6YYA:10 a=3hXQTgT0DALnAf7DM8QA:9 a=JmEJIw0BOmCsy-di:21 a=iTuLv9zXbIsyl9dn:21 a=CjuIK1q_8ugA:10 Sender: netfilter-devel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netfilter-devel@vger.kernel.org On Wed, Apr 29, 2020 at 09:00:20PM +0200, Pablo Neira Ayuso wrote: > On Wed, Apr 29, 2020 at 11:28:40PM +1000, Duncan Roe wrote: > > On Wed, Apr 29, 2020 at 12:55:20AM +0200, Pablo Neira Ayuso wrote: > > > On Wed, Apr 29, 2020 at 07:14:52AM +1000, Duncan Roe wrote: > > > > On Tue, Apr 28, 2020 at 12:34:07PM +0200, Pablo Neira Ayuso wrote: > > > [...] > > > > > pktb_alloc2() still has a memcpy which is not needed by people that do > > > > > not need to mangle the packet. > > > > > > > > No it does not. Please look again. There is only a memcpy if the caller > > > > specifies extra > 0, in which case she clearly intends to mangle it (perhaps > > > > depending on its contents). > > > > > > Right, it only happens if extra is specified. > > > > > > + if (extra) { > > > + pkt_data = buf; > > > + memcpy(pkt_data, data, len); > > > + memset((uint8_t *)pkt_data + len, 0, extra); > > > + } else { > > > + pkt_data = data; > > > + } > > > > > > So buf is only used if extra is specified? > > > > Yes, that's right. > > OK. Then, the user must pass the buf only if extra is set on. > > > > > Yes it's more complex. No problem with the buffer - the user gave that to > > > > pktb_alloc2(). > > > > > > I'm just hesitating about the new pktb_alloc2() approach because it > > > has many parameters, it's just looks a bit complicated to me (this > > > function takes 8 parameters). > > > > It has the original 4 from pktb_alloc() plus 2 {buffer, size} pairs. It could > > have been just one pair, with packet data appended to metadata as in > > pktb_alloc() but I thought it would be really awkward to document how to > > dimension it. > > I'm starting to think this function is hard to document, too many > parameters. The documentation looks fine to me - I'm looking at it in a web browser right now. Have you tried that? > > > I think we should not be usurping the data pointer of mnl_cb_run(). > > I can see people wanting to use it to pass a pointer to e.g. some > > kind of database that callbacks need to access. There's no > > performance gain to recycling the buffer: the CB doesn't need to > > call pktb_head_size() on every invocation, that can be done once by > > main() e.g. > > > > static size_t sizeof_head; > > ... > > int main(int argc, char *argv[]) > > { > > ... > > sizeof_head = pktb_head_size(); /* Avoid multiple calls in CB */ > > ... > > static int queue_cb(const struct nlmsghdr *nlh, void *data) > > { > > char head[sizeof_head]; > > You might also declare the pre-allocated pkt_buff as a global if you > don't want to use the data pointer in mnl_cb_run(). I'm uneasy about this. We're writing a library here. We shouldn't be dictating to the user that he must declare globals. "static" won't do in a multi-threaded program, but you could use "thread local" (malloc'd under the covers, (tiny) performance hit c/w stack (which is always thread local)). "The road to bloat is paved with tiny performance hits" [1] > > static struct pkt_buff *pkt; > > int main(int argc, char *argv[]) > { > ... > pkt = pktb_head_alloc(); > ... > } > > Then, use it from queue_cb(). > > Alternatively, you can also define a wrapper structure that you can > pass to mnl_cb_run(), e.g. > > struct my_data { > struct pkt_buff *pktb; > void *something_ese; > }; > > > > My understanding is that requirements are: > > > > > > * Users that do not want to mangle the packet, they use the buffer > > > that was passed to recvmsg(). > > > * Users that want to mangle the packet, they use the _mangle() > > > function that might reallocate the data buffer (the one you would > > > like to have). However, if this data buffer reallocation happens, > > > then pkt_buff should annotate that this pkt_buff object needs to > > > release this data buffer from pktb_free() otherwise. > > > > No, there is nothing to release. We told pktb_alloc2() where the buffer was, > > it's on the stack (usually). > > Then, I'm not sure I understand yet what extension you would like to > make to _mangle(), please, clarify. > > > > > Problem is that if mangler moves the packet, then any packet pointer the caller > > > > had is invalid (points to the un-mangled copy). This applies at all levels, e.g. > > > > nfq_udp_get_payload(). There is no way for the mangler functions to address > > > > this: it just has to be highlighted in the documentation. > > > > > > That's fine, this is exactly how the kernel works: if the function > > > might reallocate the data area, then you have to refetch pointers > > > after this. If you teach _mangle() to do reallocations, then > > > documenting this is fine. > > > > > > However, those reallocation need pktb_free() to release that new data > > > buffer, right? > > > > No way. There is no malloc() nor free() anywhere. The data buffer is > > (recommended to be) on the stack; for running under gdb it may be preferred to > > us a static buffer which has to be dimensioned hugely. > > If the user pre-allocates the heap or place it in the stack is > irrelevant, the save for the user is the memcpy() if it's only > inspecting the packet (no mangling) and the out-of-line pkt_buff > allocation / or place in the stack. > > If pktb_build_data() takes the extra parameter, I think the > showstopper you mentioned is gone. Otherwise, please tell me what you > cannot achieve with my patchset. My patchset comes with documentation and yours does not. I am not volunteering to document yours. Would you like me to post a detailed review of your patchset? It will not be pretty. > > Thanks. [1] I just amde that up. Good eh? Cheers ... Duncan.