From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by passt.top (Postfix) with ESMTP id 92C315A026F for ; Thu, 3 Aug 2023 07:40:49 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1691041248; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CDxAhqMKoiXH5J6EiH30mM758pUF5d+0dlbkUkykmsM=; b=OjxL5jSCLhxHHOFwuwOehVvNE8ihQtGyckYMDNlvG2kq8s1UOAkYYWVdhU1MbZ85Ooev8H my3wWamVD3l96Ja46xLQdZW6pfEU5xBNgT9QPzxtmJVgM2J+tAqcB0tvOE04h5HqKlXB73 pR8oJxlhdxSU/JNskQ2etf9VSumrWe0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-173-4_mKml7zMm-X1u2eoxQz_w-1; Thu, 03 Aug 2023 01:40:46 -0400 X-MC-Unique: 4_mKml7zMm-X1u2eoxQz_w-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 67445800159; Thu, 3 Aug 2023 05:40:46 +0000 (UTC) Received: from elisabeth (unknown [10.39.208.24]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C06991121325; Thu, 3 Aug 2023 05:40:45 +0000 (UTC) Date: Thu, 3 Aug 2023 07:40:40 +0200 From: Stefano Brivio To: David Gibson Subject: Re: [PATCH 01/17] netlink: Split up functionality if nl_link() Message-ID: <20230803074040.67511759@elisabeth> In-Reply-To: References: <20230724060936.952659-1-david@gibson.dropbear.id.au> <20230724060936.952659-2-david@gibson.dropbear.id.au> <20230803004729.03ca0e36@elisabeth> Organization: Red Hat MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-ID-Hash: RKHTR6UTL6SSNZHOD45UNJWCKNS7QLI2 X-Message-ID-Hash: RKHTR6UTL6SSNZHOD45UNJWCKNS7QLI2 X-MailFrom: sbrivio@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Thu, 3 Aug 2023 14:29:28 +1000 David Gibson wrote: > On Thu, Aug 03, 2023 at 12:09:16PM +1000, David Gibson wrote: > > On Thu, Aug 03, 2023 at 12:47:29AM +0200, Stefano Brivio wrote: > [snip] > > > > -void nl_link(int ns, unsigned int ifi, void *mac, int up, int mtu) > > > > +void nl_link_get_mac(int ns, unsigned int ifi, void *mac) > > > > { > > > > - int change = !MAC_IS_ZERO(mac) || up || mtu; > > > > struct req_t { > > > > struct nlmsghdr nlh; > > > > struct ifinfomsg ifm; > > > > - struct rtattr rta; > > > > - union { > > > > - unsigned char mac[ETH_ALEN]; > > > > - struct { > > > > - unsigned int mtu; > > > > - } mtu; > > > > - } set; > > > > } req = { > > > > - .nlh.nlmsg_type = change ? RTM_NEWLINK : RTM_GETLINK, > > > > - .nlh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg)), > > > > - .nlh.nlmsg_flags = NLM_F_REQUEST | (change ? NLM_F_ACK : 0), > > > > + .nlh.nlmsg_type = RTM_GETLINK, > > > > + .nlh.nlmsg_len = sizeof(req), > > > > > > I don't think there's a practical issue with this, but there were two > > > reasons why I used NLMSG_LENGTH(sizeof(struct ifinfomsg)) instead: > > > > > > - NLMSG_LENGTH() aligns to 4 bytes, not to whatever > > > architecture-dependent alignment we might have: the message might > > > actually be smaller > > > > Oof... so. On the one hand, I see the issue; if these are different, > > I'm not sure what the effect will be. On the other hand, if we use > > NLMSG_LENGTH and it *is* longer than the structure size, we'll be > > saying that this message is longer than the datagram containing it. > > I'm not sure what the effect of that will be either. > > Duh, sorry, I realized I had this backwards. NLSMSG_LENGTH() is the > non-aligned length, sizeof() may include alignment. I'll rework based > on that understanding. Right, I was about to write you that... or rather, NLMSG_LENGTH() is the (presumably) lesser-aligned length. Also, if you check pretty much any example in iproute2, nlmsg_len is always set like that, using NLMSG_LENGTH() on the payload. > > Not really sure what to do about this. > > > > > - I see that this works with gcc and clang, but, strictly > > > speaking, is the size of the struct known "before" > > > (sequence-point-wise) we're done initialising it? I have a very vague > > > memory of this not working with gcc 2.9 or suchlike -- which is not a > > > problem, as long as our new friend C11 actually supports this (but > > > I'm not entirely sure). > > > > I'm pretty sure it's ok, regardless of C11 state. It's not really a > > question of sequence points: those are about the ordering of run time > > operations. Even though the structure is being defined inline, > > determining it's size and layout will still happen at compile time, > > whereas the initialization is obviously a runtime event. Ah, sorry, yes, of course. Still I remember that failing spectacularly in a distant past. But you just checked with gcc 4-ish I guess, so I guess it would have been fine anyway. -- Stefano