Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753562AbaKJQjm (ORCPT ); Mon, 10 Nov 2014 11:39:42 -0500 Received: from mail-wi0-f170.google.com ([209.85.212.170]:47953 "EHLO mail-wi0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752272AbaKJQjl (ORCPT ); Mon, 10 Nov 2014 11:39:41 -0500 Message-ID: <5460EA45.2080202@linaro.org> Date: Mon, 10 Nov 2014 16:39:33 +0000 From: Zoltan Kiss User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: David Vrabel , netdev@vger.kernel.org, "David S. Miller" , Eric Dumazet , Konrad Rzeszutek Wilk , Boris Ostrovsky , Stefan Bader , Jay Vosburgh , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org Subject: Re: [Xen-devel] BUG in xennet_make_frags with paged skb data References: <20141106214940.GD44162@ubuntu-hedt> <545CA27F.4070400@citrix.com> <20141110143517.GA74005@ubuntu-hedt> <5460CEA5.3070201@citrix.com> In-Reply-To: <5460CEA5.3070201@citrix.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/11/14 14:41, David Vrabel wrote: > On 10/11/14 14:35, Seth Forshee wrote: >> On Fri, Nov 07, 2014 at 10:44:15AM +0000, David Vrabel wrote: >>> On 06/11/14 21:49, Seth Forshee wrote: >>>> We've had several reports of hitting the following BUG_ON in >>>> xennet_make_frags with 3.2 and 3.13 kernels (I'm currently awaiting >>>> results of testing with 3.17): >>>> >>>> /* Grant backend access to each skb fragment page. */ >>>> for (i = 0; i < frags; i++) { >>>> skb_frag_t *frag = skb_shinfo(skb)->frags + i; >>>> struct page *page = skb_frag_page(frag); >>>> >>>> len = skb_frag_size(frag); >>>> offset = frag->page_offset; >>>> >>>> /* Data must not cross a page boundary. */ >>>> BUG_ON(len + offset > PAGE_SIZE<>>> >>>> When this happens the page in question is a "middle" page in a compound >>>> page (i.e. it's a tail page but not the last tail page), and the data is >>>> fully contained within the compound page. The data does however cross >>>> the hardware page boundary, and since compound_order evaluates to 0 for >>>> tail pages the check fails. >>>> >>>> In going over this I've been unable to determine whether the BUG_ON in >>>> xennet_make_frags is incorrect or the paged skb data is wrong. I can't >>>> find that it's documented anywhere, and the networking code itself is a >>>> bit ambiguous when it comes to compound pages. On the one hand >>>> __skb_fill_page_desc specifically handles adding tail pages as paged >>>> data, but on the other hand skb_copy_bits kmaps frag->page.p which could >>>> fail with data that extends into another page. >>> >>> netfront will safely handle this case so you can remove this BUG_ON() >>> (and the one later on). But it would be better to find out were these >>> funny-looking skbs are coming from and (if necessary) fixing the bug there. >> >> There still seems to be disagreement about whether the "funny" skb is >> valid though - you imply it isn't, but Eric says it is. I've been trying >> to track down where these skbs originate, and so far I've determined >> that they come from a socket spliced to a pipe spliced to a socket. It >> looks like the particular page/offset/len tuple originates at least as >> far back as the first socket, as the tuple is simply copied from an skb >> into the pipe and from the pipe into the final skb. > > Apologies for the lack of clarity. I meant either: a) fix the producer > if these skbs are invalid; or b) remove the BUG_ON()s. Since Eric says > these are actually valid skbs, please do option (b). > > i.e., remove both BUG_ON()s. The BUG_ON suggested by Stefan would be still reasonable: BUG_ON(((page-compound_head(page))*PAGE_SIZE)+offset+len > PAGE_SIZE< > David > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/