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=-11.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 2955AC433E2 for ; Wed, 16 Sep 2020 20:18:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DCA7F21973 for ; Wed, 16 Sep 2020 20:18:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727887AbgIPUSi (ORCPT ); Wed, 16 Sep 2020 16:18:38 -0400 Received: from netrider.rowland.org ([192.131.102.5]:59195 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1728261AbgIPUQq (ORCPT ); Wed, 16 Sep 2020 16:16:46 -0400 Received: (qmail 1062543 invoked by uid 1000); 16 Sep 2020 16:16:43 -0400 Date: Wed, 16 Sep 2020 16:16:43 -0400 From: Alan Stern To: Greg KH Cc: Quentin Perret , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, gprocida@google.com, maennich@google.com, kernel-team@android.com Subject: Re: [PATCH] ehci-hcd: Move include to keep CRC stable Message-ID: <20200916201643.GA1062163@rowland.harvard.edu> References: <20200916171825.3228122-1-qperret@google.com> <20200916173127.GB3068942@kroah.com> <20200916173206.GA3070014@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200916173206.GA3070014@kroah.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 16, 2020 at 07:32:06PM +0200, Greg KH wrote: > On Wed, Sep 16, 2020 at 07:31:27PM +0200, Greg KH wrote: > > On Wed, Sep 16, 2020 at 06:18:25PM +0100, Quentin Perret wrote: > > > The CRC calculation done by genksyms is triggered when the parser hits > > > EXPORT_SYMBOL*() macros. At this point, genksyms recursively expands the > > > types of the function parameters, and uses that as the input for the CRC > > > calculation. In the case of forward-declared structs, the type expands > > > to 'UNKNOWN'. Following this, it appears that the result of the > > > expansion of each type is cached somewhere, and seems to be re-used > > > when/if the same type is seen again for another exported symbol in the > > > same C file. > > > > > > Unfortunately, this can cause CRC 'stability' issues when a struct > > > definition becomes visible in the middle of a C file. For example, let's > > > assume code with the following pattern: > > > > > > struct foo; > > > > > > int bar(struct foo *arg) > > > { > > > /* Do work ... */ > > > } > > > EXPORT_SYMBOL_GPL(bar); > > > > > > /* This contains struct foo's definition */ > > > #include "foo.h" > > > > > > int baz(struct foo *arg) > > > { > > > /* Do more work ... */ > > > } > > > EXPORT_SYMBOL_GPL(baz); > > > > > > Here, baz's CRC will be computed using the expansion of struct foo that > > > was cached after bar's CRC calculation ('UNKOWN' here). But if > > > EXPORT_SYMBOL_GPL(bar) is removed from the file (because of e.g. symbol > > > trimming using CONFIG_TRIM_UNUSED_KSYMS), struct foo will be expanded > > > late, during baz's CRC calculation, which now has visibility over the > > > full struct definition, hence resulting in a different CRC for baz. > > > > > > The proper fix for this certainly is in genksyms, but that will take me > > > some time to get right. In the meantime, we have seen one occurrence of > > > this in the ehci-hcd code which hits this problem because of the way it > > > includes C files halfway through the code together with an unlucky mix > > > of symbol trimming. > > > > > > In order to workaround this, move the include done in ehci-hub.c early > > > in ehci-hcd.c, hence making sure the struct definitions are visible to > > > the entire file. This improves CRC stability of the ehci-hcd exports > > > even when symbol trimming is enabled. > > > > > > Signed-off-by: Quentin Perret > > > --- > > > drivers/usb/host/ehci-hcd.c | 1 + > > > drivers/usb/host/ehci-hub.c | 1 - > > > 2 files changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/drivers/usb/host/ehci-hcd.c b/drivers/usb/host/ehci-hcd.c > > > index 6257be4110ca..3575b7201881 100644 > > > --- a/drivers/usb/host/ehci-hcd.c > > > +++ b/drivers/usb/host/ehci-hcd.c > > > @@ -22,6 +22,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > #include > > > #include > > > diff --git a/drivers/usb/host/ehci-hub.c b/drivers/usb/host/ehci-hub.c > > > index ce0eaf7d7c12..087402aec5cb 100644 > > > --- a/drivers/usb/host/ehci-hub.c > > > +++ b/drivers/usb/host/ehci-hub.c > > > @@ -14,7 +14,6 @@ > > > */ > > > > > > /*-------------------------------------------------------------------------*/ > > > -#include > > > > > > #define PORT_WAKE_BITS (PORT_WKOC_E|PORT_WKDISC_E|PORT_WKCONN_E) > > > > Thanks for root-causing this issue. genksyms is a fragile beast, good > > luck trying to resolve that! > > > > I'll go queue this up and mark it for stable kernels, thanks. > > I'll queue it up after I get Alan's review first :) It's fine with me. Acked-by: Alan Stern Alan Stern