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=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 BFCF6C433ED for ; Thu, 8 Apr 2021 18:13:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 87C2861056 for ; Thu, 8 Apr 2021 18:13:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232759AbhDHSNL (ORCPT ); Thu, 8 Apr 2021 14:13:11 -0400 Received: from mail.kernel.org ([198.145.29.99]:36974 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232281AbhDHSNJ (ORCPT ); Thu, 8 Apr 2021 14:13:09 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 1B6D3610C8; Thu, 8 Apr 2021 18:12:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1617905577; bh=srcP1POWJBP8EPl3rQLISqPDTr2hkWQ0RrdqLHmlAUk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=UJKA4y4ZjWUD7wjS+Ek2nii2ja6PIv6PzC+WASHTZRtqgAMksAdSQSe6eKSarpcGY 9LI6D0pLiLGBxs0z3IinkULJjYQQngN9JIkIfPsUFrbBbMDapPYLCQGagxz19h+O0n DVwq0LXM+NFP5+2L/+OW53kXDv4dbubAIwIEHFRA= Date: Thu, 8 Apr 2021 20:12:55 +0200 From: Greg KH To: Quentin Perret Cc: masahiroy@kernel.org, linux-kernel@vger.kernel.org, maennich@google.com, gprocida@google.com, kernel-team@android.com Subject: Re: [PATCH] export: Make CRCs robust to symbol trimming Message-ID: References: <20210408180105.2496212-1-qperret@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210408180105.2496212-1-qperret@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 08, 2021 at 06:01:05PM +0000, 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, and uses that as the input for the CRC calculation. In the case > of forward-declared structs, the type expands to 'UNKNOWN'. Next, the > result of the expansion of each type is cached, and is re-used when/if > the same type is seen again for another exported symbol in the 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. > > This can cause annoying issues for distro kernel (such as the Android > Generic Kernel Image) which use CONFIG_UNUSED_KSYMS_WHITELIST. Indeed, > as per the above, adding a symbol to the whitelist can change the CRC of > symbols that are already kept exported. As such, modules built against a > kernel with a trimmed ABI may not load against the same kernel built > with an extended whitelist, even though they are still strictly binary > compatible. While rebuilding the modules would obviously solve the > issue, I believe this classifies as an odd genksyms corner case, and it > gets in the way of kernel updates in the GKI context. > > To work around the issue, make sure to keep issuing the > __GENKSYMS_EXPORT_SYMBOL macros for all trimmed symbols, hence making > the genksyms parsing insensitive to symbol trimming. > > Signed-off-by: Quentin Perret > --- > include/linux/export.h | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/include/linux/export.h b/include/linux/export.h > index 6271a5d9c988..27d848712b90 100644 > --- a/include/linux/export.h > +++ b/include/linux/export.h > @@ -140,7 +140,12 @@ struct kernel_symbol { > #define ___cond_export_sym(sym, sec, ns, enabled) \ > __cond_export_sym_##enabled(sym, sec, ns) > #define __cond_export_sym_1(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns) > + > +#ifdef __GENKSYMS__ > +#define __cond_export_sym_0(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym) > +#else > #define __cond_export_sym_0(sym, sec, ns) /* nothing */ > +#endif > > #else > Anything to help make these symbol values more "stable" is good, they drive me crazy... Acked-by: Greg Kroah-Hartman