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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 455CEC433B4 for ; Sat, 8 May 2021 16:30:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0B4026140F for ; Sat, 8 May 2021 16:30:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229768AbhEHQbC (ORCPT ); Sat, 8 May 2021 12:31:02 -0400 Received: from mail-40136.protonmail.ch ([185.70.40.136]:13578 "EHLO mail-40136.protonmail.ch" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229749AbhEHQay (ORCPT ); Sat, 8 May 2021 12:30:54 -0400 Date: Sat, 08 May 2021 16:29:33 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=pm.me; s=protonmail; t=1620491388; bh=5rNl+kAanqy5Ow6PHG0Nk/iTcHfaHO7mTi0wPOmmCc0=; h=Date:To:From:Cc:Reply-To:Subject:In-Reply-To:References:From; b=WyD5ez/5my30NXYcWHdga/2UTQhj3s1Mai4b6T2yCM7pXPFqSIyVDLZr44wTS1Sz6 9JnHA0T8xPqHDnBBEngpRZ9z/HlmGq5WdFMsxgIQyE6HZQi/WcWkpStY4jdMmBFZ03 QmWx1tyQKZv31lXLsZOPWM/yc3JUTdeIzzNZZ0YuWUrQPIe41RMeeFT1ZOQu5EPlQX eeNtYz6tMydBuvU/l1YDzPcaWpKbnohxfwvDNkRMAKXeILIBtFlCwdW0UFvOHYfEB5 TLfd8iAn6ysn8OTtXLzEE5aHuj0aWwGlEnLwHk+Lfn9UG2BhGfJRD4BAXwsPqHGwlQ liOPHHS4GgHVQ== To: Nathan Chancellor From: Alexander Lobakin Cc: clang-built-linux@googlegroups.com, linux-mips@vger.kernel.org, Thomas Bogendoerfer , Kees Cook , Nathan Chancellor , Nick Desaulniers , Fangrui Song , Sami Tolvanen , Ralf Baechle , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Reply-To: Alexander Lobakin Subject: Re: [BUG mips llvm] MIPS: malformed R_MIPS_{HI16,LO16} with LLVM Message-ID: In-Reply-To: References: <20210109171058.497636-1-alobakin@pm.me> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Nathan Chancellor Date: Fri, 7 May 2021 00:47:14 -0700 > On Sat, Jan 09, 2021 at 05:11:18PM +0000, Alexander Lobakin wrote: > > Machine: MIPS32 R2 Big Endian (interAptiv (multi)) > > > > While testing MIPS with LLVM, I found a weird and very rare bug with > > MIPS relocs that LLVM emits into kernel modules. It happens on both > > 11.0.0 and latest git snapshot and applies, as I can see, only to > > references to static symbols. > > > > When the kernel loads the module, it allocates a space for every > > section and then manually apply the relocations relative to the > > new address. > > > > Let's say we have a function phy_probe() in drivers/net/phy/libphy.ko. > > It's static and referenced only in phy_register_driver(), where it's > > used to fill callback pointer in a structure. > > > > The real function address after module loading is 0xc06c1444, that > > is observed in its ELF st_value field. > > There are two relocs related to this usage in phy_register_driver(): > > > > R_MIPS_HI16 refers to 0x3c010000 > > R_MIPS_LO16 refers to 0x24339444 > > > > The address of .text is 0xc06b8000. So the destination is calculated > > as follows: > > > > 0x00000000 from hi16; > > 0xffff9444 from lo16 (sign extend as it's always treated as signed); > > 0xc06b8000 from base. > > > > =3D3D 0xc06b1444. The value is lower than the real phy_probe() address > > (0xc06c1444) by 0x10000 and is lower than the base address of > > module's .text, so it's 100% incorrect. > > > > This results in: > > > > [ 2.204022] CPU 3 Unable to handle kernel paging request at virtual > > address c06b1444, epc =3D3D=3D3D c06b1444, ra =3D3D=3D3D 803f1090 > > > > The correct instructions should be: > > > > R_MIPS_HI16 0x3c010001 > > R_MIPS_LO16 0x24339444 > > > > so there'll be 0x00010000 from hi16. > > > > I tried to catch those bugs in arch/mips/kernel/module.c (by checking > > if the destination is lower than the base address, which should never > > happen), and seems like I have only 3 such places in libphy.ko (and > > one in nf_tables.ko). > > I don't think it should be handled somehow in mentioned source code > > as it would look rather ugly and may break kernels build with GNU > > stack, which seems to not produce such bad codes. > > > > If I should report this to any other resources, please let me know. > > I chose clang-built-linux and LKML as it may not happen with userland > > (didn't tried to catch). > > > > Thanks, > > Al > > > > Hi Alexander, Hi! > Doubling back around to this as I was browsing through the LLVM 12.0.1 > blockers on LLVM's bug tracker and I noticed a commit that could resolve > this? It refers to the same relocations that you reference here. > > https://bugs.llvm.org/show_bug.cgi?id=3D3D49821 > > http://github.com/llvm/llvm-project/commit/7e83a7f1fdfcc2edde61f0a535f9d7= a=3D > 56f531db9 This really seems very related to the bug I encountered. Currently I don't have a MIPS setup to try since I've moved to another country, but I should "deploy" it again soon. So I'll definitely take a look a bit later, thanks for pointing on this commit! > I think that Debian's apt.llvm.org repository should have a build > available with that commit in it. Otherwise, building it from source is > not too complicated with my script: > > https://github.com/ClangBuiltLinux/tc-build > > $ ./build-llvm.py --build-stage1-only --install-stage1-only --projects "c= l=3D > ang;lld" --targets "Mips;X86" > > would get you a working toolchain relatively quickly. I could just build llvm-git from Arch Linux User Repository :) I did that last time when was checking if the latest snapshot also suffers from the bug, and I think it didn't take much time to build. > Cheers, > Nathan Thanks, Al