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=-5.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT 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 3E315C282DC for ; Wed, 17 Apr 2019 06:42:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 01735204FD for ; Wed, 17 Apr 2019 06:42:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="djaC53Ns" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730713AbfDQGl7 (ORCPT ); Wed, 17 Apr 2019 02:41:59 -0400 Received: from mail-ed1-f67.google.com ([209.85.208.67]:32912 "EHLO mail-ed1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725767AbfDQGl7 (ORCPT ); Wed, 17 Apr 2019 02:41:59 -0400 Received: by mail-ed1-f67.google.com with SMTP id d55so19185400ede.0; Tue, 16 Apr 2019 23:41:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=0AQgQfmPU9nu5iMXDY4QAAQM22/ta3tiiqMeA67DHQQ=; b=djaC53Nsk9F6Xrk2uCNen1+Fv6py1BvtkjOTMBZ0ZiI1HTBzUZZ+jf56O3M+NifqBo 20uuvQWELnz2M6u9iw5+N3U57C7tZtG4zn+OlnqSDP8MZXftsVfxpftVSxBFs8EDIUk+ Xp1Eo9HFPOs8LzzReK9AfjorQCZIdbhpYyFTuW6RujZAIroLJVLtkuZvxnXnKvdRVJ9V ROaMxpmonCPbUc7pKYrJ26ab/otJPZqSeHLZjNy3Bo54CbhQjjFBwfRJ3227TfFOMdpQ JaqFqUgdT7oc86HSSKLGdlCm0PgXL06B3XoaRzWyO9LrwpyuW18QTLjmUhluodcOPo3v gfxg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=0AQgQfmPU9nu5iMXDY4QAAQM22/ta3tiiqMeA67DHQQ=; b=KV8AgLqn7/rnM0OePrTH6+f9QcQiL0k6wrzGV7zx9YCc1nppZSFDYYaMPLt3Q9rg8d bpXSeTeq+kkvTsDwiq5TuXVj+Sejgts0SDEe329RBMpdkBKKosB9BBMAy7BQAR0dCk6z DQE01vEZAzhaPChtBfB4JHPOTyIJGuP6E+yGS5otP7xuAd+tKWsiJioFu7bLPxUsuT7T 09qgzqtqjyxFC3uKs2k07V4e/n/jRx9qLJr5/UOlZrlQm+TxN6cm4dhWoGXKOA3wJxoD kDoFkrrqOHlvo3AFzbUB+5Y/Zm+EK+31RZeAJyjFb6J94TUKEKGylR3YJJrm/UO0jnqK +nLA== X-Gm-Message-State: APjAAAWC0ypfr75AxqOPi9iue0jdFy044nYUsOWQy42a7aK6ErMOPHT4 JXqLKs/e5kX/XwK3nAjPPW4= X-Google-Smtp-Source: APXvYqz04Nts5+9iZLG49W+anIQjkK2tVefioULMM6bFOnRYnVDzc6jv5tyWTPIVxOh6QFO5mmJ3UQ== X-Received: by 2002:aa7:d7d2:: with SMTP id e18mr2127065eds.142.1555483316146; Tue, 16 Apr 2019 23:41:56 -0700 (PDT) Received: from archlinux-i9 ([2a01:4f9:2b:2b84::2]) by smtp.gmail.com with ESMTPSA id v24sm6853103ejx.34.2019.04.16.23.41.55 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 16 Apr 2019 23:41:55 -0700 (PDT) Date: Tue, 16 Apr 2019 23:41:53 -0700 From: Nathan Chancellor To: Greg Kroah-Hartman Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, stable@kernel.org, Will Deacon Subject: Re: [PATCH 4.9 72/76] arm64: futex: Fix FUTEX_WAKE_OP atomic ops with non-zero result value Message-ID: <20190417064153.GA14656@archlinux-i9> References: <20190415183707.712011689@linuxfoundation.org> <20190415183729.170980546@linuxfoundation.org> <20190415220151.GA23056@archlinux-i9> <20190416090052.GD4209@kroah.com> <20190416164751.GA25801@archlinux-i9> <20190417061508.GB18320@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190417061508.GB18320@kroah.com> User-Agent: Mutt/1.11.4 (2019-03-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 17, 2019 at 08:15:08AM +0200, Greg Kroah-Hartman wrote: > On Tue, Apr 16, 2019 at 09:47:51AM -0700, Nathan Chancellor wrote: > > On Tue, Apr 16, 2019 at 11:00:52AM +0200, Greg Kroah-Hartman wrote: > > > On Mon, Apr 15, 2019 at 03:01:51PM -0700, Nathan Chancellor wrote: > > > > On Mon, Apr 15, 2019 at 08:44:36PM +0200, Greg Kroah-Hartman wrote: > > > > > From: Will Deacon > > > > > > > > > > commit 045afc24124d80c6998d9c770844c67912083506 upstream. > > > > > > > > > > Rather embarrassingly, our futex() FUTEX_WAKE_OP implementation doesn't > > > > > explicitly set the return value on the non-faulting path and instead > > > > > leaves it holding the result of the underlying atomic operation. This > > > > > means that any FUTEX_WAKE_OP atomic operation which computes a non-zero > > > > > value will be reported as having failed. Regrettably, I wrote the buggy > > > > > code back in 2011 and it was upstreamed as part of the initial arm64 > > > > > support in 2012. > > > > > > > > > > The reasons we appear to get away with this are: > > > > > > > > > > 1. FUTEX_WAKE_OP is rarely used and therefore doesn't appear to get > > > > > exercised by futex() test applications > > > > > > > > > > 2. If the result of the atomic operation is zero, the system call > > > > > behaves correctly > > > > > > > > > > 3. Prior to version 2.25, the only operation used by GLIBC set the > > > > > futex to zero, and therefore worked as expected. From 2.25 onwards, > > > > > FUTEX_WAKE_OP is not used by GLIBC at all. > > > > > > > > > > Fix the implementation by ensuring that the return value is either 0 > > > > > to indicate that the atomic operation completed successfully, or -EFAULT > > > > > if we encountered a fault when accessing the user mapping. > > > > > > > > > > Cc: > > > > > Fixes: 6170a97460db ("arm64: Atomic operations") > > > > > Signed-off-by: Will Deacon > > > > > Signed-off-by: Greg Kroah-Hartman > > > > > > > > > > --- > > > > > arch/arm64/include/asm/futex.h | 16 ++++++++-------- > > > > > 1 file changed, 8 insertions(+), 8 deletions(-) > > > > > > > > > > --- a/arch/arm64/include/asm/futex.h > > > > > +++ b/arch/arm64/include/asm/futex.h > > > > > @@ -33,8 +33,8 @@ > > > > > " prfm pstl1strm, %2\n" \ > > > > > "1: ldxr %w1, %2\n" \ > > > > > insn "\n" \ > > > > > -"2: stlxr %w3, %w0, %2\n" \ > > > > > -" cbnz %w3, 1b\n" \ > > > > > +"2: stlxr %w0, %w3, %2\n" \ > > > > > +" cbnz %w0, 1b\n" \ > > > > > " dmb ish\n" \ > > > > > "3:\n" \ > > > > > " .pushsection .fixup,\"ax\"\n" \ > > > > > @@ -53,29 +53,29 @@ > > > > > static inline int > > > > > arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *uaddr) > > > > > { > > > > > - int oldval = 0, ret, tmp; > > > > > + int oldval, ret, tmp; > > > > > > > > > > pagefault_disable(); > > > > > > > > > > switch (op) { > > > > > case FUTEX_OP_SET: > > > > > - __futex_atomic_op("mov %w0, %w4", > > > > > + __futex_atomic_op("mov %w3, %w4", > > > > > ret, oldval, uaddr, tmp, oparg); > > > > > break; > > > > > case FUTEX_OP_ADD: > > > > > - __futex_atomic_op("add %w0, %w1, %w4", > > > > > + __futex_atomic_op("add %w3, %w1, %w4", > > > > > ret, oldval, uaddr, tmp, oparg); > > > > > break; > > > > > case FUTEX_OP_OR: > > > > > - __futex_atomic_op("orr %w0, %w1, %w4", > > > > > + __futex_atomic_op("orr %w3, %w1, %w4", > > > > > ret, oldval, uaddr, tmp, oparg); > > > > > break; > > > > > case FUTEX_OP_ANDN: > > > > > - __futex_atomic_op("and %w0, %w1, %w4", > > > > > + __futex_atomic_op("and %w3, %w1, %w4", > > > > > ret, oldval, uaddr, tmp, ~oparg); > > > > > break; > > > > > case FUTEX_OP_XOR: > > > > > - __futex_atomic_op("eor %w0, %w1, %w4", > > > > > + __futex_atomic_op("eor %w3, %w1, %w4", > > > > > ret, oldval, uaddr, tmp, oparg); > > > > > break; > > > > > default: > > > > > > > > > > > > > > > > > > This causes a (false) build warning with AOSP's GCC 4.9.4 (which is > > > > used to build nearly all arm64 Android kernels before 4.14): > > > > > > > > CC kernel/futex.o > > > > ../kernel/futex.c: In function 'do_futex': > > > > ../kernel/futex.c:1492:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized] > > > > return oldval == cmparg; > > > > ^ > > > > In file included from ../kernel/futex.c:69:0: > > > > ../arch/arm64/include/asm/futex.h:56:6: note: 'oldval' was declared here > > > > int oldval, ret, tmp; > > > > ^ > > > > > > > > The only reason I bring this up is Qualcomm based kernels have a Python > > > > script that emulates -Werror, meaning this will be fatal for a large > > > > number of kernels, when this eventually gets merged into them. > > > > > > Argh, really? That's a buggy compiler that you have there, as oldval > > > will be set correctly if all is good, and if not, ret will be and the > > > code will error out. > > > > > > > Correct. > > > > > Working around broken compilers is not something I really like doing :( > > > > > > > Indeed, I wouldn't have brought it up if it wasn't the compiler for all > > Android 4.9 kernels aside from the Pixel 3 (XL). > > > > > That being said, does this also show up in the 4.19.y and 5.0.y tree > > > right now? If not, why not? > > > > > > > It does. > > > > $ make ARCH=arm64 CROSS_COMPILE=/bin/aarch64-linux-gnu- defconfig kernel/futex.o > > Great, so it seems this needs to be fixed in Linus's tree first, before > I can backport it everywhere. > Well, is it worth working around this in Linus's tree? I know you hate taking patches just for stable but this compiler won't be used on 4.14+ according to [1] and support for it is planned to be discontinued in less than a year [2]. This warning doesn't happen with Clang or newer versions of GCC (I tested 6.3 in a Debian Docker image, which seems to be the oldest I can find). I suppose there could be other buggy/ancient compilers to work around... > Want me to send a patch for this or can you? > I am happy to send a patch regardless of where it goes, just want to be sure we are all on the same page. [1]: https://android.googlesource.com/platform/test/vts-testcase/kernel/+/e1622ae19e0419ceac363d31f7fd53b514b8c218 [2]: https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/+/a28e116f380a7e7c19e4639b6a35fecf5dddd4e8 Thanks, Nathan > thanks, > > greg k-h