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dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:46908 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lATAW-0008Gh-EI for qemu-devel@archiver.kernel.org; Fri, 12 Feb 2021 02:47:12 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:35384) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lAT9E-0007kO-5t for qemu-devel@nongnu.org; Fri, 12 Feb 2021 02:45:52 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:60568) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lAT99-000499-Kw for qemu-devel@nongnu.org; Fri, 12 Feb 2021 02:45:51 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1613115944; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=7Lc220DFDF86fyb3ekQl56ffqDgLqDhHmNuqeNVOOPI=; b=Pzr0AQYeIzAmPTiFAGKMAk65RWd7jlDBZb2oM0x+cfniiw+liptsnbM5JHMSaej1DKmBYZ fvJF/dgoE1nppmkEjiu94Ow2HudWEEZHXOll0vsBOSvK3vIPbokeiw+yP3w72FhZtak7Ko DcovQ6Phz6c5mTT7lOruWS2KRpDKaoI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-376-0oUmmCvMNeKWdnzrQYIIzw-1; Fri, 12 Feb 2021 02:45:42 -0500 X-MC-Unique: 0oUmmCvMNeKWdnzrQYIIzw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6B9A3196632C; Fri, 12 Feb 2021 07:45:41 +0000 (UTC) Received: from thuth.remote.csb (ovpn-112-27.ams2.redhat.com [10.36.112.27]) by smtp.corp.redhat.com (Postfix) with ESMTP id 91DC51F455; Fri, 12 Feb 2021 07:45:40 +0000 (UTC) Subject: Re: [PATCH] linux-user/syscall: Silence warning from the undefined behavior sanitizer To: Laurent Vivier , qemu-devel@nongnu.org References: <20210211132959.574168-1-thuth@redhat.com> <2baf117e-2af0-5bb7-944c-219bcb9282ba@vivier.eu> From: Thomas Huth Message-ID: Date: Fri, 12 Feb 2021 08:45:39 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <2baf117e-2af0-5bb7-944c-219bcb9282ba@vivier.eu> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=thuth@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.569, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.119, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 11/02/2021 22.28, Laurent Vivier wrote: > Le 11/02/2021 à 14:29, Thomas Huth a écrit : >> When compiling QEMU with -fsanitize=undefined, there is a warning when >> running "make check-tcg": >> >> TEST linux-test on m68k >> ../linux-user/syscall.c:10499:34: runtime error: member access within >> misaligned address 0x00008006df3c for type 'struct linux_dirent64', >> which requires 8 byte alignment >> 0x00008006df3c: note: pointer points here >> 00 00 00 00 68 03 28 00 00 00 00 00 5b 96 3e e4 61 4b 05 26 18 00 04 2e 00 00 00 00 da 3f 18 00 >> ^ >> >> It's likely not an issue in reality, since I assume that on hosts where >> the alignment really matters (like sparc64), the Linux kernel likely >> adds the right padding. Anyway, let's use the stw_p() / stq_p() accessor >> helpers here to silence the warning and thus to allow to compile the code >> with -fsanitize=undefined, too. >> >> Signed-off-by: Thomas Huth >> --- >> linux-user/syscall.c | 16 +++++++++------- >> 1 file changed, 9 insertions(+), 7 deletions(-) >> >> diff --git a/linux-user/syscall.c b/linux-user/syscall.c >> index 34760779c8..50de535ade 100644 >> --- a/linux-user/syscall.c >> +++ b/linux-user/syscall.c >> @@ -10491,20 +10491,22 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1, >> return -TARGET_EFAULT; >> ret = get_errno(sys_getdents64(arg1, dirp, count)); >> if (!is_error(ret)) { >> - struct linux_dirent64 *de; >> + char *de; >> int len = ret; >> int reclen; >> - de = dirp; >> + de = (char *)dirp; >> + #define de64(x) offsetof(struct linux_dirent64, x) > > Do we really need the cast to the "(char *)"? > > can't we use "&de->XXX" with the accessors? > We don't access the memory, only read the address, the compiler should be happy. That's what I thought and tried first, too. Unfortunately, it did not help to fix the issue, I had to take the detour via the char* I guess the compiler also checks the alignment of the pointer when it gets assigned to the next record below ("de = ..."). Thomas > >> while (len > 0) { >> - reclen = de->d_reclen; >> + reclen = lduw_he_p(de + de64(d_reclen)); > > to avoid human error, it would be better to let the compiler take the good accessor: > > ldn_he_p(&de->d_reclen, sizeof(de->d_reclen)) > >> if (reclen > len) >> break; >> - de->d_reclen = tswap16(reclen); >> - tswap64s((uint64_t *)&de->d_ino); >> - tswap64s((uint64_t *)&de->d_off); >> - de = (struct linux_dirent64 *)((char *)de + reclen); >> + stw_p(de + de64(d_reclen), reclen); >> + stq_p(de + de64(d_ino), ldq_he_p(de + de64(d_ino))); >> + stq_p(de + de64(d_off), ldq_he_p(de + de64(d_off))); > > and stwn_he_p() here too. > >> + de += reclen; >> len -= reclen; >> } >> + #undef de64 >> } >> unlock_user(dirp, arg2, ret); >> } >> > > Thank you Thomas for your help. > > Laurent >