From owner-freebsd-current@freebsd.org Sun Dec 4 16:38:56 2016 Return-Path: Delivered-To: freebsd-current@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id BCD80C6777C for ; Sun, 4 Dec 2016 16:38:56 +0000 (UTC) (envelope-from dim@FreeBSD.org) Received: from tensor.andric.com (tensor.andric.com [IPv6:2001:7b8:3a7:1:2d0:b7ff:fea0:8c26]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client CN "tensor.andric.com", Issuer "COMODO RSA Domain Validation Secure Server CA" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 657FF1C78 for ; Sun, 4 Dec 2016 16:38:56 +0000 (UTC) (envelope-from dim@FreeBSD.org) Received: from coleburn.home.andric.com (coleburn.home.andric.com [192.168.0.15]) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by tensor.andric.com (Postfix) with ESMTPSA id 96A9043360; Sun, 4 Dec 2016 17:38:52 +0100 (CET) Content-Type: multipart/signed; boundary="Apple-Mail=_94FBECC4-CEE0-45DB-A6E4-D62B19F18479"; protocol="application/pgp-signature"; micalg=pgp-sha1 Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) Subject: Re: clang/llvm 3.9.0 mysteriously zeroing variables? From: Dimitry Andric In-Reply-To: Date: Sun, 4 Dec 2016 17:38:42 +0100 Cc: Colin Percival , freebsd-current@freebsd.org Message-Id: <78FB227F-3542-452F-9A16-4FB0E0E698AC@FreeBSD.org> References: <01000158c7252f0c-6c3198b0-fbef-4a60-ade9-e3b91d9e83bd-000000@email.amazonses.com> To: Hans Petter Selasky X-Mailer: Apple Mail (2.3124) X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 04 Dec 2016 16:38:56 -0000 --Apple-Mail=_94FBECC4-CEE0-45DB-A6E4-D62B19F18479 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii On 04 Dec 2016, at 10:52, Hans Petter Selasky wrote: >=20 > On 12/04/16 01:04, Colin Percival wrote: >> Starting with r309124 (when clang/llvm 3.9.0 was imported) I'm seeing = EC2 >> instances panic on boot with a division-by-zero error; the code in = question >> is in blkfront.c, printing out the size of disks: >>=20 >>> device_printf(dev, "%juMB <%s> at %s", >>> (uintmax_t) sectors / (1048576 / sector_size), >>> device_get_desc(dev), >>> xenbus_get_node(dev)); >>=20 >> My first thought was that 'sector_size' must be either zero or very = large... >> but no, when I add printf("sector_size =3D %ju\n", = (uintmax_t)sector_size), it's >> entirely normal. What's more, adding that printf makes the = division-by-zero >> panic go away. >>=20 >> I'd think I was just hallucinating, but earlier today I heard that a = similarly >> "impossible" panic had been observed in the NFS client code when = compiled with >> clang/llvm 3.9.0. >>=20 >> So... is anyone else seeing unexpected panics or other odd behaviour = starting >> after clang/llvm 3.9.0 was imported? >>=20 >=20 > Hi, >=20 > Can you look at the code with "objdump -Dx --source" and see what is = going on there? Might it be the "sector" variable is shadowed? I don't see anything in the generated code for the call that can cause this, except for sector_size really being zero, or the result of 1048576/sector_size being zero. On i386, you get this: .loc 1 1349 19 # = /usr/src/sys/dev/xen/blkfront/blkfront.c:1349:19 movl -56(%ebp), %ecx # -56(%rbp) =3D sectors .Ltmp1148: #DEBUG_VALUE: xbd_connect:sectors <- %ECX .loc 1 1349 38 is_stmt 0 # = /usr/src/sys/dev/xen/blkfront/blkfront.c:1349:38 movl $1048576, %eax # imm =3D 0x100000 xorl %edx, %edx divl -52(%ebp) # -52(%ebp) =3D sector_size movl %eax, %edi .loc 1 1349 27 # = /usr/src/sys/dev/xen/blkfront/blkfront.c:1349:27 xorl %edx, %edx movl %ecx, %eax divl %edi movl %eax, -32(%ebp) # 4-byte Spill On amd64, it looks pretty similar: .loc 1 1349 19 # = /usr/src/sys/dev/xen/blkfront/blkfront.c:1349:19 movq -112(%rbp), %rcx # -112(%rbp) =3D sectors .Ltmp1128: #DEBUG_VALUE: xbd_connect:sectors <- %RCX .loc 1 1349 38 is_stmt 0 # = /usr/src/sys/dev/xen/blkfront/blkfront.c:1349:38 movl $1048576, %eax # imm =3D 0x100000 xorl %edx, %edx divq -88(%rbp) # -88(%rbp) =3D sector_size movq %rax, %rsi .loc 1 1349 27 # = /usr/src/sys/dev/xen/blkfront/blkfront.c:1349:27 xorl %edx, %edx movq %rcx, %rax divq %rsi movq %rax, %r15 Colin, does it panic for you in the first or the second div? -Dimitry --Apple-Mail=_94FBECC4-CEE0-45DB-A6E4-D62B19F18479 Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP using GPGMail -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.0.30 iEYEARECAAYFAlhERpwACgkQsF6jCi4glqOtUQCgiet4dxDTxMKC9N78yT5IFPaW alQAnRA1AEKTbdsjoypsjCDietcUOTbi =IFrk -----END PGP SIGNATURE----- --Apple-Mail=_94FBECC4-CEE0-45DB-A6E4-D62B19F18479--