Date: Tue, 20 Dec 2016 13:14:33 +0100 From: Dimitry Andric <dim@FreeBSD.org> To: Jakub Palider <jpa@semihalf.com> Cc: Hans Petter Selasky <hps@selasky.org>, Colin Percival <cperciva@tarsnap.com>, freebsd-current@freebsd.org Subject: Re: clang/llvm 3.9.0 mysteriously zeroing variables? Message-ID: <8618D217-9DD6-4732-A1C1-D980C4FD3E9E@FreeBSD.org> In-Reply-To: <CAL7QUyNeHiUANEtBzT1gGU9En_tOvy%2Bey5qGR9p_dLhWgsJsAw@mail.gmail.com> References: <01000158c7252f0c-6c3198b0-fbef-4a60-ade9-e3b91d9e83bd-000000@email.amazonses.com> <e0646eb8-d793-1ffb-bd12-febbce86a4f8@selasky.org> <78FB227F-3542-452F-9A16-4FB0E0E698AC@FreeBSD.org> <CAL7QUyNeHiUANEtBzT1gGU9En_tOvy%2Bey5qGR9p_dLhWgsJsAw@mail.gmail.com>
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[-- Attachment #1 --] See here: https://lists.freebsd.org/pipermail/svn-src-head/2016-December/094657.html and here: https://lists.freebsd.org/pipermail/svn-src-head/2016-December/094695.html I committed a fix on Dec 14, and MFCd it on Dec 18. -Dimitry > On 20 Dec 2016, at 11:54, Jakub Palider <jpa@semihalf.com> wrote: > > Hi, > > do you still observe this behaviour? Which type of EC2 instances were affected? > I tried to reproduce with kernel/tools from Dec 15 and did not manage to crash the machine. > > Jakub > > On Sun, Dec 4, 2016 at 5:38 PM, Dimitry Andric <dim@freebsd.org> wrote: > On 04 Dec 2016, at 10:52, Hans Petter Selasky <hps@selasky.org> wrote: > > > > 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: > >> > >>> device_printf(dev, "%juMB <%s> at %s", > >>> (uintmax_t) sectors / (1048576 / sector_size), > >>> device_get_desc(dev), > >>> xenbus_get_node(dev)); > >> > >> My first thought was that 'sector_size' must be either zero or very large... > >> but no, when I add printf("sector_size = %ju\n", (uintmax_t)sector_size), it's > >> entirely normal. What's more, adding that printf makes the division-by-zero > >> panic go away. > >> > >> 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. > >> > >> So... is anyone else seeing unexpected panics or other odd behaviour starting > >> after clang/llvm 3.9.0 was imported? > >> > > > > Hi, > > > > 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) = 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 = 0x100000 > xorl %edx, %edx > divl -52(%ebp) # -52(%ebp) = 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) = 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 = 0x100000 > xorl %edx, %edx > divq -88(%rbp) # -88(%rbp) = 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 > > [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.0.30 iEYEARECAAYFAlhZIK4ACgkQsF6jCi4glqNJqQCdEKmFHPiarjp/V+2UDozJ8RpE 4REAoNnyJoRpVKS5HRKLD4MVBZebXiK7 =Fq0Q -----END PGP SIGNATURE-----help
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