On 04 Dec 2016, at 10:52, Hans Petter Selasky <h...@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
Description: Message signed with OpenPGP using GPGMail