core.c: Prevent SIGSEGV during migration

Don Slutz Thu, 20 Nov 2014 10:34:02 -0800

On 11/19/14 07:29, Markus Armbruster wrote:

Don Slutz <dsl...@verizon.com> writes:

The other callers to blk_set_enable_write_cache() in this file
already check for s->blk == NULL.

Signed-off-by: Don Slutz <dsl...@verizon.com>
---

I think this is a bugfix that should be back ported to stable
releases.

I also think this should be done in xen's copy of QEMU for 4.5 with
back port(s) to active stable releases.

Note: In 2.1 and earlier the routine is
bdrv_set_enable_write_cache(); variable is s->bs.

Got a reproducer?


yes.  Migrating a guest from xen 4.2 or 4.3 to xen 4.4 (or 4.5-unstable) on
CentOS 6.3 with xen_emul_unplug=unnecessary and no cdrom defined.


I'm asking because I believe s->identify_set implies s->blk.
s->identify_set is initialized to zero, and gets set to non-zero exactly
on the first successful IDENTIFY DEVICE or IDENTIFY PACKET DEVICE, in
ide_identify(), ide_atapi_identify() or ide_cfata_identify(),
respectively.  Only called via cmd_identify() / cmd_identify_packet()
via ide_exec_cmd().  The latter immediately fails when !s->blk:

     s = idebus_active_if(bus);
     /* ignore commands to non existent slave */
     if (s != bus->ifs && !s->blk) {
         return;
     }


I do think that you are right.  I have now spent more time on why I am
seeing this.

Even if I'm right, your patch is fine, because it makes this spot more
obviously correct, and consistent with the other uses of
blk_set_enable_write_cache().  The case for stable is weak, though.


I had not fully tracked down what is happening before sending the bugfix.
I have now done more debugging, and have tracked it down to xen 4.4
now using "-nodefaults" with QEMU.

I needed to add output to QEMU to track this down because I have long
command lines...

(all I get for ps -ef):

root 14864 1 82 16:42 ? 00:00:09/usr/lib/xen/bin/qemu-system-i386 -xen-domid 20 -chardevsocket,id=libxl-cmd,path=/var/run/xen/qmp-libxl-20,server,nowait -monchardev=libxl-cmd,mode=control -name C63-min-tools -machinexenfv,vmware_hw=7,xen_platform_pci=240 -hostbridge 82443 -deviceagp-bridge,id=agp,msi=off,msi-x=off,addr=0x12.0 -monitor pty -monitor vc-boot menu=on -devicevmware-pci-bridge,msi=on,msi-x=on,id=pciBridge1.0,addr=0x11.0 -devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.0,multifunction=on,addr=0x15.0-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.1,multifunction=on,addr=0x15.1-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.2,multifunction=on,addr=0x15.2-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.3,multifunction=on,addr=0x15.3-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.4,multifunction=on,addr=0x15.4-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.5,multifunction=on,addr=0x15.5-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.6,multifunction=on,addr=0x15.6-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge5.7,multifunction=on,addr=0x15.7-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.0,multifunction=on,addr=0x16.0-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.1,multifunction=on,addr=0x16.1-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.2,multifunction=on,addr=0x16.2-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.3,multifunction=on,addr=0x16.3-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.4,multifunction=on,addr=0x16.4-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.5,multifunction=on,addr=0x16.5-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.6,multifunction=on,addr=0x16.6-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge6.7,multifunction=on,addr=0x16.7-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.0,multifunction=on,addr=0x17.0-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.1,multifunction=on,addr=0x17.1-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.2,multifunction=on,addr=0x17.2-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.3,multifunction=on,addr=0x17.3-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.4,multifunction=on,addr=0x17.4-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.5,multifunction=on,addr=0x17.5-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.6,multifunction=on,addr=0x17.6-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge7.7,multifunction=on,addr=0x17.7-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.0,multifunction=on,addr=0x18.0-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.1,multifunction=on,addr=0x18.1-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.2,multifunction=on,addr=0x18.2-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.3,multifunction=on,addr=0x18.3-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.4,multifunction=on,addr=0x18.4-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.5,multifunction=on,addr=0x18.5-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.6,multifunction=on,addr=0x18.6-devicevmware-pcie-bridge,msi=on,msi-x=on,id=pciBridge8.7,multifunction=on,addr=0x18.7-vnc 0.0.0.0:7,to=99 -serial pty -vga vmware -globalvmware-svga.vgamem_mb=32 -boot order=cda -devicevmxnet3,id=nic0,netdev=net0,mac=00:0c:29:86:44:a0,addr=0x3.0x0 -netdevtype=tap,id=net0,ifname=vif20.0-emu,script=no,downscript=no -devicee1000_vmw,id=nic1,netdev=net1,mac=00:0c:29:86:44:aa,addr=0x4.0x0 -netdevtype=tap,id=net1,ifname=vif20.1-emu,script=no,downscript=no -devicevmxnet3,id=nic2,netdev=net2,mac=00:0c:29:86:44:b4,addr=0x5.0x0 -netdevtype=tap,id=net2,ifname=vif20.2-emu,script=no,downscript=no -devicevmxnet3,id=nic3,netdev=net3,mac=00:0c:29:86:44:be,addr=0x6.0x0 -netdevtype=tap,id=net3,ifname=vif20.3-emu,script=no,downscript=no -devicevmxnet3,id=nic4,netdev=net4,mac=00:0c:29:86:44:c8,addr=0x8.0x0 -netdevtype=tap,id=net4,ifname=vif20.4-emu,script=no,downscript=no -devicevmxnet3,id=nic5,netdev=net5,mac=00:0c:29:86:44:d2,addr=0x9.0x0 -netdev t



Which is missing that option.

The ide that was aborting in this case is the cdrom at hdc that is added
if you do not specify "-nodefaults".

Since this is a "changed" machine config, I am no longer as sure as what
versions this needs to be in.

If I put my QEMU hat on, it does not look like a back port is needed.However

for xen it would be nice.

I do not know how the QEMU community feels about migration from a config
without "-nodefaults" to one with "-nodefaults" as the only difference.

   -Don Slutz

  hw/ide/core.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/ide/core.c b/hw/ide/core.c
index 00e21cf..d4af5e2 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -2401,7 +2401,7 @@ static int ide_drive_post_load(void *opaque, int 
version_id)
  {
      IDEState *s = opaque;

- if (s->identify_set) {

+    if (s->blk && s->identify_set) {
          blk_set_enable_write_cache(s->blk, !!(s->identify_data[85] & (1 << 
5)));
      }
      return 0;

Re: [Qemu-devel] [BUGFIX][PATCH for 2.2 1/1] hw/ide/core.c: Prevent SIGSEGV during migration

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