[libvirt] Quantifying libvirt errors in launching the libguestfs appliance

Cole Robinson crobinso at redhat.com
Wed Jan 13 23:45:43 UTC 2016


On 01/13/2016 05:18 AM, Richard W.M. Jones wrote:
> As people may know, we frequently encounter errors caused by libvirt
> when running the libguestfs appliance.
> 
> I wanted to find out exactly how frequently these happen and classify
> the errors, so I ran the 'virt-df' tool overnight 1700 times.  This
> tool runs several parallel qemu:///session libvirt connections both
> creating a short-lived appliance guest.
> 
> Note that I have added Cole's patch to fix https://bugzilla.redhat.com/1271183
> "XML-RPC error : Cannot write data: Transport endpoint is not connected"
> 
> Results:
> 
> The test failed 538 times (32% of the time), which is pretty dismal.
> To be fair, virt-df is aggressive about how it launches parallel
> libvirt connections.  Most other virt-* tools use only a single
> libvirt connection and are consequently more reliable.
> 
> Of the failures, 518 (96%) were of the form:
> 
>   process exited while connecting to monitor: qemu: could not load kernel '/home/rjones/d/libguestfs/tmp/.guestfs-1000/appliance.d/kernel': Permission denied
> 
> which is https://bugzilla.redhat.com/921135 or maybe
> https://bugzilla.redhat.com/1269975.  It's not clear to me if these
> bugs have different causes, but if they do then potentially we're
> seeing a mix of both since my test has no way to distinguish them.
> 

I just experimented with this, I think it's the issue I suggested at:

https://bugzilla.redhat.com/show_bug.cgi?id=1269975#c4

I created two VMs, kernel1 and kernel2, just booting off a kernel in
$HOME/session-kernel/vmlinuz. Then I added this patch:

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index f083f3f..5d9f0fa 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -4901,6 +4901,13 @@ qemuProcessLaunch(virConnectPtr conn,
                                       incoming ? incoming->path : NULL) < 0)
         goto cleanup;

+    if (STREQ(vm->def->name, "kernel1")) {
+        for (int z = 0; z < 30; z++) {
+            printf("kernel1: sleeping %d of 30\n", z + 1);
+            sleep(1);
+        }
+    }
+
     /* Security manager labeled all devices, therefore
      * if any operation from now on fails, we need to ask the caller to
      * restore labels.


Which is right after selinux labels are set on VM startup. This is then easy
to reproduce with:

virsh start kernel1 (sleeps)
virsh start kernel2 && virsh destroy kernel2

The shared vmlinuz is reset to user_home_t after kernel2 is shut down, so
kernel1 fails to start after the patch's timeout

When we detect similar issues with <disk> devices, like when the media already
has the expected label, we encode 'relabel=no' in the disk XML, which tells
libvirt not to run restorecon on the disks path when the VM is shutdown.
However kernel/initrd XML doesn't have support for this XML, so it won't work
there. Adding that could be one fix.

But I think there's longer term plans for this type of issue by using ACLs, or
virtlockd or something, Michal had patches but I don't know the specifics.

Unfortunately even hardlinks share selinux labels so I don't think there's any
workaround on the libguestfs side short of using a separate copy of the
appliance kernel for each VM

- Cole




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