[Libguestfs] guestfs_launch() fails when C application is started as a systemd service

[Peter Dimitrov]

> That makes no sense because we are supposed to have just forked
successfully

I just realized libguestfs uses fork. Now we know why qemu-img worked - I
launched it with popen.

> So it must be something to do with collectd and how it runs programs.
> Is it using LD_PRELOAD trickery, or replacing libc, or using seccomp?

If I understand the question correctly - it's about how collectd loads its
plugins? If so it uses:

static int plugin_load_file(const char *file, _Bool global) {
  void (*reg_handle)(void);
  int flags = RTLD_NOW;
    if (global)
      flags |= RTLD_GLOBAL;
  void *dlh = *dlopen*(file, flags);
  //...
  reg_handle = (void (*)(void))*dlsym*(dlh, "module_register");
  //...
  *(*reg_handle)();*
}


Does this give any clues?

Best Regards,
Peter


On Wed, Nov 7, 2018 at 12:56 PM Richard W.M. Jones <rjones at redhat.com>
wrote:

> On Wed, Nov 07, 2018 at 12:32:48PM +0200, Peter Dimitrov wrote:
> > Thank you, Rich,
> > This was the issue indeed. export LIBGUESTFS_BACKEND=direct fixed it.
> >
> > The next step I tried was to integrate libguestfs in collectd virt plugin
> > to collect this data automatically.
> > In this case I'm having an unknown error in add_libvirt_dom() (same with
> > add_domain) when it's invoking qemu-img to create overlay image.
> >
> > There is no difference between manual and service execution.
> > I tried setting LIBGUESTFS_BACKEND to direct,
> > libvirt, libvirt:qemu:///session with no success.
> > Also tried using a different tmp dir just in case - nothing.
> >
> > Maybe something is wrong with how collectd runs its plugins (dynamic
> > linking)?
> > Invoking virt-df from collectd's plugin gives the same error message.
> > I tried running the same qemu-img command from collectd and it passes
> > though! Confusing...
>
> The log indicates something a bit strange is going on:
>
> > libguestfs: command: run: qemu-img
> > libguestfs: command: run: \ create
> > libguestfs: command: run: \ -f qcow2
> > libguestfs: command: run: \ -o
> > backing_file=/home/peterd/TVE/wer.qcow2,backing_fmt=qcow2
> > libguestfs: command: run: \ /tmp/libguestfsUIZbDK/overlay1.qcow2
> > Formatting '/tmp/libguestfsUIZbDK/overlay1.qcow2', fmt=qcow2
> > size=107374182400 backing_file=/home/peterd/TVE/wer.qcow2
> backing_fmt=qcow2
> > encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16
> > libguestfs: error: command: waitpid: No child processes
> > libguestfs: error: qemu-img: /tmp/libguestfsUIZbDK/overlay1.qcow2:
> qemu-img
> > exited for an unknown reason (status -1), see debug messages above
>
> Obviously waitpid(2) is failing with ECHILD here:
>
>
> https://github.com/libguestfs/libguestfs/blob/3430c2dd654b19a55d213a9302ac5e4b6a387bee/lib/command.c#L741
>
> That makes no sense because we are supposed to have just forked
> successfully:
>
>
> https://github.com/libguestfs/libguestfs/blob/3430c2dd654b19a55d213a9302ac5e4b6a387bee/lib/command.c#L479
>
> called from:
>
>
> https://github.com/libguestfs/libguestfs/blob/3430c2dd654b19a55d213a9302ac5e4b6a387bee/lib/command.c#L764
>
> Notice also that qemu-img *does* run (you can see the output from the
> command).
>
> So it must be something to do with collectd and how it runs programs.
> Is it using LD_PRELOAD trickery, or replacing libc, or using seccomp?
> My guess is that any program which launched a subprocess and then
> waited for it would fail in the same way.
>
> Rich.
>
> --
> Richard Jones, Virtualization Group, Red Hat
> http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> virt-df lists disk usage of guests without needing to install any
> software inside the virtual machine.  Supports Linux and Windows.
> http://people.redhat.com/~rjones/virt-df/
>
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