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

[Richard W.M. Jones]

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
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