Re: [ANNOUNCE] libblkio v0.1.0 preview release

["Richard W.M. Jones" <rjones@redhat.com>]

On Thu, Apr 29, 2021 at 04:08:22PM +0100, Daniel P. Berrangé wrote:
> On Thu, Apr 29, 2021 at 04:00:38PM +0100, Richard W.M. Jones wrote:
> > On Thu, Apr 29, 2021 at 03:41:29PM +0100, Stefan Hajnoczi wrote:
> > > On Thu, Apr 29, 2021 at 03:22:59PM +0100, Richard W.M. Jones wrote:
> > > > On Thu, Apr 29, 2021 at 03:05:50PM +0100, Stefan Hajnoczi wrote:
> > > > > The purpose of this preview release is to discuss both the API design
> > > > > and general direction of the project. API documentation is available
> > > > > here:
> > > > > 
> > > > >   https://gitlab.com/libblkio/libblkio/-/blob/v0.1.0/docs/blkio.rst
> > > > 
> > > > libvirt originally, and now libnbd, keep a per-thread error message
> > > > (stored in thread-local storage).  It's a lot nicer than having to
> > > > pass &errmsg to every function.  You can just write:
> > > > 
> > > >  if (nbd_connect_tcp (nbd, "remote", "nbd") == -1) {
> > > >    fprintf (stderr,
> > > >             "failed to connect to remote server: %s (errno = %d)\n",
> > > >             nbd_get_error (), nbd_get_errno ());
> > > >    exit (EXIT_FAILURE);
> > > >  }
> > > > 
> > > > (https://libguestfs.org/libnbd.3.html#ERROR-HANDLING)
> > > > 
> > > > It means you can extend the range of error information available in
> > > > future.  Also you can return a 'const char *' and the application
> > > > doesn't have to worry about lifetimes, at least in the common case.
> > > 
> > > Thanks for sharing the idea, I think it would work well for libblkio
> > > too.
> > > 
> > > Do you ignore the dlclose(3) memory leak?
> > 
> > I believe this mechanism in libnbd ensures there is no leak in the
> > ordinary shared library (not dlopen/dlclose) case:
> > 
> > https://gitlab.com/nbdkit/libnbd/-/blob/f9257a9fdc68706a4079deb4ced61e1d468f28d6/lib/errors.c#L35
> > 
> > However I'm not sure what happens in the dlopen case, so I'm going to
> > test that out now ...
> 
> pthread_key destructors are a disaster waiting to happen with
> dlclose, if the dlclose happens while non-main threads are
> still running. When those threads exit, they'll run the
> destructor which points to a function that is no longer
> resident in memory.
> 
> IOW if you have destrutors, then you need to make sure your
> library uses "-z nodelete" when linking, to turn dlclose()
> into a no-op.

I suspect letting the string leak is a better idea for libnbd.

Still trying to write a nice reproducer ..

Rich.

>   commit 8e44e5593eb9b89fbc0b54fde15f130707a0d81e
>   Author: Daniel P. Berrangé <berrange@redhat.com>
>   Date:   Thu Sep 1 17:57:06 2011 +0100
> 
>     Prevent crash from dlclose() of libvirt.so
>     
>     When libvirt calls virInitialize it creates a thread local
>     for the virErrorPtr storage, and registers a callback to
>     cleanup memory when a thread exits. When libvirt is dlclose()d
>     or otherwise made non-resident, the callback function is
>     removed from memory, but the thread local may still exist
>     and if a thread later exists, it will invoke the callback
>     and SEGV. There may also be other thread locals with callbacks
>     pointing to libvirt code, so it is in general never safe to
>     unload libvirt.so from memory once initialized.
>     
>     To allow dlclose() to succeed, but keep libvirt.so resident
>     in memory, link with '-z nodelete'. This issue was first
>     found with the libvirt CIM provider, but can potentially
>     hit many of the dynamic language bindings which all ultimately
>     involve dlopen() in some way, either on libvirt.so itself,
>     or on the glue code for the binding which in turns links
>     to libvirt
> 
> 
> 
> Regards,
> Daniel
> -- 
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-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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