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 <[email protected]> > 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 > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v
