Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation

* Mathieu Desnoyers:

Here is one scenario: we have 2 early adopter libraries using rseq which
are deployed in an environment with an older glibc (which does not
support rseq).

Of course, none of those libraries can be dlclose'd unless they somehow
track all registered threads.

Well, you can always make them NODELETE so that dlclose is not an issue.
If the library is small enough, that shouldn't be a problem.

But let's focus on how exactly those libraries can handle lazily
registering rseq. They can use pthread_key, and pthread_setspecific on
first use by the thread to setup a destructor function to be invoked
at thread exit. But each early adopter library is unaware of the
other, so if we just use a "is_initialized" flag, the first destructor
to run will unregister rseq while the second library may still be
using it.

I don't think you need unregistering if the memory is initial-exec TLS
memory.  Initial-exec TLS memory is tied directly to the TCB and cannot
be freed while the thread is running, so it should be safe to put the
rseq area there even if glibc knows nothing about it.  Then you'll only
need a mechanism to find the address of the actually active rseq area
(which you probably have to store in a TLS variable for performance
reasons).  And that part you need whether you have reference counter or
not.

The same problem arises if we have an application early adopter which
explicitly deal with rseq, with a library early adopter. The issue is
similar, except that the application will explicitly want to unregister
rseq before exiting the thread, which leaves a race window where rseq
is unregistered, but the library may still need to use it.

The reference counter solves this: only the last rseq user for a thread
performs unregistration.

If you do explicit unregistration, you will run into issues related to
destructor ordering.  You should really find a way to avoid that.

Thanks,
Florian