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

[Florian Weimer <fweimer at redhat dot com>]

* Mathieu Desnoyers:

> ----- On Nov 22, 2018, at 11:28 AM, Florian Weimer fweimer@redhat.com wrote:
>
>> * 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.
>
> That's indeed what I do with lttng-ust, mainly due to use of pthread_key.
>
>> 
>>> 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.
>
> Is it true for user-supplied stacks as well ?

I'm not entirely sure because the glibc terminology is confusing, but I
think it places intial-exec TLS into the static TLS area (so that it has
a fixed offset from the TCB).  The static TLS area is placed on the
user-supplied stack.

>> 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.
>
> I'm not sure I follow your thoughts here. Currently, the __rseq_abi
> TLS symbol identifies a structure registered to the kernel. The
> "currently active" rseq critical section is identified by the field
> "rseq_cs" within the __rseq_abi structure.
>
> So here when you say "actually active rseq area", do you mean the
> currently registered struct rseq (__rseq_abi) or the currently running
> rseq critical section ? (pointed to by __rseq_abi.rseq_cs)

__rseq_abi.

> One issue here is that early adopter libraries cannot always use
> the IE model. I tried using it for other TLS variables in lttng-ust, and
> it ended up hanging our CI tests when tracing a sample application with
> lttng-ust under a Java virtual machine: being dlopen'd in a process that
> possibly already exhausts the number of available backup TLS IE entries
> seems to have odd effects. This is why I'm worried about using the IE model
> within lttng-ust.

You can work around this by preloading the library.  I'm not sure if
this is a compelling reason not to use initial-exec TLS memory.

>>> 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.
>
> The per-thread reference counter is a way to avoid issues that arise from
> lack of destructor ordering. Is it an acceptable approach for you, or
> you have something else in mind ?

Only for the involved libraries.  It will not help if other TLS
destructors run and use these libraries.

Thanks,
Florian