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Re: Problem with atexit and _dl_fini



On 19.05.19 18:23, Florian Weimer wrote:
* Nat!:

So my problem is, that I observe that my atexit calls are not executed
in the correct order.

i.e. atexit( a); atexit( b);  should result in b(), a() being called in
that order. To quote the man page,

"the registered functions are invoked in reverse order".


When I register with `atexit` I can see my functions being added
properly within `__internal_atexit` in the

correct order. Finally after my functions, the elf-loader ? also adds
itself there. So it is being called first by

`__run_exit_handlers`.


Then comes the part where it goes wrong. I registered my two function
with `__internal_atexit`, but for some reason

`_dl_fini` is calling `__cxa_finalize` and that is calling the wrong
function first.
When atexit is called from a DSO, glibc calls the registered function
before the DSO is unloaded.  This choice was made because after
unloading, the function pointer becomes invalid.

I haven't checked, but I suspect atexit still works this way even if
it doesn't have to (because the DSO is never unloaded).


I understand, but the behavior is wrong :) The C standard (or the C++ standard for this matter) http://www.cplusplus.com/reference/cstdlib/atexit/ states that


```

If more than one atexit function has been specified by different calls to this function, they are all executed in reverse order as a stack (i.e. the last function specified is the first to be executed at exit).

```

I think its been shown that glibc can violate this C standard, so for me the argument would be over here already. That one should unwind in the reverse order is, I assume, not a interesting discussion topic. Currently atexit as a reliable mechanism is broken.


But I also don't think the way this is currently handled in glibc, can't be of much use to anyone.

Case 1: a regular exe linked with shared libraries, nothing gets unloaded at exit, so what's the point ?

Case 2: someone manually unloads a shared library, that contains atexit code. The bug is either using `atexit` for a shared library that gets unloaded, or unloading a shared library that contains atexit code. But it's not really glibcs business IMO.

Case 3: some automatism unloads shared libraries. Then the automatism should check if atexit code is affected and not unload, because the shared library is still clearly needed. It's a bug in the automatism.

If one was hellbent on trying to support atexit for unloading shared libraries, an atexit contained in a shared library should up the reference count of the shared library during the atexit call and decrement after the callback has executed.

Ciao

   Nat!



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