dlopen(), atexit() crash on FreeBSD (testcase included)

[Alexander Kabaev]

On Mon, 31 Dec 2007 17:35:10 +0100
"Markus Hoenicka" <markus.hoenicka at mhoenicka.de> wrote:

> Hi,
> 
> I've been redirected by Giorgos Keramidas to this list after reporting
> a problem on the freebsd-questions list. I'd greatly appreciate if you
> could have a look at the following problem. Apparently programs are
> doomed to segfault on FreeBSD if dlopen()ed modules install exit
> handlers via atexit(). Similar problem reports have cropped up before,
> see e.g.
> 
> http://www.imagemagick.org/pipermail/magick-developers/2006-March/002523.html
> 
> My system runs:
> 
> FreeBSD yeti.mininet 6.1-RELEASE FreeBSD 6.1-RELEASE #1: Mon Aug 28
> 22:24:48 CEST 2006
> markus at yeti.mininet:/usr/src/sys/i386/compile/YETI  i386
> 
> I'm one of the developers of libdbi, a database abstraction layer for
> C, see http://libdbi.sourceforge.net.
> 
> libdbi is a library for programs which are supposed to be able to
> access different database engines with a unified API. libdbi
> essentially maps generic API calls to the specific database client
> library calls of a particular database engine. To do this, libdbi
> loads available database drivers at runtime via dlopen() calls. Each
> of these drivers is linked against one database client
> library. E.g. the Firebird driver is linked against
> libfbclient.so. When libdbi is properly shut down, it unloads all
> loaded drivers by calling dlclose() on each of them.
> 
> This design works well on all supported platforms and with all
> supported database engines, with one exception: the Firebird driver on
> FreeBSD invariably causes a segfault when the application linked
> against libdbi exits:
> 
> #0  0x28514fe4 in ?? ()
> #1  0x281507c3 in __cxa_finalize () from /lib/libc.so.6
> #2  0x281503fe in exit () from /lib/libc.so.6
> #3  0x0804a40f in main (argc=1, argv=0xbfbfe754) at test_dbi.c:419
> 
> The reason appears to be that the Firebird client libraries install
> exit handlers via atexit(). Remember that due to libdbi's design to
> load all available drivers whether or not they are used later, libdbi
> will cause a crash even if no Firebird database is accessed - it is
> sufficient that the driver has been loaded. As per Giorgos' suggestion
> it is simple to circumvent this segfault by avoiding the call to
> dlclose() before exiting, but I wonder whether there is a more robust
> solution for this problem.
> 
> The attached minimal testcase is sufficient to illustrate the
> problem. atexitmod.c defines a module which is loaded by datest.c Make
> sure to fix the hardcoded path in datest.c before building the app. To
> build the test program and watch it crash, do the following:
> 
> gcc -shared -o atexitmod.so atexitmod.c
> gcc -o datest datest.c
> ./datest
> 
> Commenting out either the atexit() call in atexitmod.c or the
> dlclose() call in datest.c prevent the segfault.
> 
> If you find some solution, please cc me as I'm not subscribed to
> freebsd-hackers.
> 
> regards,
> Markus

As designed. atexit should not be used by shared objects that do not
expect themselves to live until actual exit() happens. ELF provides
proper _init/_fini sections to support shared object
initialization/destruction.

-- 
Alexander Kabaev
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