Counting static __cxa_atexit calls
Michael Matz
matz@suse.de
Wed Aug 24 12:53:33 GMT 2022
Hello,
On Wed, 24 Aug 2022, Florian Weimer wrote:
> > Isn't this merely moving the failure point from exception-at-ctor to
> > dlopen-fails?
>
> Yes, and that is a soft error that can be handled (likewise for
> pthread_create).
Makes sense. Though that actually hints at a design problem with ELF
static ctors/dtors: they should be able to soft-fail (leading to dlopen or
pthread_create error returns). So, maybe the _best_ way to deal with this
is to extend the definition of the various object-initionalization means
in ELF to allow propagating failure.
> > Probably a note section, which the link editor could either transform into
> > a dynamic tag or leave as note(s) in the PT_NOTE segment. The latter
> > wouldn't require any specific tooling support in the link editor. But the
> > consumer would have to iterate through all the notes to add the
> > individual counts together. Might be acceptable, though.
>
> I think we need some level of link editor support to avoid drastically
> over-counting multiple static calls that get merged into one
> implementation as the result of vague linkage. Not sure how to express
> that at the ELF level?
Hmm. The __cxa_atexit calls are coming from the per-file local static
initialization_and_destruction routine which doesn't have vague linkage,
so its contribution to the overall number of cxa_atexit calls doesn't
change from .o to final-exe. Can you show an example of what you're
worried about?
A completely different way would be to not use cxa_atexit at all: allocate
memory statically for the object and dtor addresses in .rodata (instead of
in .text right now), and iterate over those at static_destruction time.
(For the thread-local ones it would need to store arguments to
__tls_get_addr).
Doing that or defining failure modes for ELF init/fini seems a better
design than hacking around the current limitation via counting static
cxa_atexit calls.
Ciao,
Michael.
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