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From: Michael Matz <matz@suse.de>
To: Florian Weimer <fweimer@redhat.com>
Cc: binutils@sourceware.org, gcc@gcc.gnu.org, libc-alpha@sourceware.org
Subject: Re: Counting static __cxa_atexit calls
Date: Wed, 24 Aug 2022 12:53:33 +0000 (UTC)	[thread overview]
Message-ID: <alpine.LSU.2.20.2208241218410.15960@wotan.suse.de> (raw)
In-Reply-To: <87k06x7sme.fsf@oldenburg.str.redhat.com>

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.

  reply	other threads:[~2022-08-24 12:53 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-23 11:58 Florian Weimer
2022-08-23 12:28 ` Nick Clifton
2022-08-23 13:40 ` Michael Matz
2022-08-24 12:06   ` Florian Weimer
2022-08-24 12:53     ` Michael Matz [this message]
2022-08-24 14:31       ` Florian Weimer
2022-08-24 15:25         ` Michael Matz

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