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Message-ID: <20250527190349.GQ1827@brightrain.aerifal.cx>
Date: Tue, 27 May 2025 15:03:49 -0400
From: Rich Felker <dalias@...c.org>
To: Markus Wichmann <nullplan@....net>
Cc: musl@...ts.openwall.com
Subject: Re: Deadlock in dynamic linker?
On Tue, May 27, 2025 at 06:59:12PM +0200, Markus Wichmann wrote:
> Am Tue, May 27, 2025 at 11:20:07AM -0400 schrieb Rich Felker:
> > On Sat, May 24, 2025 at 07:45:45AM +0200, Markus Wichmann wrote:
> > > I'm thinking something like this: Thread A initializes liba.so. liba.so
> > > has initializers and finalizers, so thread A adds liba.so to the fini
> > > list before calling the initializers. The liba initializer calls
> > > dlopen("libb.so"). libb.so also has initializers.
> > >
> > > While thread A is not holding the init_fini_lock, thread B calls exit().
> > > That progresses until __libc_exit_fini() sets shutting_down to 1. Then
> > > it tries to destroy all the libraries, but the loop stops when it comes
> > > to liba.
> > >
> > > liba.so has a ctor_visitor, namely thread A, so thread B cannot advance.
> > > Thread A meanwhile is hanging in the infinite wait loop trying to
> > > initialize libb.so. The situation cannot change, and the process hangs
> > > indefinitely.
> >
> > I see. In particular you're assuming the dlopen of libb happened after
> > the exit started.
> >
>
> I had completely neglected to look at the global ldso lock, actually.
> But looking at it again, I am actually assuming that the dlopen() is
> *starting* before the __libc_exit_fini() (so that thread B hangs waiting
> for the lock), but that thread B then overtakes thread A between the
> latter's release of the global lock and the taking of the init_fini_lock.
>
> This does mean that taking the init_fini_lock before releasing the
> global lock would entirely prevent the issue. Not sure if that's
> acceptable, though.
No, it wouldn't, because the lock is released every time a condwait is
done, and exit can start during any of those condwaits.
Rich
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