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Message-ID: <20190109114536.GI21289@port70.net>
Date: Wed, 9 Jan 2019 12:45:37 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: pthread_key_create bug?
* Rich Felker <dalias@...c.org> [2019-01-08 19:29:53 -0500]:
> On Tue, Jan 08, 2019 at 11:10:06PM +0100, Markus Wichmann wrote:
> > On Mon, Jan 07, 2019 at 07:00:18PM -0500, Rich Felker wrote:
> > > I think you're right, though we don't generally use weak references in
> > > musl on the basis (perhaps somewhat dubious) that they're an
> > > additional toolchain feature that might cause problems reusing the
> > > code in non-ELF contexts (this may affect midipix; I'm not sure).
> > > That's why weak aliases to dummy functions are used for this purpose
> > > everywhere else.
> > >
> >
> > I don't quite get you. Weak aliases are just weak references with a
> > non-zero default value. Any toolchain being able to handle weak aliases
> > should be able to handle weak references, right?
>
> No, not necessarily, and no they're not equivalent to weak definitions
> with a zero value. A weak definition with a zero value would provide a
> definition for other translation units to see/use, preventing one
> with a non-weak reference from pulling in the real definition. Weak
> references are a property of the reference. Weak definitions are a
> property of the definition. They are not in any way equivalent.
>
> In any case, the policy is that we don't use weak references (*). If
> there were a strong reason to want them, we could review the reasons
> and see if they are still relevant.
>
> (*) This is only partly true. There are weak references to some
> special ELF symbols defined by the linker, because providing a weak
> definition would override the linker-provided value. For these, there
> will never be definitions provided by source files, so the weak
> reference is mostly equivalent to a weak definition by your above
> logic. Furthermore, we know they'll always be defined for
> position-independent linking (_DYNAMIC has to exist for shared
> libraries and PIE) so there is no concern about the issue below
> (PC-relative references to an undefined weakly-referenced symbol).
note that weak references are not even well supported on elf targets:
weak references can be undefined and then the symbol address is 0 at
runtime, which should be possible to check with if (weakref == 0)..,
but this does not work reliably, see e.g.
https://groups.google.com/forum/#!topic/generic-abi/eZv8VQskSD0
so the specified weak reference semantics is broken in important use
cases: tls, copy reloc, plt address.
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