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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