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Date: Mon, 31 Aug 2015 15:37:55 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: STB_GNU_UNIQUE not handled as original spec intended
On Mon, Aug 31, 2015 at 03:51:16PM +0300, Alexander Monakov wrote:
> Hello,
>
> At present the dynamic linker in musl handles STB_GNU_UNIQUE not in the way
> the original spec intended. Which is not necessarily a bad thing, but the
> confusion about what that symbol binding type is supposed to convey probably
> is. Given that the commit that introduced handling into dynlink.c said:
>
> commit e152ee9778846c1f233641b2d3562ccdb081c6a9
> Author: Rich Felker <dalias@...ifal.cx>
> Date: Wed Jul 24 11:53:23 2013 -0400
>
> support STB_GNU_UNIQUE symbol bindings in dynamic linker
>
> these are needed for some C++ library binaries including most builds
> of libstdc++. I'm not entirely clear on the rationale. this patch does
> not implement any special semantics for them, but as far as I can
> tell, no special treatment is needed in correctly-linked programs;
> this binding seems to exist only for catching incorrectly-linked
> programs.
>
> ....it seems that either reasons for STB_GNU_UNIQUE were unclear at that time,
> or I'm missing what "correctly-linked programs" was supposed to mean. :)
I roughly agree. I think it's probably a mix -- partly a lack of full
understanding of how the feature works, and partly a very broad
concept of what "incorrectly linked" might mean.
> STB_GNU_UNIQUE is marking a data symbol that should be unique in a running
> program, *even when DSOs defining that symbol are all loaded with RTLD_LOCAL*.
> Apart from behavior under dlopen(..., ... | RTLD_LOCAL), I don't see any way
> it's different from a normal binding.
It should be noted that supporting this precludes a simple version of
the proposed dynamic linker optimization to have a separate
global-namespace linked list -- even non-global libs would have to be
searched if they contain STB_GNU_UNIQUE symbols, at least in some
cases. I don't actually want to think too much about how this would
need to work unless we are actually interested in supporting it.
> The original cause for the new binding type was a desire to support dlopen'ed
> plugins implemented in C++ that reference data expected to be unified in
> normal link (via what C++ calls "vague linkage"). These are emails from when
> the binding was introduced:
I understand that failure to support this violates the C++ semantics,
but RTLD_LOCAL _inherently_ violates C global semantics anyway. The
whole point of RTLD_LOCAL is that you want to, at least to some
extent, ignore the language's concept of a global symbol namespace and
instead isolate your module/plugin/whatever so that it can't step on
others' symbol names.
> https://gcc.gnu.org/ml/gcc-patches/2009-07/msg01240.html
> https://www.redhat.com/archives/posix-c++-wg/2009-August/msg00002.html
>
> At the moment, my personal view is that STB_GNU_UNIQUE made things messier.
> The way it overrides RTLD_LOCAL sometimes makes it harder to reason about
> program behavior, and the way it's opt-out rather than opt-in makes it easier
> to accidentally write code that works on Linux with modern toolchain, but
> fails with old toolchain, or other OSes without a similar binding type. Here
> are some emails from people dissatisfied with the development:
>
> https://sourceware.org/ml/binutils/2011-10/msg00276.html
> https://sourceware.org/ml/libc-alpha/2011-10/msg00066.html
I agree completely. So I'm fairly strongly in favor of leaving musl's
(non-)handling of this feature the way it is.
Rich
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