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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