[PATCH] [ELF] Export strong defined symbol if it coalesces away a weak symbol defined in a shared library

[Simon Atanasyan]

Hi Bigcheese, shankarke, kledzik, ruiu,

Now LLD does not export a strong defined symbol if it coalesces away a weak symbol defined in a shared library. This bug affects all ELF architectures and leads to segfault:

  % cat foo.c
  extern int __attribute__((weak)) flag;
  int foo() { return flag; }

  % cat main.c
  int flag = 1;
  int foo();
  int main() { return foo() == 1 ? 0 : -1; }

  % clang -c -fPIC foo.c main.c
  % lld -flavor gnu -target x86_64 -shared -o libfoo.so ... foo.o
  % lld -flavor gnu -target x86_64 -o a.out ... main.o libfoo.so
  % ./a.out
  Segmentation fault

The problem is caused by the fact that we lose all information about coalesced symbols after the `Resolver::resolve()` method is finished.

The patch solves the problem by overriding the `LinkingContext::notifySymbolTableCoalesce()` method and adding new reference `kindDynExport` to all defined atoms which coalesce weak atoms defined in a shared library.

Notes:
- I use const_cast's where I need to add a new edge to the graph to make the patch smaller. It looks like we need to make the atom's graph mutable but I prefer to do that in a separate patch.
- The `ELFDefinedAtom` class did not maintain its own container for references and used a shared reference container populated when the ELF file read from disk. Now we can add a 'marker' reference to the defined atom in an arbitrary moment. That is why I add a separate container for 'marker' references to the `ELFDefinedAtom` class.

http://reviews.llvm.org/D5164

Files:
  include/lld/Core/Reference.h
  include/lld/ReaderWriter/ELFLinkingContext.h
  lib/ReaderWriter/ELF/Atoms.h
  lib/ReaderWriter/ELF/DefaultTargetHandler.h
  lib/ReaderWriter/ELF/ELFFile.h
  lib/ReaderWriter/ELF/ELFLinkingContext.cpp
  lib/ReaderWriter/Reader.cpp
  test/elf/X86_64/dynsym-weak.test
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