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Re: Assertion failure in elflink.c:2824 (binutils 2.30, powerpc)


On Wed, Jul 10, 2019 at 02:18:36PM -0500, Justin Hibbits wrote:
> Hi,
> 
> Building LLVM 6.0 on FreeBSD/powerpc (devel/llvm60 port) the assertion
> in the subject trips (displays twice) when linking libLTO.so.1.  The
> issue has been filed in FreeBSD's bugzilla, at
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237068 .  It appears
> the 'llvm::hashing::detail::get_execution_seed()::seed@@JL_LLVM_6.0'
> symbol is being weakly aliased to an indirect symbol
> __bss_start@@JL_LLVM_6.0.  Since __bss_start@@JL_LLVM_6.0 is an
> indirect symbol, it fails the assertion.
> 
> My thought, shown in comment #51, and Mark Millard's comment #52, is
> that 'def' may need to itself be resolved to its target as well, by
> duplicating the while loop resolving h's indirection.
> 
> Is this correct thinking?

I haven't looked under a debugger at your testcase but I think I know
what is going on here.  You have a shared library with a weakly
defined llvm::hashing::detail::get_execution_seed()::seed which
happens to be at the same location as __bss_start in that library.  At
the time the linker loads symbols for that library, it sees they are
both versioned and thus introduces non-versioned indirect symbols for
them.  The linker considers the symbols as possibly being aliases,
setting up h->u.alias and h->is_weakalias such that
__bss_start@@JL_LLVM_6.0 is the definition.  No real problem so far,
the definition is bfd_link_hash_defined, except that the zero size, no
type __bss_start symbol possibly should not be considered an alias in
the first place.

Later, __bss_start as defined by the linker script is entered into the
linker symbol table.  This is similar to __bss_start being defined by
a regular object file in that ELF symbol resolution rules say that the
value of __bss_start in the library is overridden by __bss_start in
the executable/library being produced.  So to accomplish the override,
ld flips __bss_start from being an indirect symbol pointing at
__bss_start@@JL_LLVM_6.0 to __bss_start@@JL_LLVM_6.0 being an indirect
symbol pointing at __bss_start.  That's how we get an unexpected
indirect symbol and hit the assert.

What should happen I think, is for the def->def_regular code above the
assert to run in this case.  The symbols are no longer aliases.

Please try the following.  I'll add a comment and commit this in a few
days or on hearing back from you that this doesn't break anything on
FreeBSD.

diff --git a/bfd/elflink.c b/bfd/elflink.c
index 65a5f5da74..9689d132ab 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -2922,7 +2922,8 @@ _bfd_elf_fix_symbol_flags (struct elf_link_hash_entry *h,
       /* If the real definition is defined by a regular object file,
 	 don't do anything special.  See the longer description in
 	 _bfd_elf_adjust_dynamic_symbol, below.  */
-      if (def->def_regular)
+      if (def->def_regular
+	  || def->root.type != bfd_link_hash_defined)
 	{
 	  h = def;
 	  while ((h = h->u.alias) != def)
@@ -2935,7 +2936,6 @@ _bfd_elf_fix_symbol_flags (struct elf_link_hash_entry *h,
 	  BFD_ASSERT (h->root.type == bfd_link_hash_defined
 		      || h->root.type == bfd_link_hash_defweak);
 	  BFD_ASSERT (def->def_dynamic);
-	  BFD_ASSERT (def->root.type == bfd_link_hash_defined);
 	  (*bed->elf_backend_copy_indirect_symbol) (eif->info, def, h);
 	}
     }

-- 
Alan Modra
Australia Development Lab, IBM


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