Fix treatment of symbol versions with unused as-needed libraries.

When we have a weak reference to a symbol defined in an
as-needed library, and that library ends up not-needed, gold
simply clears the version information in the symbol table, even
if the symbol could have been resolved by a needed library later
in the link order. This results in a loss of version information,
which can cause the program to bind to the wrong version at run
time.

This patch lets a dynamic definition override an earlier one if
the earlier one is from a not-needed library, so that we can
retain the version information from the binding to the needed
library. In order to do that, the tracking of needed/not-needed
had to be moved up to symbol resolution time, instead of during
Symbol_table::set_dynsym_indexes().

In cases where we still end up discarding version information,
I've added a warning.

For the original problem report and discussion, see:

https://stackoverflow.com/questions/50751421/undefined-behavior-in-shared-lib-using-libpthread-but-not-having-it-in-elf-as-d

gold/
	* resolve.cc (Symbol_table::resolve): Rename tobinding to
	orig_tobinding.  Call set_is_needed() for objects that resolve
	non-weak references.
	(Symbol_table::should_override): Allow a dynamic definition to
	override an earlier one in a not-needed library.
	* symtab.cc (Symbol_table::set_dynsym_indexes): Remove separate
	processing for as-needed symbols.  Add warning when discarding
	version informatin.
	* testsuite/Makefile.am (weak_as_needed): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/weak_as_needed.sh: New test script.
	* testsuite/weak_as_needed_a.c: New source file.
	* testsuite/weak_as_needed_b.c: New source file.
	* testsuite/weak_as_needed_b.script: New version script.
	* testsuite/weak_as_needed_c.c: New source file.
	* testsuite/weak_as_needed_c.script: New version script.

1 files changed, 8 insertions, 25 deletions

diff --git a/gold/symtab.cc b/gold/symtab.cc
index 238834d..c43d127 100644
--- a/gold/symtab.cc
+++ b/gold/symtab.cc
@@ -2543,8 +2543,6 @@ Symbol_table::set_dynsym_indexes(unsigned int index,
 				 Stringpool* dynpool,
 				 Versions* versions)
 {
-  std::vector<Symbol*> as_needed_sym;
-
   // First process all the symbols which have been forced to be local,
   // as they must appear before all global symbols.
   unsigned int forced_local_count = 0;
@@ -2611,15 +2609,6 @@ Symbol_table::set_dynsym_indexes(unsigned int index,
 	  syms->push_back(sym);
 	  dynpool->add(sym->name(), false, NULL);
 
-	  // If the symbol is defined in a dynamic object and is
-	  // referenced strongly in a regular object, then mark the
-	  // dynamic object as needed.  This is used to implement
-	  // --as-needed.
-	  if (sym->is_from_dynobj()
-	      && sym->in_reg()
-	      && !sym->is_undef_binding_weak())
-	    sym->object()->set_is_needed();
-
 	  // Record any version information, except those from
 	  // as-needed libraries not seen to be needed.  Note that the
 	  // is_needed state for such libraries can change in this loop.
@@ -2630,24 +2619,18 @@ Symbol_table::set_dynsym_indexes(unsigned int index,
 		  || sym->object()->is_needed())
 		versions->record_version(this, dynpool, sym);
 	      else
-		as_needed_sym.push_back(sym);
+		{
+		  gold_warning(_("discarding version information for "
+				 "%s@%s, defined in unused shared library %s "
+				 "(linked with --as-needed)"),
+			       sym->name(), sym->version(),
+			       sym->object()->name().c_str());
+		  sym->clear_version();
+		}
 	    }
 	}
     }
 
-  // Process version information for symbols from as-needed libraries.
-  for (std::vector<Symbol*>::iterator p = as_needed_sym.begin();
-       p != as_needed_sym.end();
-       ++p)
-    {
-      Symbol* sym = *p;
-
-      if (sym->object()->is_needed())
-	versions->record_version(this, dynpool, sym);
-      else
-	sym->clear_version();
-    }
-
   // Finish up the versions.  In some cases this may add new dynamic
   // symbols.
   index = versions->finalize(this, index, syms);