Warn when a script redefines a symbol

Note that we don't even warn if scripts adjust a symbol as in
ld-elf/var1 and ld-scripts/pr14962.

include/
	* bfdlink.h (struct bfd_link_info): Add warn_multiple_definition.
ld/
	* ldexp.c (exp_fold_tree_1): Warn on script defining a symbol
	defined in an object file.
	* ldmain.c (multiple_definition): Heed info->warn_multiple_definition.
	* testsuite/ld-scripts/defined5.d: Expect a warning.

include/ChangeLog

@@ -1,3 +1,7 @@
+2021-02-21  Alan Modra  <amodra@gmail.com>
+
+	* bfdlink.h (struct bfd_link_info): Add warn_multiple_definition.
+
 2021-02-17  Nick Alcock  <nick.alcock@oracle.com>
 
 	* ctf-api.h (ctf_arc_lookup_symbol_name): New.

include/bfdlink.h

@@ -465,12 +465,16 @@ struct bfd_link_info
      statics.  */
   unsigned int task_link: 1;
 
-  /* TRUE if ok to have multiple definition.  */
+  /* TRUE if ok to have multiple definitions, without warning.  */
   unsigned int allow_multiple_definition: 1;
 
-  /* TRUE if ok to have prohibit multiple definition of absolute symbols.  */
+  /* TRUE if multiple definition of absolute symbols (eg. from -R) should
+     be reported.  */
   unsigned int prohibit_multiple_definition_absolute: 1;
 
+  /* TRUE if multiple definitions should only warn.  */
+  unsigned int warn_multiple_definition: 1;
+
   /* TRUE if ok to have version with no definition.  */
   unsigned int allow_undefined_version: 1;
 

ld/ChangeLog

@@ -1,3 +1,10 @@
+2021-02-21  Alan Modra  <amodra@gmail.com>
+
+	* ldexp.c (exp_fold_tree_1): Warn on script defining a symbol
+	defined in an object file.
+	* ldmain.c (multiple_definition): Heed info->warn_multiple_definition.
+	* testsuite/ld-scripts/defined5.d: Expect a warning.
+
 2021-02-19  Alan Modra  <amodra@gmail.com>
 
 	* testsuite/lib/ld-lib.exp: Whitespace.

ld/ldexp.c

@@ -1186,16 +1186,19 @@ exp_fold_tree_1 (etree_type *tree)
 		{
 		  if (expld.result.section == NULL)
 		    expld.result.section = expld.section;
-		  if (!update_definedness (tree->assign.dst, h) && 0)
+		  if (!update_definedness (tree->assign.dst, h)
+		      && expld.assign_name != NULL)
 		    {
-		      /* Symbol was already defined.  For now this error
-			 is disabled because it causes failures in the ld
-			 testsuite: ld-elf/var1, ld-scripts/defined5, and
-			 ld-scripts/pr14962.  Some of these no doubt
-			 reflect scripts used in the wild.  */
+		      /* Symbol was already defined, and the script isn't
+			 modifying the symbol value for some reason as in
+			 ld-elf/var1 and ld-scripts/pr14962.
+			 For now this is only a warning.  */
+		      unsigned int warn = link_info.warn_multiple_definition;
+		      link_info.warn_multiple_definition = 1;
 		      (*link_info.callbacks->multiple_definition)
 			(&link_info, h, link_info.output_bfd,
 			 expld.result.section, expld.result.value);
+		      link_info.warn_multiple_definition = warn;
 		    }
 		  if (expld.phase == lang_fixed_phase_enum)
 		    {

ld/ldmain.c

@@ -1074,7 +1074,9 @@ multiple_definition (struct bfd_link_info *info,
       nval = oval;
       obfd = NULL;
     }
-  einfo (_("%X%P: %C: multiple definition of `%pT'"),
+  if (!info->warn_multiple_definition)
+    einfo ("%X");
+  einfo (_("%P: %C: multiple definition of `%pT'"),
 	 nbfd, nsec, nval, name);
   if (obfd != NULL)
     einfo (_("; %D: first defined here"), obfd, osec, oval);

ld/testsuite/ld-scripts/defined5.d

@@ -1,10 +1,11 @@
 #ld: -Tdefined5.t
+#warning: .*multiple definition of `defined'.*
 #nm: -B
-#source: defined5.s
 #xfail: [is_xcoff_format]
 # xcoff outputs value of "defined" from the object file
 
-# Check that arithmetic on DEFINED works.
+# Check that a script can override an object file symbol, if multiple
+# definitions are allowed.  See pr12356.
 #...
 0+1000 D defined
 #pass