[ELF] Support -z undefs

[MaskRay]

Summary

-z undefs is the inverse of -z defs. It allows unresolved references
from object files. This can be used to cancel --no-undefined or -z defs.

[MaskRay]

Some findings related to the topic. There are two kinds of unresolved references: 1) from object files 2) from shared objects. --unresolved-symbols= toggles both bits while -z (un)defs and --(no-)allow-shlib-undefined toggles one bit, respectively. I personally favor the fine-grained options over --unresolved-symbols=

It seems we can actually model --unresolved-symbols=ignore-in-object-files in terms of --allow-shlib-undefined but I think that is probably unnecessary. I can't find any use of "ignore-in-object-files" outside lld and binutils.

// ld/emultempl/elf32.em
    case 'z':
      if (strcmp (optarg, "defs") == 0)
	link_info.unresolved_syms_in_objects = RM_GENERATE_ERROR;
      else if (strcmp (optarg, "undefs") == 0)
	link_info.unresolved_syms_in_objects = RM_IGNORE;

// ld/lexsup.c
	case OPTION_NO_UNDEFINED:
	  link_info.unresolved_syms_in_objects
	    = how_to_report_unresolved_symbols;
	  break;
	case OPTION_ALLOW_SHLIB_UNDEFINED:
	  link_info.unresolved_syms_in_shared_libs = RM_IGNORE;
	  break;
	case OPTION_NO_ALLOW_SHLIB_UNDEFINED:
	  link_info.unresolved_syms_in_shared_libs
	    = how_to_report_unresolved_symbols;
	  break;
	case OPTION_UNRESOLVED_SYMBOLS:
	  if (strcmp (optarg, "ignore-all") == 0)
	    {
	      link_info.unresolved_syms_in_objects = RM_IGNORE;
	      link_info.unresolved_syms_in_shared_libs = RM_IGNORE;
	    }
	  else if (strcmp (optarg, "report-all") == 0)
	    {
	      link_info.unresolved_syms_in_objects
		= how_to_report_unresolved_symbols;
	      link_info.unresolved_syms_in_shared_libs
		= how_to_report_unresolved_symbols;
	    }
	  else if (strcmp (optarg, "ignore-in-object-files") == 0)
	    {
	      link_info.unresolved_syms_in_objects = RM_IGNORE;
	      link_info.unresolved_syms_in_shared_libs
		= how_to_report_unresolved_symbols;
	    }
	  else if (strcmp (optarg, "ignore-in-shared-libs") == 0)
	    {
	      link_info.unresolved_syms_in_objects
		= how_to_report_unresolved_symbols;
	      link_info.unresolved_syms_in_shared_libs = RM_IGNORE;
	    }

[MaskRay]

ELF/Driver.cpp

390

Unrelated to this change.

% ld.lld -z max-page-sizez a.o
error: unknown -z value: max-page-sizez
% ld.lld -z common-page-sizez a.o
# No diagnostic "error: unknown -z value: common-page-sizez"

[ruiu]

LGTM

ELF/Driver.cpp

390

Yeah, it is perhaps better to tighten the check a bit to catch that error too.

ELF/Driver.cpp

return s == "combreloc" || s == "copyreloc" || s == "defs" ||
          s == "execstack" || s == "global" || s == "hazardplt" ||
          s == "ifunc-noplt" || s == "initfirst" || s == "interpose" ||
          s == "keep-text-section-prefix" || s == "lazy" || s == "muldefs" ||
          s == "separate-code" || s == "nocombreloc" || s == "nocopyreloc" ||
          s == "nodefaultlib" || s == "nodelete" || s == "nodlopen" ||
          s == "noexecstack" || s == "nokeep-text-section-prefix" ||
          s == "norelro" || s == "noseparate-code" || s == "notext" ||
          s == "now" || s == "origin" || s == "relro" || s == "retpolineplt" ||
-         s == "rodynamic" || s == "text" || s == "wxneeded" ||
+         s == "rodynamic" || s == "text" || s == "undefs" || s == "wxneeded" ||
          s.startswith("common-page-size") || s.startswith("max-page-size=") ||
          s.startswith("stack-size=");
 }
 
 // Report an error for an unknown -z option.
 static void checkZOptions(opt::InputArgList &args) {
   for (auto *arg : args.filtered(OPT_z))
     if (!isKnownZFlag(arg->getValue()))
       error("unknown -z value: " + StringRef(arg->getValue()));
case OPT_unresolved_symbols: {
       error("unknown --unresolved-symbols value: " + s);
       continue;
     }
     case OPT_no_undefined:
       return errorOrWarn;
     case OPT_z:
       if (StringRef(arg->getValue()) == "defs")
         return errorOrWarn;
+      if (StringRef(arg->getValue()) == "undefs")
+        return UnresolvedPolicy::Ignore;
       continue;
     }
   }
 
   // -shared implies -unresolved-symbols=ignore-all because missing
   // symbols are likely to be resolved at runtime using other DSOs.
   if (config->shared)
     return UnresolvedPolicy::Ignore;

test/ELF/zdefs.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+
+## Unresolved references from object files are allowed by default for -shared.
 # RUN: ld.lld -shared %t.o -o %t1.so
 
+## -z defs disallows unresolved references.
 # RUN: not ld.lld -z defs -shared %t.o -o %t1.so 2>&1 | FileCheck -check-prefix=ERR %s
 # ERR: error: undefined symbol: foo
 # ERR: >>> referenced by {{.*}}:(.text+0x1)
 
+## -z undefs allows unresolved references.
+# RUN: ld.lld -z defs -z undefs -shared %t.o -o /dev/null 2>&1 | count 0
+
 callq foo@PLT