[ELF] Refine --export-dynamic-symbol semantics to be compatible GNU ld 2.35

[MaskRay]

Summary

GNU ld from binutils 2.35 onwards will likely support
--export-dynamic-symbol but with different semantics.
https://sourceware.org/pipermail/binutils/2020-May/111302.html

Differences:

1. -export-dynamic-symbol is not supported
2. --export-dynamic-symbol takes a glob argument
3. --export-dynamic-symbol can suppress binding the references to the definition within the shared object if (-Bsymbolic or -Bsymbolic-functions)
4. --export-dynamic-symbol does not imply -u

I don't think the first three points can affect any user.
For the fourth point, Not implying -u can lead to some archive members unfetched.
Add -u foo to restore the previous behavior.

Exact semantics:

• -no-pie or -pie: matched non-local defined symbols will be added to the dynamic symbol table.
• -shared: matched non-local STV_DEFAULT symbols will not be bound to definitions within the shared object even if they would otherwise be due to -Bsymbolic, -Bsymbolic-functions, or --dynamic-list.

[MaskRay]

Improve tests

[psmith]

Looks reasonable to me. Our documentation and help for dynamic-list is a bit sparse, it may be worth emphasising that it implies -bsymbolic (if I've understood correctly).

lld/ELF/Options.td

148

May be worth adding that this implies -bsymbolic apart from symbols in dynamic-list.

lld/docs/ld.lld.1

159

May be worth adding that this implies -bsymbolic apart from symbols in dynamic-list.

[MaskRay]

Improve documentation

[psmith]

Thanks for the update. I'm happy with the changes.

[grimar]

(I have no comments for this. Happy if other people are happy).

[MaskRay]

Add MetaVarName<"<file>"> to dynamic_list

lld/ELF/Config.h

struct Configuration {
   bool exportDynamic;
   bool fixCortexA53Errata843419;
   bool fixCortexA8;
   bool formatBinary = false;
   bool gcSections;
   bool gdbIndex;
   bool gnuHash = false;
   bool gnuUnique;
-  bool hasDynamicList = false;
   bool hasDynSymTab;
   bool ignoreDataAddressEquality;
   bool ignoreFunctionAddressEquality;
   bool ltoCSProfileGenerate;
   bool ltoDebugPassManager;
   bool ltoEmitAsm;
   bool ltoNewPassManager;
   bool ltoUniqueBBSectionNames;
struct Configuration {
   bool printGcSections;
   bool printIcfSections;
   bool relocatable;
   bool relrPackDynRelocs;
   bool saveTemps;
   llvm::Optional<uint32_t> shuffleSectionSeed;
   bool singleRoRx;
   bool shared;
+  bool symbolic;
   bool isStatic = false;
   bool sysvHash = false;
   bool target1Rel;
   bool trace;
   bool thinLTOEmitImportsFiles;
   bool thinLTOIndexOnly;
   bool timeTraceEnabled;
   bool tocOptimize;

lld/ELF/Driver.cpp

static void readConfigs(opt::InputArgList &args) {
   for (opt::Arg *arg : args.filtered(OPT_warn_backrefs_exclude)) {
     StringRef pattern(arg->getValue());
     if (Expected<GlobPattern> pat = GlobPattern::create(pattern))
       config->warnBackrefsExclude.push_back(std::move(*pat));
     else
       error(arg->getSpelling() + ": " + toString(pat.takeError()));
   }
 
-  // Parses -dynamic-list and -export-dynamic-symbol. They make some
-  // symbols private. Note that -export-dynamic takes precedence over them
-  // as it says all symbols should be exported.
-  if (!config->exportDynamic) {
-    for (auto *arg : args.filtered(OPT_dynamic_list))
-      if (Optional<MemoryBufferRef> buffer = readFile(arg->getValue()))
-        readDynamicList(*buffer);
+  // When producing an executable, --dynamic-list specifies non-local defined
+  // symbols whith are required to be exported. When producing a shared object,
+  // symbols not specified by --dynamic-list are non-preemptible.
+  config->symbolic =
+      args.hasArg(OPT_Bsymbolic) || args.hasArg(OPT_dynamic_list);
 
-    for (auto *arg : args.filtered(OPT_export_dynamic_symbol))
-      config->dynamicList.push_back(
-          {arg->getValue(), /*isExternCpp=*/false, /*hasWildcard=*/false});
-  }
-
-  // If --export-dynamic-symbol=foo is given and symbol foo is defined in
-  // an object file in an archive file, that object file should be pulled
-  // out and linked. (It doesn't have to behave like that from technical
-  // point of view, but this is needed for compatibility with GNU.)
-  for (auto *arg : args.filtered(OPT_export_dynamic_symbol))
-    config->undefined.push_back(arg->getValue());
+  // --export-dynamic-symbol specifies additional --dynamic-list symbols if any
+  // other option expresses a symbolic intention: -no-pie, -pie, -Bsymbolic,
+  // -Bsymbolic-functions (if STT_FUNC), --dynamic-list.
+        {arg->getValue(), /*isExternCpp=*/false, /*hasWildcard=*/true});
 
   for (auto *arg : args.filtered(OPT_version_script))
     if (Optional<std::string> path = searchScript(arg->getValue())) {
       if (Optional<MemoryBufferRef> buffer = readFile(*path))
         readVersionScript(*buffer);
     } else {
       error(Twine("cannot find version script ") + arg->getValue());
     }

lld/ELF/Options.td

 def discard_locals: F<"discard-locals">,
   HelpText<"Delete temporary local symbols">;
 
 def discard_none: F<"discard-none">,
   HelpText<"Keep all symbols in the symbol table">;
 
 defm dynamic_linker: Eq<"dynamic-linker", "Which dynamic linker to use">;
 
 defm dynamic_list: Eq<"dynamic-list", "Read a list of dynamic symbols">;
 
 defm eh_frame_hdr: B<"eh-frame-hdr",
     "Request creation of .eh_frame_hdr section and PT_GNU_EH_FRAME segment header",
     "Do not create .eh_frame_hdr section">;
 
 def emit_relocs: F<"emit-relocs">, HelpText<"Generate relocations in output">;
 
 def enable_new_dtags: F<"enable-new-dtags">,

 defm execute_only: BB<"execute-only",
     "Mark executable sections unreadable",
     "Mark executable sections readable (default)">;
 
 defm export_dynamic: B<"export-dynamic",
     "Put symbols in the dynamic symbol table",
     "Do not put symbols in the dynamic symbol table (default)">;
 
-defm export_dynamic_symbol:
-  Eq<"export-dynamic-symbol", "Put a symbol in the dynamic symbol table">;
+defm export_dynamic_symbol : EEq<"export-dynamic-symbol",
+    "Put matched symbols in the dynamic symbol table (executable). Don't bind "
+    "matched defined symbols locally (shared)">,
+    MetaVarName<"glob">;
 
 defm fatal_warnings: B<"fatal-warnings",
     "Treat warnings as errors",
     "Do not treat warnings as errors (default)">;
 
 defm filter: Eq<"filter", "Set DT_FILTER field to the specified name">;
 
 defm fini: Eq<"fini", "Specify a finalizer function">, MetaVarName<"<symbol>">;

lld/ELF/ScriptParser.cpp

 
 static ExprValue bitOr(ExprValue a, ExprValue b) {
   moveAbsRight(a, b);
   return {a.sec, a.forceAbsolute,
           (a.getValue() | b.getValue()) - a.getSecAddr(), a.loc};
 }
 
 void ScriptParser::readDynamicList() {
-  config->hasDynamicList = true;
   expect("{");
   std::vector<SymbolVersion> locals;
   std::vector<SymbolVersion> globals;
   std::tie(locals, globals) = readSymbols();
   expect(";");
 
   if (!atEOF()) {
     setError("EOF expected, but got " + next());

lld/ELF/Symbols.cpp

bool elf::computeIsPreemptible(const Symbol &sym) {
   // At this point copy relocations have not been created yet, so any
   // symbol that is not defined locally is preemptible.
   if (!sym.isDefined())
     return true;
 
   if (!config->shared)
     return false;
 
-  // If the dynamic list is present, it specifies preemptable symbols in a DSO.
-  if (config->hasDynamicList)
+  // If -Bsymbolic or --dynamic-list is specified, or -Bsymbolic-functions is
+  // specified and the symbol is STT_FUNC, the symbol is preemptible iff it is
+  // in the dynamic list.
+  if (config->symbolic || (config->bsymbolicFunctions && sym.isFunc()))
     return sym.inDynamicList;
-
-  // -Bsymbolic means that definitions are not preempted.
-  if (config->bsymbolic || (config->bsymbolicFunctions && sym.isFunc()))
-    return false;
   return true;
 }
 
 void elf::reportBackrefs() {
   for (auto &it : backwardReferences) {
     const Symbol &sym = *it.first;
     warn("backward reference detected: " + sym.getName() + " in " +
          toString(it.second) + " refers to " + toString(sym.file));

lld/docs/ReleaseNotes.rst

* New ``--time-trace`` option records a time trace file that can be viewed in
   (`D71060 <https://reviews.llvm.org/D71060>`_)
 * ...
 
 Breaking changes
 ----------------
 
 * One-dash form of some long option (``--thinlto-*``, ``--lto-*``, ``--shuffle-sections=``)
   are no longer supported.
+* ``--export-dynamic-symbol`` no longer implies ``-u``.
 
 COFF Improvements
 -----------------
 
 * ...
 
 MinGW Improvements
 ------------------

lld/docs/ld.lld.1

 .It Fl -discard-none
 Keep all symbols in the symbol table.
 .It Fl -dynamic-linker Ns = Ns Ar value
 Specify the dynamic linker to be used for a dynamically linked executable.
 This is recorded in an ELF segment of type
 .Dv PT_INTERP .
 .It Fl -dynamic-list Ns = Ns Ar file
 Read a list of dynamic symbols from
 .Ar file .
 .It Fl -eh-frame-hdr
 Request creation of
 .Li .eh_frame_hdr
 section and
 .Dv PT_GNU_EH_FRAME
 segment header.
 .It Fl -emit-relocs , Fl q
 Generate relocations in the output.

 Report unresolved symbols as errors.
 .It Fl -execute-only
 Mark executable sections unreadable.
 This option is currently only supported on AArch64.
 .It Fl -exclude-libs Ns = Ns Ar value
 Exclude static libraries from automatic export.
 .It Fl -export-dynamic , Fl E
 Put symbols in the dynamic symbol table.
-.It Fl -export-dynamic-symbol Ns = Ns Ar symbol
-Include
-.Ar symbol
-in the dynamic symbol table.
+.It Fl -export-dynamic-symbol Ns = Ns Ar glob
+When creating an executable, symbols matching
+.Ar glob
+will be added to the dynamic symbol table. When creating a shared object,
+matched symbols will not be bound locally even if they would otherwise be due to
+.Cm -Bsymbolic
+,
+.Cm -Bsymbolic-functions
+or
+.Cm --dynamic-list
 .It Fl -fatal-warnings
 Treat warnings as errors.
 .It Fl -filter Ns = Ns Ar value , Fl F Ar value
 Set the
 .Dv DT_FILTER
 field to the specified value.
 .It Fl -fini Ns = Ns Ar symbol
 Specify a finalizer function.

lld/test/ELF/export-dynamic-symbol.s

 # REQUIRES: x86
 
-# RUN: rm -f %t.a
-# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/archive2.s -o %t1.o
-# RUN: llvm-ar rcs %t.a %t1.o
-# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t2.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
 
-# RUN: ld.lld -shared -o %t.so --export-dynamic-symbol foo %t.a %t2.o
-# RUN: llvm-readelf -dyn-symbols %t.so | FileCheck %s
+## For an executable, --export-dynamic-symbol exports a symbol if it is non-local and defined.
+# RUN: ld.lld -pie --export-dynamic-symbol foo %t.o -o %t
+# RUN: llvm-nm -D -p %t | FileCheck %s
 
-# RUN: ld.lld -shared -o %t.so --export-dynamic --export-dynamic-symbol foo %t.a %t2.o
-# RUN: llvm-readelf -dyn-symbols %t.so | FileCheck %s
+## --export-dynamic exports all non-local defined symbols.
+## --export-dynamic-symbol is shadowed.
+# RUN: ld.lld -pie --export-dynamic --export-dynamic-symbol foo %t.o -o %t.start
+# RUN: llvm-nm -D -p %t.start | FileCheck --check-prefixes=CHECK,START %s
 
-# CHECK: foo
+# CHECK-NOT:  .
+# START:      T _start
+# CHECK:      T foo
+# CHECK-NOT:  .
 
-.global _start
+## --export-dynamic-symbol does not imply -u: %t1.a(%t1.o) is not fetched.
+## This is compatible with GNU ld since binutils 2.35 onwards.
+# RUN: echo '.globl foo, bar; foo: bar:' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
+# RUN: rm -f %t1.a && llvm-ar rc %t1.a %t1.o
+# RUN: ld.lld --export-dynamic-symbol bar %t1.a %t.o -o %t.nofetch
+# RUN: llvm-nm %t.nofetch | FileCheck /dev/null --implicit-check-not=bar
+
+## --export-dynamic-symbol can make a symbol preemptible even if it would be otherwise
+## non-preemptible (due to -Bsymbolic, -Bsymbolic-functions or --dynamic-list).
+# RUN: ld.lld -shared -Bsymbolic --export-dynamic-symbol nomatch %t.o -o %t.nopreempt
+# RUN: llvm-objdump -d %t.nopreempt | FileCheck --check-prefix=NOPLT %s
+# RUN: ld.lld -shared -Bsymbolic --export-dynamic-symbol foo %t.o -o %t.preempt
+# RUN: llvm-objdump -d %t.preempt | FileCheck --check-prefix=PLT %s
+
+# RUN: ld.lld -shared -Bsymbolic-functions --export-dynamic-symbol nomatch %t.o -o %t.nopreempt2
+# RUN: llvm-objdump -d %t.nopreempt2 | FileCheck --check-prefix=NOPLT %s
+# RUN: ld.lld -shared -Bsymbolic-functions --export-dynamic-symbol foo %t.o -o %t.preempt2
+# RUN: llvm-objdump -d %t.preempt2 | FileCheck --check-prefix=PLT %s
+
+# RUN: echo '{};' > %t.list
+# RUN: ld.lld -shared --dynamic-list %t.list --export-dynamic-symbol foo %t.o -o %t.preempt3
+# RUN: llvm-objdump -d %t.preempt3 | FileCheck --check-prefix=PLT %s
+
+# PLT:       <foo@plt>
+# NOPLT-NOT: <foo@plt>
+
+.global _start, foo
+.type foo, @function
 _start:
-  nop
+  call foo
+foo:

lld/test/ELF/warn-backrefs.s

 
 ## This is a limitation. The resolution rules are different but
 ## --warn-backrefs does not warn.
 # RUN: ld.lld --fatal-warnings --warn-backrefs %t2.a %t1.o %t2.so %t2.a -o /dev/null
 
 ## In GNU linkers, -u does not make a backward reference.
 # RUN: ld.lld --fatal-warnings --warn-backrefs -u foo %t2.a %t1.o -o /dev/null
 
-## In GNU gold, --export-dynamic-symbol does not make a backward reference.
-# RUN: ld.lld --fatal-warnings --warn-backrefs --export-dynamic-symbol foo %t2.a %t1.o -o /dev/null
+## -u does not make a backward reference.
+# RUN: ld.lld --fatal-warnings --warn-backrefs -u foo %t2.a %t1.o -o /dev/null
 
 # RUN: not ld.lld --warn-backrefs-exclude='[' 2>&1 | FileCheck --check-prefix=INVALID %s
 # INVALID: error: --warn-backrefs-exclude: invalid glob pattern: [
 
 .globl _start, foo
 _start:
   call foo