[lld-macho] -flat_namespace for dylibs should make all externs interposable

All references to interposable symbols can be redirected at runtime to
point to a different symbol definition (with the same name). For
example, if both dylib A and B define symbol _foo, and we load A before
B at runtime, then all references to _foo within dylib B will point to
the definition in dylib A.

ld64 makes all extern symbols interposable when linking with
`-flat_namespace`.

TODO 1: Support `-interposable` and `-interposable_list`, which should
just be a matter of parsing those CLI flags and setting the
`Defined::interposable` bit.

TODO 2: Set Reloc::FinalDefinitionInLinkageUnit correctly with this info
(we are currently not setting it at all, so we're erring on the
conservative side, but we should help the LTO backend generate more
optimal code.)

Reviewed By: modimo, MaskRay

Differential Revision: https://reviews.llvm.org/D119294

lld/MachO/SymbolTable.cpp

@@ -96,10 +96,15 @@ Defined *SymbolTable::addDefined(StringRef name, InputFile *file,
     // of a name conflict, we fall through to the replaceSymbol() call below.
   }
 
+  // With -flat_namespace, all extern symbols in dylibs are interposable.
+  // FIXME: Add support for `-interposable` (PR53680).
+  bool interposable = config->namespaceKind == NamespaceKind::flat &&
+                      config->outputType != MachO::MH_EXECUTE &&
+                      !isPrivateExtern;
   Defined *defined = replaceSymbol<Defined>(
       s, name, file, isec, value, size, isWeakDef, /*isExternal=*/true,
       isPrivateExtern, isThumb, isReferencedDynamically, noDeadStrip,
-      overridesWeakDef, isWeakDefCanBeHidden);
+      overridesWeakDef, isWeakDefCanBeHidden, interposable);
   return defined;
 }
 

lld/MachO/Symbols.cpp

@@ -44,12 +44,14 @@ Defined::Defined(StringRefZ name, InputFile *file, InputSection *isec,
                  uint64_t value, uint64_t size, bool isWeakDef, bool isExternal,
                  bool isPrivateExtern, bool isThumb,
                  bool isReferencedDynamically, bool noDeadStrip,
-                 bool canOverrideWeakDef, bool isWeakDefCanBeHidden)
+                 bool canOverrideWeakDef, bool isWeakDefCanBeHidden,
+                 bool interposable)
     : Symbol(DefinedKind, name, file), overridesWeakDef(canOverrideWeakDef),
       privateExtern(isPrivateExtern), includeInSymtab(true), thumb(isThumb),
       referencedDynamically(isReferencedDynamically), noDeadStrip(noDeadStrip),
-      weakDefCanBeHidden(isWeakDefCanBeHidden), weakDef(isWeakDef),
-      external(isExternal), isec(isec), value(value), size(size) {
+      interposable(interposable), weakDefCanBeHidden(isWeakDefCanBeHidden),
+      weakDef(isWeakDef), external(isExternal), isec(isec), value(value),
+      size(size) {
   if (isec) {
     isec->symbols.push_back(this);
     // Maintain sorted order.

lld/MachO/Symbols.h

@@ -118,7 +118,8 @@ public:
   Defined(StringRefZ name, InputFile *file, InputSection *isec, uint64_t value,
           uint64_t size, bool isWeakDef, bool isExternal, bool isPrivateExtern,
           bool isThumb, bool isReferencedDynamically, bool noDeadStrip,
-          bool canOverrideWeakDef = false, bool isWeakDefCanBeHidden = false);
+          bool canOverrideWeakDef = false, bool isWeakDefCanBeHidden = false,
+          bool interposable = false);
 
   bool isWeakDef() const override { return weakDef; }
   bool isExternalWeakDef() const {
@@ -158,6 +159,14 @@ public:
   // metadata. This is information only for the static linker and not written
   // to the output.
   bool noDeadStrip : 1;
+  // Whether references to this symbol can be interposed at runtime to point to
+  // a different symbol definition (with the same name). For example, if both
+  // dylib A and B define an interposable symbol _foo, and we load A before B at
+  // runtime, then all references to _foo within dylib B will point to the
+  // definition in dylib A.
+  //
+  // Only extern symbols may be interposable.
+  bool interposable : 1;
 
   bool weakDefCanBeHidden : 1;
 

lld/MachO/SyntheticSections.cpp

@@ -235,6 +235,8 @@ void macho::addNonLazyBindingEntries(const Symbol *sym,
     in.rebase->addEntry(isec, offset);
     if (defined->isExternalWeakDef())
       in.weakBinding->addEntry(sym, isec, offset, addend);
+    else if (defined->interposable)
+      in.binding->addEntry(sym, isec, offset, addend);
   } else {
     // Undefined symbols are filtered out in scanRelocations(); we should never
     // get here
@@ -420,6 +422,7 @@ static int16_t ordinalForDylibSymbol(const DylibSymbol &dysym) {
 static int16_t ordinalForSymbol(const Symbol &sym) {
   if (const auto *dysym = dyn_cast<DylibSymbol>(&sym))
     return ordinalForDylibSymbol(*dysym);
+  assert(cast<Defined>(&sym)->interposable);
   return BIND_SPECIAL_DYLIB_FLAT_LOOKUP;
 }
 

lld/MachO/Writer.cpp

@@ -594,6 +594,9 @@ static void prepareBranchTarget(Symbol *sym) {
         in.weakBinding->addEntry(sym, in.lazyPointers->isec,
                                  sym->stubsIndex * target->wordSize);
       }
+    } else if (defined->interposable) {
+      if (in.stubs->addEntry(sym))
+        in.lazyBinding->addEntry(sym);
     }
   } else {
     llvm_unreachable("invalid branch target symbol type");
@@ -605,7 +608,7 @@ static bool needsBinding(const Symbol *sym) {
   if (isa<DylibSymbol>(sym))
     return true;
   if (const auto *defined = dyn_cast<Defined>(sym))
-    return defined->isExternalWeakDef();
+    return defined->isExternalWeakDef() || defined->interposable;
   return false;
 }
 

lld/test/MachO/flat-namespace-interposable.s

@@ -0,0 +1,80 @@
+# REQUIRES: x86
+# RUN: rm -rf %t; split-file %s %t
+
+## With -flat_namespace, non-weak extern symbols in dylibs become interposable.
+## Check that we generate the correct bindings for them. The test also includes
+## other symbol types like weak externs to verify we continue to do the same
+## (correct) thing even when `-flat_namespace` is enabled, instead of generating
+## spurious bindings.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/foo.o %t/foo.s
+# RUN: %lld -lSystem -flat_namespace -o %t/foo %t/foo.o
+# RUN: %lld -lSystem -dylib -flat_namespace -o %t/foo.dylib %t/foo.o
+# RUN: %lld -lSystem -bundle -flat_namespace -o %t/foo.bundle %t/foo.o
+# RUN: llvm-objdump --macho --bind --lazy-bind --weak-bind %t/foo | FileCheck \
+# RUN:   %s --check-prefix=EXEC --implicit-check-not=_private_extern
+# RUN: llvm-objdump --macho --bind --lazy-bind --weak-bind %t/foo.dylib | \
+# RUN:   FileCheck %s --check-prefix=DYLIB --implicit-check-not=_private_extern
+# RUN: llvm-objdump --macho --bind --lazy-bind --weak-bind %t/foo.bundle | \
+# RUN:   FileCheck %s --check-prefix=DYLIB --implicit-check-not=_private_extern
+
+## Executables with -flat_namespace don't have interposable externs.
+# EXEC:       Bind table:
+# EXEC-NEXT:  segment  section          address  type     addend dylib   symbol
+# EXEC-EMPTY:
+# EXEC-NEXT:  Lazy bind table:
+# EXEC-NEXT:  segment  section          address  dylib    symbol
+# EXEC-EMPTY:
+# EXEC-NEXT:  Weak bind table:
+# EXEC-NEXT:  segment  section          address  type     addend   symbol
+# EXEC-NEXT:  __DATA   __la_symbol_ptr  {{.*}}   pointer       0   _weak_extern
+# EXEC-NEXT:  __DATA   __data           {{.*}}   pointer       0   _weak_extern
+# EXEC-EMPTY:
+
+# DYLIB:       Bind table:
+# DYLIB-NEXT:  segment      section        address  type     addend dylib            symbol
+# DYLIB-DAG:   __DATA       __data         {{.*}}   pointer       0 flat-namespace   _extern
+# DYLIB-DAG:   __DATA       __thread_ptrs  {{.*}}   pointer       0 flat-namespace   _tlv
+# DYLIB-DAG:   __DATA_CONST __got          {{.*}}   pointer       0 flat-namespace   dyld_stub_binder
+# DYLIB-EMPTY:
+# DYLIB-NEXT:  Lazy bind table:
+# DYLIB-NEXT:  segment  section            address  dylib            symbol
+# DYLIB-NEXT:  __DATA   __la_symbol_ptr    {{.*}}   flat-namespace   _extern
+# DYLIB-EMPTY:
+# DYLIB-NEXT:  Weak bind table:
+# DYLIB-NEXT:  segment  section            address  type    addend   symbol
+# DYLIB-NEXT:  __DATA   __la_symbol_ptr    {{.*}}   pointer      0   _weak_extern
+# DYLIB-NEXT:  __DATA   __data             {{.*}}   pointer      0   _weak_extern
+
+#--- foo.s
+
+.globl _main, _extern, _weak_extern, _tlv
+.weak_definition _weak_extern
+.private_extern _private_extern
+
+_extern:
+  retq
+_weak_extern:
+  retq
+_private_extern:
+  retq
+_local:
+  retq
+
+_main:
+  callq _extern
+  callq _weak_extern
+  callq _private_extern
+  callq _local
+  mov _tlv@TLVP(%rip), %rax
+  retq
+
+.data
+.quad _extern
+.quad _weak_extern
+.quad _local
+
+.section __DATA,__thread_vars,thread_local_variables
+_tlv:
+
+.subsections_via_symbols