[asan] Ensure __asan_register_elf_globals is called in COMDAT asan.module_ctor (#67745)

On ELF platforms, when there is no global variable and the unique module ID is
non-empty, COMDAT asan.module_ctor is created with no
`__asan_register_elf_globals` calls. If this COMDAT is the prevailing copy
selected by the linker, the linkage unit will have no
`__asan_register_elf_globals` call: the redzone will not be poisoned and ODR
violation checker will not work (#67677).

This behavior is benign for -fno-sanitize-address-globals-dead-stripping because
asan.module_ctor functions that call `__asan_register_globals`
(`InstrumentGlobalsWithMetadataArray`) do not use COMDAT.

To fix #67677:

* Use COMDAT for -fsanitize-address-globals-dead-stripping on ELF platforms.
* Call `__asan_register_elf_globals` even if there is no global variable.
* If the unique module ID is empty, don't call SetComdatForGlobalMetadata:
  placing `@.str` in a COMDAT would incorrectly discard internal COMDAT `@.str`
  in other compile units.

Alternatively, when there is no global variable, asan.module_ctor is not COMDAT
and does not call `__asan_register_elf_globals`. However, the asan.module_ctor
function cannot be eliminated by the linker.

Tested the following script. Only ELF -fsanitize-address-globals-dead-stripping has changed behaviors.
```
echo > a.cc  # no global variable, empty uniqueModuleId
echo 'void f() {}' > b.cc  # with global variable, with uniqueModuleId
echo 'int g;' > c.cc  # with global variable
for t in x86_64-linux-gnu arm64-apple-macosx x86_64-windows-msvc; do
  for gc in -f{,no-}sanitize-address-globals-dead-stripping; do
    for f in a.cc b.cc c.cc; do
      echo /tmp/Rel/bin/clang -S --target=$t -fsanitize=address $gc $f -o -
      /tmp/Rel/bin/clang -S --target=$t -fsanitize=address $gc $f -o - | sed -n '/asan.module_ctor/,/ret/p'
    done
  done
done
```

---

Identical to commit 16eed8c.
6420d33 is an incorrect revert for genuine
purely internal issues.

Author: MaskRay

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -819,11 +819,11 @@ class ModuleAddressSanitizer {
 private:
   void initializeCallbacks(Module &M);
 
-  bool InstrumentGlobals(IRBuilder<> &IRB, Module &M, bool *CtorComdat);
+  void instrumentGlobals(IRBuilder<> &IRB, Module &M, bool *CtorComdat);
   void InstrumentGlobalsCOFF(IRBuilder<> &IRB, Module &M,
                              ArrayRef<GlobalVariable *> ExtendedGlobals,
                              ArrayRef<Constant *> MetadataInitializers);
-  void InstrumentGlobalsELF(IRBuilder<> &IRB, Module &M,
+  void instrumentGlobalsELF(IRBuilder<> &IRB, Module &M,
                             ArrayRef<GlobalVariable *> ExtendedGlobals,
                             ArrayRef<Constant *> MetadataInitializers,
                             const std::string &UniqueModuleId);
@@ -2177,7 +2177,7 @@ void ModuleAddressSanitizer::InstrumentGlobalsCOFF(
     appendToCompilerUsed(M, MetadataGlobals);
 }
 
-void ModuleAddressSanitizer::InstrumentGlobalsELF(
+void ModuleAddressSanitizer::instrumentGlobalsELF(
     IRBuilder<> &IRB, Module &M, ArrayRef<GlobalVariable *> ExtendedGlobals,
     ArrayRef<Constant *> MetadataInitializers,
     const std::string &UniqueModuleId) {
@@ -2187,7 +2187,7 @@ void ModuleAddressSanitizer::InstrumentGlobalsELF(
   // false negative odr violations at link time. If odr indicators are used, we
   // keep the comdat sections, as link time odr violations will be dectected on
   // the odr indicator symbols.
-  bool UseComdatForGlobalsGC = UseOdrIndicator;
+  bool UseComdatForGlobalsGC = UseOdrIndicator && !UniqueModuleId.empty();
 
   SmallVector<GlobalValue *, 16> MetadataGlobals(ExtendedGlobals.size());
   for (size_t i = 0; i < ExtendedGlobals.size(); i++) {
@@ -2237,7 +2237,7 @@ void ModuleAddressSanitizer::InstrumentGlobalsELF(
 
   // We also need to unregister globals at the end, e.g., when a shared library
   // gets closed.
-  if (DestructorKind != AsanDtorKind::None) {
+  if (DestructorKind != AsanDtorKind::None && !MetadataGlobals.empty()) {
     IRBuilder<> IrbDtor(CreateAsanModuleDtor(M));
     IrbDtor.CreateCall(AsanUnregisterElfGlobals,
                        {IRB.CreatePointerCast(RegisteredFlag, IntptrTy),
@@ -2343,10 +2343,8 @@ void ModuleAddressSanitizer::InstrumentGlobalsWithMetadataArray(
 // redzones and inserts this function into llvm.global_ctors.
 // Sets *CtorComdat to true if the global registration code emitted into the
 // asan constructor is comdat-compatible.
-bool ModuleAddressSanitizer::InstrumentGlobals(IRBuilder<> &IRB, Module &M,
+void ModuleAddressSanitizer::instrumentGlobals(IRBuilder<> &IRB, Module &M,
                                                bool *CtorComdat) {
-  *CtorComdat = false;
-
   // Build set of globals that are aliased by some GA, where
   // getExcludedAliasedGlobal(GA) returns the relevant GlobalVariable.
   SmallPtrSet<const GlobalVariable *, 16> AliasedGlobalExclusions;
@@ -2364,11 +2362,6 @@ bool ModuleAddressSanitizer::InstrumentGlobals(IRBuilder<> &IRB, Module &M,
   }
 
   size_t n = GlobalsToChange.size();
-  if (n == 0) {
-    *CtorComdat = true;
-    return false;
-  }
-
   auto &DL = M.getDataLayout();
 
   // A global is described by a structure
@@ -2391,8 +2384,11 @@ bool ModuleAddressSanitizer::InstrumentGlobals(IRBuilder<> &IRB, Module &M,
 
   // We shouldn't merge same module names, as this string serves as unique
   // module ID in runtime.
-  GlobalVariable *ModuleName = createPrivateGlobalForString(
-      M, M.getModuleIdentifier(), /*AllowMerging*/ false, kAsanGenPrefix);
+  GlobalVariable *ModuleName =
+      n != 0
+          ? createPrivateGlobalForString(M, M.getModuleIdentifier(),
+                                         /*AllowMerging*/ false, kAsanGenPrefix)
+          : nullptr;
 
   for (size_t i = 0; i < n; i++) {
     GlobalVariable *G = GlobalsToChange[i];
@@ -2517,27 +2513,34 @@ bool ModuleAddressSanitizer::InstrumentGlobals(IRBuilder<> &IRB, Module &M,
   }
   appendToCompilerUsed(M, ArrayRef<GlobalValue *>(GlobalsToAddToUsedList));
 
-  std::string ELFUniqueModuleId =
-      (UseGlobalsGC && TargetTriple.isOSBinFormatELF()) ? getUniqueModuleId(&M)
-                                                        : "";
-
-  if (!ELFUniqueModuleId.empty()) {
-    InstrumentGlobalsELF(IRB, M, NewGlobals, Initializers, ELFUniqueModuleId);
+  if (UseGlobalsGC && TargetTriple.isOSBinFormatELF()) {
+    // Use COMDAT and register globals even if n == 0 to ensure that (a) the
+    // linkage unit will only have one module constructor, and (b) the register
+    // function will be called. The module destructor is not created when n ==
+    // 0.
     *CtorComdat = true;
-  } else if (UseGlobalsGC && TargetTriple.isOSBinFormatCOFF()) {
-    InstrumentGlobalsCOFF(IRB, M, NewGlobals, Initializers);
-  } else if (UseGlobalsGC && ShouldUseMachOGlobalsSection()) {
-    InstrumentGlobalsMachO(IRB, M, NewGlobals, Initializers);
+    instrumentGlobalsELF(IRB, M, NewGlobals, Initializers,
+                         getUniqueModuleId(&M));
+  } else if (n == 0) {
+    // When UseGlobalsGC is false, COMDAT can still be used if n == 0, because
+    // all compile units will have identical module constructor/destructor.
+    *CtorComdat = TargetTriple.isOSBinFormatELF();
   } else {
-    InstrumentGlobalsWithMetadataArray(IRB, M, NewGlobals, Initializers);
+    *CtorComdat = false;
+    if (UseGlobalsGC && TargetTriple.isOSBinFormatCOFF()) {
+      InstrumentGlobalsCOFF(IRB, M, NewGlobals, Initializers);
+    } else if (UseGlobalsGC && ShouldUseMachOGlobalsSection()) {
+      InstrumentGlobalsMachO(IRB, M, NewGlobals, Initializers);
+    } else {
+      InstrumentGlobalsWithMetadataArray(IRB, M, NewGlobals, Initializers);
+    }
   }
 
   // Create calls for poisoning before initializers run and unpoisoning after.
   if (HasDynamicallyInitializedGlobals)
     createInitializerPoisonCalls(M, ModuleName);
 
   LLVM_DEBUG(dbgs() << M);
-  return true;
 }
 
 uint64_t
@@ -2601,10 +2604,10 @@ bool ModuleAddressSanitizer::instrumentModule(Module &M) {
     assert(AsanCtorFunction || ConstructorKind == AsanCtorKind::None);
     if (AsanCtorFunction) {
       IRBuilder<> IRB(AsanCtorFunction->getEntryBlock().getTerminator());
-      InstrumentGlobals(IRB, M, &CtorComdat);
+      instrumentGlobals(IRB, M, &CtorComdat);
     } else {
       IRBuilder<> IRB(*C);
-      InstrumentGlobals(IRB, M, &CtorComdat);
+      instrumentGlobals(IRB, M, &CtorComdat);
     }
   }
 

llvm/test/Instrumentation/AddressSanitizer/basic.ll

@@ -210,8 +210,10 @@ define void @test_swifterror_3() sanitize_address {
 
 ;; ctor/dtor have the nounwind attribute. See uwtable.ll, they additionally have
 ;; the uwtable attribute with the module flag "uwtable".
-; CHECK: define internal void @asan.module_ctor() #[[#ATTR:]] {{(comdat )?}}{
+; CHECK: define internal void @asan.module_ctor() #[[#ATTR:]] comdat {
 ; CHECK: call void @__asan_init()
+;; __asan_register_elf_globals is called even if this module does not contain instrumented global variables.
+; CHECK: call void @__asan_register_elf_globals(i64 ptrtoint (ptr @___asan_globals_registered to i64), i64 ptrtoint (ptr @__start_asan_globals to i64), i64 ptrtoint (ptr @__stop_asan_globals to i64))
 
 ; CHECK: attributes #[[#ATTR]] = { nounwind }
 

llvm/test/Instrumentation/AddressSanitizer/global_metadata_array.ll

@@ -1,8 +1,12 @@
-; RUN: opt < %s -passes=asan -asan-globals-live-support=0 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefix=CHECK %s
-; RUN: opt < %s -passes=asan -asan-globals-live-support=0 -mtriple=x86_64-apple-macosx10.11.0 -S | FileCheck --check-prefix=CHECK %s
-; RUN: opt < %s -passes=asan -asan-globals-live-support=0 -mtriple=x86_64-pc-windows-msvc19.0.24215 -S | FileCheck --check-prefix=CHECK %s
-; RUN: opt < %s -passes=asan -asan-globals-live-support=0 -asan-mapping-scale=5 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefixes=CHECK,CHECK-S5 %s
+; RUN: rm -rf %t && split-file %s %t && cd %t
+; RUN: opt < a.ll -passes=asan -asan-globals-live-support=0 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < a.ll -passes=asan -asan-globals-live-support=0 -mtriple=x86_64-apple-macosx10.11.0 -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < a.ll -passes=asan -asan-globals-live-support=0 -mtriple=x86_64-pc-windows-msvc19.0.24215 -S | FileCheck --check-prefix=CHECK %s
+; RUN: opt < a.ll -passes=asan -asan-globals-live-support=0 -asan-mapping-scale=5 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefixes=CHECK,CHECK-S5 %s
 
+; RUN: opt < empty.ll -passes=asan -asan-globals-live-support=0 -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefix=ELF-NOGC %s
+
+;--- a.ll
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 
 ; Globals:
@@ -59,3 +63,10 @@ attributes #1 = { nounwind sanitize_address "less-precise-fpmad"="false" "frame-
 !7 = !{!"/tmp/asan-globals.cpp", i32 7, i32 5}
 !8 = !{!"/tmp/asan-globals.cpp", i32 12, i32 14}
 !9 = !{!"/tmp/asan-globals.cpp", i32 14, i32 25}
+
+;; In the presence of instrumented global variables, asan.module_ctor do not use comdat.
+; CHECK: define internal void @asan.module_ctor() #[[#ATTR:]] {
+
+; ELF-NOGC: define internal void @asan.module_ctor() #[[#ATTR:]] comdat {
+
+;--- empty.ll

llvm/test/Instrumentation/AddressSanitizer/global_with_comdat.ll

@@ -4,10 +4,14 @@
 ; We keep using comdats for garbage collection if odr indicators are
 ; enabled as indicator symbols will cause link time odr violations.
 ; This is to fix PR 47925.
-;
-; RUN: opt < %s -passes=asan -asan-globals-live-support=1 -asan-use-odr-indicator=0 -S | FileCheck %s --check-prefixes=CHECK,NOCOMDAT
+; RUN: rm -rf %t && split-file %s %t && cd %t
+; RUN: opt < a.ll -passes=asan -asan-globals-live-support=1 -asan-use-odr-indicator=0 -S | FileCheck %s --check-prefixes=CHECK,NOCOMDAT
 ; Check that enabling odr indicators enables comdat for globals.
-; RUN: opt < %s -passes=asan -asan-globals-live-support=1 -S | FileCheck %s --check-prefixes=CHECK,COMDAT
+; RUN: opt < a.ll -passes=asan -asan-globals-live-support=1 -S | FileCheck %s --check-prefixes=CHECK,COMDAT
+
+; RUN: opt < no_module_id.ll -passes=asan -S | FileCheck %s --check-prefix=NOMODULEID
+
+;--- a.ll
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
@@ -87,3 +91,43 @@ attributes #1 = { nounwind sanitize_address "less-precise-fpmad"="false" "frame-
 !7 = !{!"/tmp/asan-globals.cpp", i32 7, i32 5}
 !8 = !{!"/tmp/asan-globals.cpp", i32 12, i32 14}
 !9 = !{!"/tmp/asan-globals.cpp", i32 14, i32 25}
+
+;--- no_module_id.ll
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; NOMODULEID:      $asan.module_ctor = comdat any
+; NOMODULEID:      $asan.module_dtor = comdat any
+
+;; Don't place the instrumented globals in a comdat when the unique module ID is empty.
+; NOMODULEID:      @.str = internal constant { [4 x i8], [28 x i8] } { [4 x i8] c"str\00", [28 x i8] zeroinitializer }, align 32
+; NOMODULEID:      @_ZL3buf = internal global { [4 x i8], [28 x i8] } zeroinitializer, align 32
+; NOMODULEID:      @__asan_global_.str = private global {{.*}}, section "asan_globals", !associated !0
+; NOMODULEID:      @__asan_global__ZL3buf = private global {{.*}}, section "asan_globals", !associated !1
+; NOMODULEID:      @llvm.compiler.used = appending global [4 x ptr] [ptr @.str, ptr @_ZL3buf, ptr @__asan_global_.str, ptr @__asan_global__ZL3buf]
+
+; NOMODULEID:      define internal void @asan.module_ctor() #[[#]] comdat {
+; NOMODULEID-NEXT:   call void @__asan_init()
+; NOMODULEID-NEXT:   call void @__asan_version_mismatch_check_v8()
+; NOMODULEID-NEXT:   call void @__asan_register_elf_globals(i64 ptrtoint (ptr @___asan_globals_registered to i64), i64 ptrtoint (ptr @__start_asan_globals to i64), i64 ptrtoint (ptr @__stop_asan_globals to i64))
+; NOMODULEID-NEXT:   ret void
+; NOMODULEID-NEXT: }
+
+; NOMODULEID:      !0 = !{ptr @.str}
+; NOMODULEID:      !1 = !{ptr @_ZL3buf}
+
+@.str = private unnamed_addr constant [4 x i8] c"str\00", align 1
+@_ZL3buf = internal unnamed_addr global [4 x i8] zeroinitializer, align 1
+@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @_GLOBAL__sub_I_a.cc, ptr null }]
+
+declare void @ext(ptr noundef)
+
+; Function Attrs: uwtable
+define internal void @_GLOBAL__sub_I_a.cc() #2 section ".text.startup" {
+entry:
+  %0 = load i8, ptr @_ZL3buf, align 1
+  %inc = add i8 %0, 1
+  store i8 %inc, ptr @_ZL3buf, align 1
+  tail call void @ext(ptr noundef nonnull @.str)
+  ret void
+}