[KernelAddressSanitizer] Make globals constructors compatible with kernel

[melver]

Summary

This makes -fsanitize=kernel-address emit the correct globals
constructors for the kernel. We had to do the following:

• Disable generation of constructors that rely on linker features such as dead-global elimination.

• Only emit constructors for globals *not* in explicit sections. The kernel uses sections for special globals, which we should not touch.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203493

Tested:

1. With 'clang/test/CodeGen/asan-globals.cpp'.
2. With test_kasan.ko, we can see:
   
   BUG: KASAN: global-out-of-bounds in kasan_global_oob+0xb3/0xba [test_kasan]

[melver]

Add missing comment.

[glider]

Could you please add an IR test for this?

[melver]

In D80805#2063225, @glider wrote:

Could you please add an IR test for this?

Was planning to -- sorry, I should have mentioned: before I go add tests, does this look sane to you?

[andreyknvl]

The change looks good to me.

[glider]

In D80805#2063679, @melver wrote:

In D80805#2063225, @glider wrote:

Could you please add an IR test for this?

Was planning to -- sorry, I should have mentioned: before I go add tests, does this look sane to you?

I thought we didn't have support for __attribute__((constructor)) in the kernel? Does GCC emit constructors for globals with KASAN?

[melver]

In D80805#2068575, @glider wrote:

In D80805#2063679, @melver wrote:

In D80805#2063225, @glider wrote:

Could you please add an IR test for this?

Was planning to -- sorry, I should have mentioned: before I go add tests, does this look sane to you?

I thought we didn't have support for __attribute__((constructor)) in the kernel? Does GCC emit constructors for globals with KASAN?

I don't think this has much to do with attribute((constructor)), since that's never used explicitly. But implicit constructors are emitted. GCC emits them with KASAN. The disassembled constructors for globals to initialize KASAN with both GCC and Clang are now pretty much identical (apart from symbol names).

The kernel calls them here: https://elixir.bootlin.com/linux/latest/source/init/main.c#L1046

[glider]

Cool, didn't know that; then the change sure looks reasonable.

[melver]

Update IR test.

[glider]

LGTM

[thakis]

Hi, this broke check-clang on mac: http://45.33.8.238/mac/14998/step_7.txt

Please take a look, and if it takes a while to fix please revert while you investigate.

[nickdesaulniers]

clang/test/CodeGen/asan-globals.cpp

14 ↗

(On Diff #268898)

/Users/thakis/src/llvm-project/clang/test/CodeGen/asan-globals.cpp:14:28: error: argument to 'section' attribute is not valid for this target: mach-o section specifier requires a segment and section separated by a comma
int attribute((section(".foo.bar"))) sectioned_global;

^

http://45.33.8.238/mac/14998/step_7.txt

Using an explicit Linux target triple would avoid this.

See also attribute-section-data-common.c for the OSX portable example.

[thakis]

clang/test/CodeGen/asan-globals.cpp

14 ↗

(On Diff #268898)

Using an explicit Linux target triple would avoid this.

It'd also make the test fewer things, since it currently tests asan on mac (when run on mac).

If the attribute-section-data-common.c approach works here, great. If not, maybe the linux-specific bits could go in a new test file?

Are you planning on fixing the test, or should we revert for now so we can figure things out without the tree being red?

[melver]

In D80805#2077396, @thakis wrote:

Hi, this broke check-clang on mac: http://45.33.8.238/mac/14998/step_7.txt

Please take a look, and if it takes a while to fix please revert while you investigate.

Sent https://reviews.llvm.org/D81306

llvm/include/llvm/Transforms/Utils/ModuleUtils.h

 
 /// Same as appendToGlobalCtors(), but for global dtors.
 void appendToGlobalDtors(Module &M, Function *F, int Priority,
                          Constant *Data = nullptr);
 
 FunctionCallee declareSanitizerInitFunction(Module &M, StringRef InitName,
                                             ArrayRef<Type *> InitArgTypes);
 
+Function *createSanitizerCtor(Module &M, StringRef CtorName);
+
 /// Creates sanitizer constructor function, and calls sanitizer's init
 /// function from it.
 /// \return Returns pair of pointers to constructor, and init functions
 /// respectively.
 std::pair<Function *, FunctionCallee> createSanitizerCtorAndInitFunctions(
     Module &M, StringRef CtorName, StringRef InitName,
     ArrayRef<Type *> InitArgTypes, ArrayRef<Value *> InitArgs,
     StringRef VersionCheckName = StringRef());

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

 
 char ASanGlobalsMetadataWrapperPass::ID = 0;
 
 /// AddressSanitizer: instrument the code in module to find memory bugs.
 struct AddressSanitizer {
   AddressSanitizer(Module &M, const GlobalsMetadata *GlobalsMD,
                    bool CompileKernel = false, bool Recover = false,
                    bool UseAfterScope = false)
-      : UseAfterScope(UseAfterScope || ClUseAfterScope), GlobalsMD(*GlobalsMD) {
-    this->Recover = ClRecover.getNumOccurrences() > 0 ? ClRecover : Recover;
-    this->CompileKernel =
-        ClEnableKasan.getNumOccurrences() > 0 ? ClEnableKasan : CompileKernel;
-
+      : CompileKernel(ClEnableKasan.getNumOccurrences() > 0 ? ClEnableKasan
+                                                            : CompileKernel),
+        Recover(ClRecover.getNumOccurrences() > 0 ? ClRecover : Recover),
+        UseAfterScope(UseAfterScope || ClUseAfterScope), GlobalsMD(*GlobalsMD) {
     C = &(M.getContext());
     LongSize = M.getDataLayout().getPointerSizeInBits();
     IntptrTy = Type::getIntNTy(*C, LongSize);
     TargetTriple = Triple(M.getTargetTriple());
 
     Mapping = getShadowMapping(TargetTriple, LongSize, this->CompileKernel);
   }
 
private:
   bool UseAfterScope;
 };
 
 class ModuleAddressSanitizer {
 public:
   ModuleAddressSanitizer(Module &M, const GlobalsMetadata *GlobalsMD,
                          bool CompileKernel = false, bool Recover = false,
                          bool UseGlobalsGC = true, bool UseOdrIndicator = false)
-      : GlobalsMD(*GlobalsMD), UseGlobalsGC(UseGlobalsGC && ClUseGlobalsGC),
+      : GlobalsMD(*GlobalsMD),
+        CompileKernel(ClEnableKasan.getNumOccurrences() > 0 ? ClEnableKasan
+                                                            : CompileKernel),
+        Recover(ClRecover.getNumOccurrences() > 0 ? ClRecover : Recover),
+        UseGlobalsGC(UseGlobalsGC && ClUseGlobalsGC && !this->CompileKernel),
         // Enable aliases as they should have no downside with ODR indicators.
         UsePrivateAlias(UseOdrIndicator || ClUsePrivateAlias),
         UseOdrIndicator(UseOdrIndicator || ClUseOdrIndicator),
         // Not a typo: ClWithComdat is almost completely pointless without
         // ClUseGlobalsGC (because then it only works on modules without
         // globals, which are rare); it is a prerequisite for ClUseGlobalsGC;
         // and both suffer from gold PR19002 for which UseGlobalsGC constructor
         // argument is designed as workaround. Therefore, disable both
         // ClWithComdat and ClUseGlobalsGC unless the frontend says it's ok to
         // do globals-gc.
-        UseCtorComdat(UseGlobalsGC && ClWithComdat) {
-    this->Recover = ClRecover.getNumOccurrences() > 0 ? ClRecover : Recover;
-    this->CompileKernel =
-        ClEnableKasan.getNumOccurrences() > 0 ? ClEnableKasan : CompileKernel;
-
+        UseCtorComdat(UseGlobalsGC && ClWithComdat && !this->CompileKernel) {
     C = &(M.getContext());
     int LongSize = M.getDataLayout().getPointerSizeInBits();
     IntptrTy = Type::getIntNTy(*C, LongSize);
     TargetTriple = Triple(M.getTargetTriple());
     Mapping = getShadowMapping(TargetTriple, LongSize, this->CompileKernel);
   }
 
   bool instrumentModule(Module &);
case Comdat::NoDuplicates:
       break;
     case Comdat::Largest:
     case Comdat::SameSize:
       return false;
     }
   }
 
   if (G->hasSection()) {
+    // The kernel uses explicit sections for mostly special global variables
+    // that we should not instrument. E.g. the kernel may rely on their layout
+    // without redzones, or remove them at link time ("discard.*"), etc.
+    if (CompileKernel)
+      return false;
+
     StringRef Section = G->getSection();
 
     // Globals from llvm.metadata aren't emitted, do not instrument them.
     if (Section == "llvm.metadata") return false;
     // Do not instrument globals from special LLVM sections.
     if (Section.find("__llvm") != StringRef::npos || Section.find("__LLVM") != StringRef::npos) return false;
 
     // Do not instrument function pointers to initialization and termination
int ModuleAddressSanitizer::GetAsanVersion(const Module &M) const {
   // shadow.
   Version += (LongSize == 32 && isAndroid);
   return Version;
 }
 
 bool ModuleAddressSanitizer::instrumentModule(Module &M) {
   initializeCallbacks(M);
 
-  if (CompileKernel)
-    return false;
-
   // Create a module constructor. A destructor is created lazily because not all
   // platforms, and not all modules need it.
+  if (CompileKernel) {
+    // The kernel always builds with its own runtime, and therefore does not
+    // need the init and version check calls.
+    AsanCtorFunction = createSanitizerCtor(M, kAsanModuleCtorName);
+  } else {
     std::string AsanVersion = std::to_string(GetAsanVersion(M));
     std::string VersionCheckName =
         ClInsertVersionCheck ? (kAsanVersionCheckNamePrefix + AsanVersion) : "";
-  std::tie(AsanCtorFunction, std::ignore) = createSanitizerCtorAndInitFunctions(
-      M, kAsanModuleCtorName, kAsanInitName, /*InitArgTypes=*/{},
+    std::tie(AsanCtorFunction, std::ignore) =
+        createSanitizerCtorAndInitFunctions(M, kAsanModuleCtorName,
+                                            kAsanInitName, /*InitArgTypes=*/{},
                                             /*InitArgs=*/{}, VersionCheckName);
+  }
 
   bool CtorComdat = true;
-  // TODO(glider): temporarily disabled globals instrumentation for KASan.
   if (ClGlobals) {
     IRBuilder<> IRB(AsanCtorFunction->getEntryBlock().getTerminator());
     InstrumentGlobals(IRB, M, &CtorComdat);
   }
 
   const uint64_t Priority = GetCtorAndDtorPriority(TargetTriple);
 
   // Put the constructor and destructor in comdat if both

llvm/lib/Transforms/Utils/ModuleUtils.cpp

llvm::declareSanitizerInitFunction(Module &M, StringRef InitName,
                                    ArrayRef<Type *> InitArgTypes) {
   assert(!InitName.empty() && "Expected init function name");
   return M.getOrInsertFunction(
       InitName,
       FunctionType::get(Type::getVoidTy(M.getContext()), InitArgTypes, false),
       AttributeList());
 }
 
+Function *llvm::createSanitizerCtor(Module &M, StringRef CtorName) {
+  Function *Ctor = Function::Create(
+      FunctionType::get(Type::getVoidTy(M.getContext()), false),
+      GlobalValue::InternalLinkage, CtorName, &M);
+  BasicBlock *CtorBB = BasicBlock::Create(M.getContext(), "", Ctor);
+  ReturnInst::Create(M.getContext(), CtorBB);
+  return Ctor;
+}
+
 std::pair<Function *, FunctionCallee> llvm::createSanitizerCtorAndInitFunctions(
     Module &M, StringRef CtorName, StringRef InitName,
     ArrayRef<Type *> InitArgTypes, ArrayRef<Value *> InitArgs,
     StringRef VersionCheckName) {
   assert(!InitName.empty() && "Expected init function name");
   assert(InitArgs.size() == InitArgTypes.size() &&
          "Sanitizer's init function expects different number of arguments");
   FunctionCallee InitFunction =
       declareSanitizerInitFunction(M, InitName, InitArgTypes);
-  Function *Ctor = Function::Create(
-      FunctionType::get(Type::getVoidTy(M.getContext()), false),
-      GlobalValue::InternalLinkage, CtorName, &M);
-  BasicBlock *CtorBB = BasicBlock::Create(M.getContext(), "", Ctor);
-  IRBuilder<> IRB(ReturnInst::Create(M.getContext(), CtorBB));
+  Function *Ctor = createSanitizerCtor(M, CtorName);
+  IRBuilder<> IRB(Ctor->getEntryBlock().getTerminator());
   IRB.CreateCall(InitFunction, InitArgs);
   if (!VersionCheckName.empty()) {
     FunctionCallee VersionCheckFunction = M.getOrInsertFunction(
         VersionCheckName, FunctionType::get(IRB.getVoidTy(), {}, false),
         AttributeList());
     IRB.CreateCall(VersionCheckFunction, {});
   }
   return std::make_pair(Ctor, InitFunction);