From 5265adc73721963d3bf605a7ad5eab6a7e0850b8 Mon Sep 17 00:00:00 2001
From: Marco Elver <elver@google.com>
Date: Tue, 24 Jan 2023 11:51:22 +0100
Subject: [SanitizerBinaryMetadata] Declare callbacks extern weak

Declare callbacks extern weak (if no existing declaration exists), and
only call if the function address is non-null.

This allows to attach semantic metadata to binaries where no user of
that metadata exists, avoiding to have to link empty stub callbacks.

Once the binary is linked (statically or dynamically) against a tool
runtime that implements the callbacks, the respective callbacks will be
called. This vastly simplifies gradual deployment of tools using the
metadata, esp. avoiding having to recompile large codebases with
different compiler flags (which negatively impacts compiler caches).

Reviewed By: dvyukov, vitalybuka

Differential Revision: https://reviews.llvm.org/D142408
---
 llvm/lib/Transforms/Utils/ModuleUtils.cpp | 53 +++++++++++++++++++++++--------
 1 file changed, 40 insertions(+), 13 deletions(-)

(limited to 'llvm/lib/Transforms/Utils/ModuleUtils.cpp')

diff --git a/llvm/lib/Transforms/Utils/ModuleUtils.cpp b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
index 5148df5..6d17a466 100644
--- a/llvm/lib/Transforms/Utils/ModuleUtils.cpp
+++ b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
@@ -170,14 +170,17 @@ void llvm::setKCFIType(Module &M, Function &F, StringRef MangledType) {
   }
 }
 
-FunctionCallee
-llvm::declareSanitizerInitFunction(Module &M, StringRef InitName,
-                                   ArrayRef<Type *> InitArgTypes) {
+FunctionCallee llvm::declareSanitizerInitFunction(Module &M, StringRef InitName,
+                                                  ArrayRef<Type *> InitArgTypes,
+                                                  bool Weak) {
   assert(!InitName.empty() && "Expected init function name");
-  return M.getOrInsertFunction(
-      InitName,
-      FunctionType::get(Type::getVoidTy(M.getContext()), InitArgTypes, false),
-      AttributeList());
+  auto *VoidTy = Type::getVoidTy(M.getContext());
+  auto *FnTy = FunctionType::get(VoidTy, InitArgTypes, false);
+  auto FnCallee = M.getOrInsertFunction(InitName, FnTy);
+  auto *Fn = cast<Function>(FnCallee.getCallee());
+  if (Weak && Fn->isDeclaration())
+    Fn->setLinkage(Function::ExternalWeakLinkage);
+  return FnCallee;
 }
 
 Function *llvm::createSanitizerCtor(Module &M, StringRef CtorName) {
@@ -197,14 +200,33 @@ Function *llvm::createSanitizerCtor(Module &M, StringRef CtorName) {
 std::pair<Function *, FunctionCallee> llvm::createSanitizerCtorAndInitFunctions(
     Module &M, StringRef CtorName, StringRef InitName,
     ArrayRef<Type *> InitArgTypes, ArrayRef<Value *> InitArgs,
-    StringRef VersionCheckName) {
+    StringRef VersionCheckName, bool Weak) {
   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);
+      declareSanitizerInitFunction(M, InitName, InitArgTypes, Weak);
   Function *Ctor = createSanitizerCtor(M, CtorName);
-  IRBuilder<> IRB(Ctor->getEntryBlock().getTerminator());
+  IRBuilder<> IRB(M.getContext());
+
+  BasicBlock *RetBB = &Ctor->getEntryBlock();
+  if (Weak) {
+    RetBB->setName("ret");
+    auto *EntryBB = BasicBlock::Create(M.getContext(), "entry", Ctor, RetBB);
+    auto *CallInitBB =
+        BasicBlock::Create(M.getContext(), "callfunc", Ctor, RetBB);
+    auto *InitFn = cast<Function>(InitFunction.getCallee());
+    auto *InitFnPtr =
+        PointerType::get(InitFn->getType(), InitFn->getAddressSpace());
+    IRB.SetInsertPoint(EntryBB);
+    Value *InitNotNull =
+        IRB.CreateICmpNE(InitFn, ConstantPointerNull::get(InitFnPtr));
+    IRB.CreateCondBr(InitNotNull, CallInitBB, RetBB);
+    IRB.SetInsertPoint(CallInitBB);
+  } else {
+    IRB.SetInsertPoint(RetBB->getTerminator());
+  }
+
   IRB.CreateCall(InitFunction, InitArgs);
   if (!VersionCheckName.empty()) {
     FunctionCallee VersionCheckFunction = M.getOrInsertFunction(
@@ -212,6 +234,10 @@ std::pair<Function *, FunctionCallee> llvm::createSanitizerCtorAndInitFunctions(
         AttributeList());
     IRB.CreateCall(VersionCheckFunction, {});
   }
+
+  if (Weak)
+    IRB.CreateBr(RetBB);
+
   return std::make_pair(Ctor, InitFunction);
 }
 
@@ -220,7 +246,7 @@ llvm::getOrCreateSanitizerCtorAndInitFunctions(
     Module &M, StringRef CtorName, StringRef InitName,
     ArrayRef<Type *> InitArgTypes, ArrayRef<Value *> InitArgs,
     function_ref<void(Function *, FunctionCallee)> FunctionsCreatedCallback,
-    StringRef VersionCheckName) {
+    StringRef VersionCheckName, bool Weak) {
   assert(!CtorName.empty() && "Expected ctor function name");
 
   if (Function *Ctor = M.getFunction(CtorName))
@@ -228,12 +254,13 @@ llvm::getOrCreateSanitizerCtorAndInitFunctions(
     // globals. This will make moving to a concurrent model much easier.
     if (Ctor->arg_empty() ||
         Ctor->getReturnType() == Type::getVoidTy(M.getContext()))
-      return {Ctor, declareSanitizerInitFunction(M, InitName, InitArgTypes)};
+      return {Ctor,
+              declareSanitizerInitFunction(M, InitName, InitArgTypes, Weak)};
 
   Function *Ctor;
   FunctionCallee InitFunction;
   std::tie(Ctor, InitFunction) = llvm::createSanitizerCtorAndInitFunctions(
-      M, CtorName, InitName, InitArgTypes, InitArgs, VersionCheckName);
+      M, CtorName, InitName, InitArgTypes, InitArgs, VersionCheckName, Weak);
   FunctionsCreatedCallback(Ctor, InitFunction);
   return std::make_pair(Ctor, InitFunction);
 }
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