-fsanitize=function: fix alignment fault on Arm targets.

Function pointers are checked by loading a prefix structure from just
before the function's entry point. However, on Arm, the function
pointer is not always exactly equal to the address of the entry point,
because Thumb function pointers have the low bit set to tell the BX
instruction to enter them in Thumb state. So the generated code loads
from an odd address and suffers an alignment fault.

Fixed by clearing the low bit of the function pointer before
subtracting 8.

Differential Revision: https://reviews.llvm.org/D151308

clang/lib/CodeGen/CGExpr.cpp

@@ -5364,8 +5364,30 @@ RValue CodeGenFunction::EmitCall(QualType CalleeType, const CGCallee &OrigCallee
 
       llvm::Value *CalleePtr = Callee.getFunctionPointer();
 
+      // On 32-bit Arm, the low bit of a function pointer indicates whether
+      // it's using the Arm or Thumb instruction set. The actual first
+      // instruction lives at the same address either way, so we must clear
+      // that low bit before using the function address to find the prefix
+      // structure.
+      //
+      // This applies to both Arm and Thumb target triples, because
+      // either one could be used in an interworking context where it
+      // might be passed function pointers of both types.
+      llvm::Value *AlignedCalleePtr;
+      if (CGM.getTriple().isARM() || CGM.getTriple().isThumb()) {
+        llvm::Value *CalleeAddress =
+            Builder.CreatePtrToInt(CalleePtr, IntPtrTy);
+        llvm::Value *Mask = llvm::ConstantInt::get(IntPtrTy, ~1);
+        llvm::Value *AlignedCalleeAddress =
+            Builder.CreateAnd(CalleeAddress, Mask);
+        AlignedCalleePtr =
+            Builder.CreateIntToPtr(AlignedCalleeAddress, CalleePtr->getType());
+      } else {
+        AlignedCalleePtr = CalleePtr;
+      }
+
       llvm::Value *CalleePrefixStruct = Builder.CreateBitCast(
-          CalleePtr, llvm::PointerType::getUnqual(PrefixStructTy));
+          AlignedCalleePtr, llvm::PointerType::getUnqual(PrefixStructTy));
       llvm::Value *CalleeSigPtr =
           Builder.CreateConstGEP2_32(PrefixStructTy, CalleePrefixStruct, -1, 0);
       llvm::Value *CalleeSig =

clang/test/CodeGen/ubsan-function.cpp

@@ -1,11 +1,15 @@
-// RUN: %clang_cc1 -triple x86_64-linux-gnu -emit-llvm -o - %s -fsanitize=function -fno-sanitize-recover=all | FileCheck %s
-// RUN: %clang_cc1 -triple aarch64-linux-gnu -emit-llvm -o - %s -fsanitize=function -fno-sanitize-recover=all | FileCheck %s
-// RUN: %clang_cc1 -triple aarch64_be-linux-gnu -emit-llvm -o - %s -fsanitize=function -fno-sanitize-recover=all | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -emit-llvm -o - %s -fsanitize=function -fno-sanitize-recover=all | FileCheck %s --check-prefixes=CHECK,64
+// RUN: %clang_cc1 -triple aarch64-linux-gnu -emit-llvm -o - %s -fsanitize=function -fno-sanitize-recover=all | FileCheck %s --check-prefixes=CHECK,64
+// RUN: %clang_cc1 -triple aarch64_be-linux-gnu -emit-llvm -o - %s -fsanitize=function -fno-sanitize-recover=all | FileCheck %s --check-prefixes=CHECK,64
+// RUN: %clang_cc1 -triple arm-none-eabi -emit-llvm -o - %s -fsanitize=function -fno-sanitize-recover=all | FileCheck %s --check-prefixes=CHECK,ARM,32
 
 // CHECK: define{{.*}} void @_Z3funv() #0 !func_sanitize ![[FUNCSAN:.*]] {
 void fun() {}
 
 // CHECK-LABEL: define{{.*}} void @_Z6callerPFvvE(ptr noundef %f)
+// ARM:   ptrtoint ptr {{.*}} to i32, !nosanitize !5
+// ARM:   and i32 {{.*}}, -2, !nosanitize !5
+// ARM:   inttoptr i32 {{.*}} to ptr, !nosanitize !5
 // CHECK: getelementptr <{ i32, i32 }>, ptr {{.*}}, i32 -1, i32 0, !nosanitize
 // CHECK: load i32, ptr {{.*}}, align {{.*}}, !nosanitize
 // CHECK: icmp eq i32 {{.*}}, -1056584962, !nosanitize
@@ -16,7 +20,8 @@ void fun() {}
 // CHECK: icmp eq i32 {{.*}}, -1522505972, !nosanitize
 // CHECK: br i1 {{.*}}, label %[[LABEL3:.*]], label %[[LABEL2:[^,]*]], {{.*}}!nosanitize
 // CHECK: [[LABEL2]]:
-// CHECK: call void @__ubsan_handle_function_type_mismatch_abort(ptr @[[#]], i64 %[[#]]) #[[#]], !nosanitize
+// 64:    call void @__ubsan_handle_function_type_mismatch_abort(ptr @[[#]], i64 %[[#]]) #[[#]], !nosanitize
+// 32:    call void @__ubsan_handle_function_type_mismatch_abort(ptr @[[#]], i32 %[[#]]) #[[#]], !nosanitize
 // CHECK-NEXT: unreachable, !nosanitize
 // CHECK-EMPTY:
 // CHECK-NEXT: [[LABEL3]]: