[asan] Move instrumented null-terminated strings to a special section, LLVM part

On Darwin, simple C null-terminated constant strings normally end up in the __TEXT,__cstring section of the resulting Mach-O binary. When instrumented with ASan, these strings are transformed in a way that they cannot be in __cstring (the linker unifies the content of this section and strips extra NUL bytes, which would break instrumentation), and are put into a generic __const section. This breaks some of the tools that we have: Some tools need to scan all C null-terminated strings in Mach-O binaries, and scanning all the contents of __const has a large performance penalty. This patch instead introduces a special section, __asan_cstring which will now hold the instrumented null-terminated strings.

Differential Revision: https://reviews.llvm.org/D25026



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285619 91177308-0d34-0410-b5e6-96231b3b80d8

Author: kubamracek

lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -1509,6 +1509,14 @@ bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M) {
     NewGlobal->copyAttributesFrom(G);
     NewGlobal->setAlignment(MinRZ);
 
+    // Move null-terminated C strings to "__asan_cstring" section on Darwin.
+    if (TargetTriple.isOSBinFormatMachO() && !G->hasSection() &&
+        G->isConstant()) {
+      auto Seq = dyn_cast<ConstantDataSequential>(G->getInitializer());
+      if (Seq && Seq->isCString())
+        NewGlobal->setSection("__TEXT,__asan_cstring,regular");
+    }
+
     // Transfer the debug info.  The payload starts at offset zero so we can
     // copy the debug info over as is.
     SmallVector<DIGlobalVariable *, 1> GVs;

test/Instrumentation/AddressSanitizer/global_cstring_darwin.ll

@@ -0,0 +1,21 @@
+; This test checks that instrumented global C (null terminated) strings are put into a special section on Darwin.
+; RUN: opt < %s -asan -asan-module -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.10.0"
+
+; Should be put into __asan_cstring section:
+@.str.1 = private unnamed_addr constant [13 x i8] c"Hello world.\00", align 1
+@.str.2 = private unnamed_addr constant [4 x i8] c"%s\0A\00", align 1
+
+; CHECK: @.str.1 = internal unnamed_addr constant { [13 x i8], [51 x i8] } { [13 x i8] c"Hello world.\00", [51 x i8] zeroinitializer }, section "__TEXT,__asan_cstring,regular", align 32
+; CHECK: @.str.2 = internal unnamed_addr constant { [4 x i8], [60 x i8] } { [4 x i8] c"%s\0A\00", [60 x i8] zeroinitializer }, section "__TEXT,__asan_cstring,regular", align 32
+
+; Shouldn't be put into special section:
+@.str.3 = private unnamed_addr constant [4 x i8] c"\00\01\02\03", align 1
+@.str.4 = private unnamed_addr global [7 x i8] c"Hello.\00", align 1
+@.str.5 = private unnamed_addr constant [8 x i8] c"Hello.\00\00", align 1
+
+; CHECK: @.str.3 = internal unnamed_addr constant { [4 x i8], [60 x i8] } { [4 x i8] c"\00\01\02\03", [60 x i8] zeroinitializer }, align 32
+; CHECK: @.str.4 = private unnamed_addr global { [7 x i8], [57 x i8] } { [7 x i8] c"Hello.\00", [57 x i8] zeroinitializer }, align 32
+; CHECK: @.str.5 = internal unnamed_addr constant { [8 x i8], [56 x i8] } { [8 x i8] c"Hello.\00\00", [56 x i8] zeroinitializer }, align 32