[PATCH] LowerBitSets: Avoid reusing byte set addresses.

[Peter Collingbourne]

Hi kcc, jfb,

Each use of the byte array uses a different alias. This makes the
backend less likely to reuse previously computed byte array addresses,
improving the security of the CFI mechanism based on this pass.

http://reviews.llvm.org/D8455

Files:
  lib/Transforms/IPO/LowerBitSets.cpp
  test/Transforms/LowerBitSets/simple.ll

Index: lib/Transforms/IPO/LowerBitSets.cpp
===================================================================
--- lib/Transforms/IPO/LowerBitSets.cpp
+++ lib/Transforms/IPO/LowerBitSets.cpp
@@ -389,7 +389,17 @@
       BAI = createByteArray(BSI);
     }
 
-    Value *ByteAddr = B.CreateGEP(BAI->ByteArray, BitOffset);
+    Constant *ByteArray = BAI->ByteArray;
+    if (!LinkerSubsectionsViaSymbols) {
+      // Each use of the byte array uses a different alias. This makes the
+      // backend less likely to reuse previously computed byte array addresses,
+      // improving the security of the CFI mechanism based on this pass.
+      ByteArray = GlobalAlias::create(
+          BAI->ByteArray->getType()->getElementType(), 0,
+          GlobalValue::PrivateLinkage, "bits_use", ByteArray, M);
+    }
+
+    Value *ByteAddr = B.CreateGEP(ByteArray, BitOffset);
     Value *Byte = B.CreateLoad(ByteAddr);
 
     Value *ByteAndMask = B.CreateAnd(Byte, BAI->Mask);
Index: test/Transforms/LowerBitSets/simple.ll
===================================================================
--- test/Transforms/LowerBitSets/simple.ll
+++ test/Transforms/LowerBitSets/simple.ll
@@ -38,6 +38,10 @@
 
 !llvm.bitsets = !{ !0, !1, !2, !3, !4, !5, !6, !7 }
 
+; CHECK: @bits_use{{[0-9]*}} = private alias i8* @bits{{[0-9]*}}
+; CHECK: @bits_use{{[0-9]*}} = private alias i8* @bits{{[0-9]*}}
+; CHECK: @bits_use{{[0-9]*}} = private alias i8* @bits{{[0-9]*}}
+
 ; CHECK: @a = alias getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 0)
 ; CHECK: @b = alias getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 2)
 ; CHECK: @c = alias getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 4)
@@ -57,8 +61,8 @@
 
 ; CHECK-DARWIN: [[G]] = private constant
 
-; CHECK: @bits = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
-; CHECK: @bits1 = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
+; CHECK: @bits{{[0-9]*}} = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
+; CHECK: @bits{{[0-9]*}} = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
 
 declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
 
@@ -76,7 +80,7 @@
   ; CHECK: [[R6:%[^ ]*]] = icmp ult i32 [[R5]], 68
   ; CHECK: br i1 [[R6]]
 
-  ; CHECK: [[R8:%[^ ]*]] = getelementptr i8, i8* @bits, i32 [[R5]]
+  ; CHECK: [[R8:%[^ ]*]] = getelementptr i8, i8* @bits_use{{[0-9]*}}, i32 [[R5]]
   ; CHECK: [[R9:%[^ ]*]] = load i8, i8* [[R8]]
   ; CHECK: [[R10:%[^ ]*]] = and i8 [[R9]], 1
   ; CHECK: [[R11:%[^ ]*]] = icmp ne i8 [[R10]], 0
@@ -119,7 +123,7 @@
   ; CHECK: [[T6:%[^ ]*]] = icmp ult i32 [[T5]], 66
   ; CHECK: br i1 [[T6]]
 
-  ; CHECK: [[T8:%[^ ]*]] = getelementptr i8, i8* @bits1, i32 [[T5]]
+  ; CHECK: [[T8:%[^ ]*]] = getelementptr i8, i8* @bits_use{{[0-9]*}}, i32 [[T5]]
   ; CHECK: [[T9:%[^ ]*]] = load i8, i8* [[T8]]
   ; CHECK: [[T10:%[^ ]*]] = and i8 [[T9]], 2
   ; CHECK: [[T11:%[^ ]*]] = icmp ne i8 [[T10]], 0

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/
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