Add the heuristic to differentiate SSPStrong from SSPRequired.

The requirements of the strong heuristic are:

* A Protector is required for functions which contain an array, regardless of
  type or length.

* A Protector is required for functions which contain a structure/union which
  contains an array, regardless of type or length.  Note, there is no limit to
  the depth of nesting.

* A protector is required when the address of a local variable (i.e., stack
  based variable) is exposed. (E.g., such as through a local whose address is
  taken as part of the RHS of an assignment or a local whose address is taken as
  part of a function argument.)

llvm-svn: 173231

llvm/docs/LangRef.rst

@@ -825,7 +825,11 @@ example:
     placed on the stack before the local variables that's checked upon
     return from the function to see if it has been overwritten. A
     heuristic is used to determine if a function needs stack protectors
-    or not.
+    or not. The heuristic used will enable protectors for functions with:
+    - Character arrays larger than ``ssp-buffer-size`` (default 8).
+    - Aggregates containing character arrays larger than ``ssp-buffer-size``.
+    - Calls to alloca() with variable sizes or constant sizes greater than
+      ``ssp-buffer-size``.
 
     If a function that has an ``ssp`` attribute is inlined into a
     function that doesn't have an ``ssp`` attribute, then the resulting
@@ -841,8 +845,15 @@ example:
     an ``sspreq`` attribute.
 ``sspstrong``
     This attribute indicates that the function should emit a stack smashing
-    protector. Currently this attribute has the same effect as
-    ``sspreq``. This overrides the ``ssp`` function attribute.
+    protector. This attribute causes a strong heuristic to be used when
+    determining if a function needs stack protectors.  The strong heuristic
+    will enable protectors for functions with:
+    - Arrays of any size and type
+    - Aggregates containing an array of any size and type.
+    - Calls to alloca().
+    - Local variables that have had their address taken.
+
+    This overrides the ``ssp`` function attribute.
 
     If a function that has an ``sspstrong`` attribute is inlined into a
     function that doesn't have an ``sspstrong`` attribute, then the

llvm/lib/CodeGen/StackProtector.cpp

@@ -16,6 +16,8 @@
 
 #define DEBUG_TYPE "stack-protector"
 #include "llvm/CodeGen/Passes.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/IR/Attributes.h"
@@ -32,6 +34,10 @@
 #include "llvm/Target/TargetOptions.h"
 using namespace llvm;
 
+STATISTIC(NumFunProtected, "Number of functions protected");
+STATISTIC(NumAddrTaken, "Number of local variables that have their address"
+                        " taken.");
+
 namespace {
   class StackProtector : public FunctionPass {
     /// TLI - Keep a pointer of a TargetLowering to consult for determining
@@ -43,6 +49,12 @@ namespace {
 
     DominatorTree *DT;
 
+    /// VisitedPHIs - The set of PHI nodes visited when determining
+    /// if a variable's reference has been taken.  This set 
+    /// is maintained to ensure we don't visit the same PHI node multiple
+    /// times.
+    SmallPtrSet<const PHINode*, 16> VisitedPHIs;
+
     /// InsertStackProtectors - Insert code into the prologue and epilogue of
     /// the function.
     ///
@@ -58,11 +70,15 @@ namespace {
     /// ContainsProtectableArray - Check whether the type either is an array or
     /// contains an array of sufficient size so that we need stack protectors
     /// for it.
-    bool ContainsProtectableArray(Type *Ty, bool InStruct = false) const;
+    bool ContainsProtectableArray(Type *Ty, bool Strong = false,
+                                  bool InStruct = false) const;
+
+    /// \brief Check whether a stack allocation has its address taken.
+    bool HasAddressTaken(const Instruction *AI);
 
     /// RequiresStackProtector - Check whether or not this function needs a
     /// stack protector based upon the stack protector level.
-    bool RequiresStackProtector() const;
+    bool RequiresStackProtector();
   public:
     static char ID;             // Pass identification, replacement for typeid.
     StackProtector() : FunctionPass(ID), TLI(0) {
@@ -96,15 +112,21 @@ bool StackProtector::runOnFunction(Function &Fn) {
 
   if (!RequiresStackProtector()) return false;
 
+  ++NumFunProtected;
   return InsertStackProtectors();
 }
 
 /// ContainsProtectableArray - Check whether the type either is an array or
 /// contains a char array of sufficient size so that we need stack protectors
 /// for it.
-bool StackProtector::ContainsProtectableArray(Type *Ty, bool InStruct) const {
+bool StackProtector::ContainsProtectableArray(Type *Ty, bool Strong,
+                                              bool InStruct) const {
   if (!Ty) return false;
   if (ArrayType *AT = dyn_cast<ArrayType>(Ty)) {
+    // In strong mode any array, regardless of type and size, triggers a
+    // protector
+    if (Strong)
+      return true;
     const TargetMachine &TM = TLI->getTargetMachine();
     if (!AT->getElementType()->isIntegerTy(8)) {
       Triple Trip(TM.getTargetTriple());
@@ -126,45 +148,103 @@ bool StackProtector::ContainsProtectableArray(Type *Ty, bool InStruct) const {
 
   for (StructType::element_iterator I = ST->element_begin(),
          E = ST->element_end(); I != E; ++I)
-    if (ContainsProtectableArray(*I, true))
+    if (ContainsProtectableArray(*I, Strong, true))
       return true;
 
   return false;
 }
 
-/// RequiresStackProtector - Check whether or not this function needs a stack
-/// protector based upon the stack protector level. The heuristic we use is to
-/// add a guard variable to functions that call alloca, and functions with
-/// buffers larger than SSPBufferSize bytes.
-bool StackProtector::RequiresStackProtector() const {
+bool StackProtector::HasAddressTaken(const Instruction *AI) {
+  for (Value::const_use_iterator UI = AI->use_begin(), UE = AI->use_end();
+        UI != UE; ++UI) {
+    const User *U = *UI;
+    if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
+      if (AI == SI->getValueOperand())
+        return true;
+    } else if (const PtrToIntInst *SI = dyn_cast<PtrToIntInst>(U)) {
+      if (AI == SI->getOperand(0))
+        return true;
+    } else if (isa<CallInst>(U)) {
+      return true;
+    } else if (isa<InvokeInst>(U)) {
+      return true;
+    } else if (const SelectInst *SI = dyn_cast<SelectInst>(U)) {
+      if (HasAddressTaken(SI))
+        return true;
+    } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
+      // Keep track of what PHI nodes we have already visited to ensure
+      // they are only visited once.
+      if (VisitedPHIs.insert(PN))
+        if (HasAddressTaken(PN))
+          return true;
+    } else if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
+      if (HasAddressTaken(GEP))
+        return true;
+    } else if (const BitCastInst *BI = dyn_cast<BitCastInst>(U)) {
+      if (HasAddressTaken(BI))
+        return true;
+    }
+  }
+  return false;
+}
+
+/// \brief Check whether or not this function needs a stack protector based
+/// upon the stack protector level.
+///
+/// We use two heuristics: a standard (ssp) and strong (sspstrong).
+/// The standard heuristic which will add a guard variable to functions that
+/// call alloca with a either a variable size or a size >= SSPBufferSize,
+/// functions with character buffers larger than SSPBufferSize, and functions
+/// with aggregates containing character buffers larger than SSPBufferSize. The
+/// strong heuristic will add a guard variables to functions that call alloca
+/// regardless of size, functions with any buffer regardless of type and size,
+/// functions with aggregates that contain any buffer regardless of type and
+/// size, and functions that contain stack-based variables that have had their
+/// address taken.
+bool StackProtector::RequiresStackProtector() {
+  bool Strong = false;
   if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                       Attribute::StackProtectReq))
     return true;
-
-  // FIXME: Dummy SSP-strong implementation.  Default to required until
-  // strong heuristic is implemented.
-  if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
-                                        Attribute::StackProtectStrong))
-    return true;
-
-  if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
-                                       Attribute::StackProtect))
+  else if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
+                                           Attribute::StackProtectStrong))
+    Strong = true;
+  else if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
+                                            Attribute::StackProtect))
     return false;
 
   for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
     BasicBlock *BB = I;
 
     for (BasicBlock::iterator
-           II = BB->begin(), IE = BB->end(); II != IE; ++II)
+           II = BB->begin(), IE = BB->end(); II != IE; ++II) {
       if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
-        if (AI->isArrayAllocation())
-          // This is a call to alloca with a variable size. Emit stack
-          // protectors.
+        if (AI->isArrayAllocation()) {
+          // SSP-Strong: Enable protectors for any call to alloca, regardless
+          // of size.
+          if (Strong)
+            return true;
+  
+          if (const ConstantInt *CI =
+               dyn_cast<ConstantInt>(AI->getArraySize())) {
+            unsigned BufferSize = TLI->getTargetMachine().Options.SSPBufferSize;
+            if (CI->getLimitedValue(BufferSize) >= BufferSize)
+              // A call to alloca with size >= SSPBufferSize requires
+              // stack protectors.
+              return true;
+          } else // A call to alloca with a variable size requires protectors.
+            return true;
+        }
+
+        if (ContainsProtectableArray(AI->getAllocatedType(), Strong))
           return true;
 
-        if (ContainsProtectableArray(AI->getAllocatedType()))
+        if (Strong && HasAddressTaken(AI)) {
+          ++NumAddrTaken; 
           return true;
+        }
       }
+    }
   }
 
   return false;