COFF: Ensure that objects produced by LLVM link with /safeseh

Summary:
We indicate that the object files are safe by emitting a @feat.00
absolute address symbol.  The address is presumably interpreted as a
bitfield of features that the compiler would like to enable.  Bit 0 is
documented in the PE COFF spec to opt in to "registered SEH", which is
what /safeseh enables.

LLVM's object files are safe by default because LLVM doesn't know how to
produce SEH handlers.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1691

llvm-svn: 190898

llvm/lib/MC/WinCOFFObjectWriter.cpp

@@ -148,8 +148,8 @@ public:
   object_t *createCOFFEntity(StringRef Name, list_t &List);
 
   void DefineSection(MCSectionData const &SectionData);
-  void DefineSymbol(MCSymbolData const &SymbolData,
-                    MCAssembler &Assembler);
+  void DefineSymbol(MCSymbolData const &SymbolData, MCAssembler &Assembler,
+                    const MCAsmLayout &Layout);
 
   void MakeSymbolReal(COFFSymbol &S, size_t Index);
   void MakeSectionReal(COFFSection &S, size_t Number);
@@ -397,7 +397,8 @@ void WinCOFFObjectWriter::DefineSection(MCSectionData const &SectionData) {
 /// This function takes a section data object from the assembler
 /// and creates the associated COFF symbol staging object.
 void WinCOFFObjectWriter::DefineSymbol(MCSymbolData const &SymbolData,
-                                       MCAssembler &Assembler) {
+                                       MCAssembler &Assembler,
+                                       const MCAsmLayout &Layout) {
   MCSymbol const &Symbol = SymbolData.getSymbol();
   COFFSymbol *coff_symbol = GetOrCreateCOFFSymbol(&Symbol);
   SymbolMap[&Symbol] = coff_symbol;
@@ -438,6 +439,12 @@ void WinCOFFObjectWriter::DefineSymbol(MCSymbolData const &SymbolData,
     const MCSymbolData &ResSymData =
       Assembler.getSymbolData(Symbol.AliasedSymbol());
 
+    if (Symbol.isVariable()) {
+      int64_t Addr;
+      if (Symbol.getVariableValue()->EvaluateAsAbsolute(Addr, Layout))
+        coff_symbol->Data.Value = Addr;
+    }
+
     coff_symbol->Data.Type         = (ResSymData.getFlags() & 0x0000FFFF) >>  0;
     coff_symbol->Data.StorageClass = (ResSymData.getFlags() & 0x00FF0000) >> 16;
 
@@ -449,7 +456,9 @@ void WinCOFFObjectWriter::DefineSymbol(MCSymbolData const &SymbolData,
        external ? COFF::IMAGE_SYM_CLASS_EXTERNAL : COFF::IMAGE_SYM_CLASS_STATIC;
     }
 
-    if (ResSymData.Fragment != NULL)
+    if (Symbol.isAbsolute() || Symbol.AliasedSymbol().isVariable())
+      coff_symbol->Data.SectionNumber = COFF::IMAGE_SYM_ABSOLUTE;
+    else if (ResSymData.Fragment != NULL)
       coff_symbol->Section =
         SectionMap[&ResSymData.Fragment->getParent()->getSection()];
 
@@ -597,7 +606,7 @@ void WinCOFFObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm,
   for (MCAssembler::const_symbol_iterator i = Asm.symbol_begin(),
                                           e = Asm.symbol_end();
        i != e; i++)
-    DefineSymbol(*i, Asm);
+    DefineSymbol(*i, Asm, Layout);
 }
 
 void WinCOFFObjectWriter::RecordRelocation(const MCAssembler &Asm,

llvm/lib/Target/X86/X86AsmPrinter.cpp

@@ -518,6 +518,26 @@ bool X86AsmPrinter::PrintAsmMemoryOperand(const MachineInstr *MI,
 void X86AsmPrinter::EmitStartOfAsmFile(Module &M) {
   if (Subtarget->isTargetEnvMacho())
     OutStreamer.SwitchSection(getObjFileLowering().getTextSection());
+
+  if (Subtarget->isTargetCOFF()) {
+    // Emit an absolute @feat.00 symbol.  This appears to be some kind of
+    // compiler features bitfield read by link.exe.
+    if (!Subtarget->is64Bit()) {
+      MCSymbol *S = MMI->getContext().GetOrCreateSymbol(StringRef("@feat.00"));
+      OutStreamer.BeginCOFFSymbolDef(S);
+      OutStreamer.EmitCOFFSymbolStorageClass(COFF::IMAGE_SYM_CLASS_STATIC);
+      OutStreamer.EmitCOFFSymbolType(COFF::IMAGE_SYM_DTYPE_NULL);
+      OutStreamer.EndCOFFSymbolDef();
+      // According to the PE-COFF spec, the LSB of this value marks the object
+      // for "registered SEH".  This means that all SEH handler entry points
+      // must be registered in .sxdata.  Use of any unregistered handlers will
+      // cause the process to terminate immediately.  LLVM does not know how to
+      // register any SEH handlers, so its object files should be safe.
+      S->setAbsolute();
+      OutStreamer.EmitAssignment(
+          S, MCConstantExpr::Create(int64_t(1), MMI->getContext()));
+    }
+  }
 }
 
 

llvm/test/CodeGen/X86/coff-feat00.ll

@@ -0,0 +1,7 @@
+; RUN: llc -O0 -mtriple=i386-pc-win32 -filetype=asm -o - %s | FileCheck %s
+
+define i32 @foo() {
+  ret i32 0
+}
+
+; CHECK: @feat.00 = 1

llvm/test/MC/COFF/feat00.s

@@ -0,0 +1,13 @@
+// RUN: llvm-mc -filetype=obj -triple i686-pc-win32 %s -o - | llvm-readobj -t | FileCheck %s
+
+"@feat.00" = 123
+
+// CHECK: Symbol {
+// CHECK:   Name: @feat.00
+// CHECK:   Value: 123
+// CHECK:   Section: (-1)
+// CHECK:   BaseType: Null (0x0)
+// CHECK:   ComplexType: Null (0x0)
+// CHECK:   StorageClass: External (0x2)
+// CHECK:   AuxSymbolCount: 0
+// CHECK: }