[PATCH] D52235: [COFF] Emit @feat.00 on 64-bit and set the CFG bit when emitting guardcf tables

[Hans Wennborg via Phabricator via llvm-commits]

hans created this revision.
hans added a reviewer: rnk.

The 0x800 bit in @feat.00 needs to be set in order to make LLD pick up the .gfid$y table. I believe this is fine to set even if we don't emit the instrumentation.

We haven't emitted @feat.00 on 64-bit before. I see that MSVC does emit it, but I'm not entirely sure what the default value should be. I went with zero since that seems as safe as not emitting the symbol in the first place.

Please take a look.


https://reviews.llvm.org/D52235

Files:
  lib/Target/X86/X86AsmPrinter.cpp
  test/CodeGen/WinCFGuard/cfguard.ll


Index: test/CodeGen/WinCFGuard/cfguard.ll
===================================================================
--- test/CodeGen/WinCFGuard/cfguard.ll
+++ test/CodeGen/WinCFGuard/cfguard.ll
@@ -1,5 +1,7 @@
 ; RUN: llc < %s | FileCheck %s
 
+; CHECK: .set @feat.00, 2048
+
 ; CHECK: .section .gfids$y
 ; CHECK: .symidx "?address_taken@@YAXXZ"
 ; CHECK: .symidx "?virt_method at Derived@@UEBAHXZ"
Index: lib/Target/X86/X86AsmPrinter.cpp
===================================================================
--- lib/Target/X86/X86AsmPrinter.cpp
+++ lib/Target/X86/X86AsmPrinter.cpp
@@ -587,21 +587,28 @@
   if (TT.isOSBinFormatCOFF()) {
     // Emit an absolute @feat.00 symbol.  This appears to be some kind of
     // compiler features bitfield read by link.exe.
+    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();
+    int64_t Feat00Flags = 0;
+
     if (TT.getArch() == Triple::x86) {
-      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.
-      OutStreamer->EmitSymbolAttribute(S, MCSA_Global);
-      OutStreamer->EmitAssignment(
-          S, MCConstantExpr::create(int64_t(1), MMI->getContext()));
+      Feat00Flags |= 1;
     }
+
+    if (M.getModuleFlag("cfguardtable"))
+      Feat00Flags |= 0x800; // Object is CFG-aware.
+
+    OutStreamer->EmitSymbolAttribute(S, MCSA_Global);
+    OutStreamer->EmitAssignment(
+        S, MCConstantExpr::create(Feat00Flags, MMI->getContext()));
   }
   OutStreamer->EmitSyntaxDirective();
 


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