[MC] Make .note.GNU-stack explicit for the trampoline case (#151754)

In the presence of trampolines, the .note.GNU-stack section is not emitted. The
absence of .note.GNU-stack results in the stack marked executable by some
linkers. But others require an explict .note.GNU-stack section.

The GNU ld 2.43 on x86 machines, for example, issues the following:

missing .note.GNU-stack section implies executable stack
NOTE: This behaviour is deprecated and will be removed in a future version of the linker

On one of the ARM machines, the absence of .note.GNU-stack results in the stack
marked as non-executable:

STACK off 0x0000000000000000 vaddr 0x0000000000000000 paddr 0x0000000000000000 align 2**4
filesz 0x0000000000000000 memsz 0x0000000000000000 flags rw-

This change just emits the explicit .note.GNU-stack and marks it executable if required.

Author: ssijaric-nv

llvm/include/llvm/MC/MCAsmInfo.h

@@ -464,10 +464,10 @@ class LLVM_ABI MCAsmInfo {
   const char *getData64bitsDirective() const { return Data64bitsDirective; }
   bool supportsSignedData() const { return SupportsSignedData; }
 
-  /// Targets can implement this method to specify a section to switch to if the
-  /// translation unit doesn't have any trampolines that require an executable
-  /// stack.
-  virtual MCSection *getNonexecutableStackSection(MCContext &Ctx) const {
+  /// Targets can implement this method to specify a section to switch to
+  /// depending on whether the translation unit has any trampolines that require
+  /// an executable stack.
+  virtual MCSection *getStackSection(MCContext &Ctx, bool Exec) const {
     return nullptr;
   }
 

llvm/include/llvm/MC/MCAsmInfoELF.h

@@ -15,7 +15,7 @@ namespace llvm {
 
 class MCAsmInfoELF : public MCAsmInfo {
   virtual void anchor();
-  MCSection *getNonexecutableStackSection(MCContext &Ctx) const override;
+  MCSection *getStackSection(MCContext &Ctx, bool Exec) const override;
   void printSwitchToSection(const MCSection &, uint32_t, const Triple &,
                             raw_ostream &) const final;
   bool useCodeAlign(const MCSection &Sec) const final;

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -2866,9 +2866,11 @@ bool AsmPrinter::doFinalization(Module &M) {
   // If we don't have any trampolines, then we don't require stack memory
   // to be executable. Some targets have a directive to declare this.
   Function *InitTrampolineIntrinsic = M.getFunction("llvm.init.trampoline");
-  if (!InitTrampolineIntrinsic || InitTrampolineIntrinsic->use_empty())
-    if (MCSection *S = MAI->getNonexecutableStackSection(OutContext))
-      OutStreamer->switchSection(S);
+  bool HasTrampolineUses =
+      InitTrampolineIntrinsic && !InitTrampolineIntrinsic->use_empty();
+  MCSection *S = MAI->getStackSection(OutContext, /*Exec=*/HasTrampolineUses);
+  if (S)
+    OutStreamer->switchSection(S);
 
   if (TM.Options.EmitAddrsig) {
     // Emit address-significance attributes for all globals.

llvm/lib/MC/MCAsmInfoELF.cpp

@@ -27,12 +27,13 @@ using namespace llvm;
 
 void MCAsmInfoELF::anchor() {}
 
-MCSection *MCAsmInfoELF::getNonexecutableStackSection(MCContext &Ctx) const {
+MCSection *MCAsmInfoELF::getStackSection(MCContext &Ctx, bool Exec) const {
   // Solaris doesn't know/doesn't care about .note.GNU-stack sections, so
   // don't emit them.
   if (Ctx.getTargetTriple().isOSSolaris())
     return nullptr;
-  return Ctx.getELFSection(".note.GNU-stack", ELF::SHT_PROGBITS, 0);
+  return Ctx.getELFSection(".note.GNU-stack", ELF::SHT_PROGBITS,
+                           Exec ? ELF::SHF_EXECINSTR : 0U);
 }
 
 bool MCAsmInfoELF::useCodeAlign(const MCSection &Sec) const {

llvm/lib/MC/MCELFStreamer.cpp

@@ -54,7 +54,7 @@ void MCELFStreamer::initSections(bool NoExecStack, const MCSubtargetInfo &STI) {
                     &STI);
 
   if (NoExecStack)
-    switchSection(Ctx.getAsmInfo()->getNonexecutableStackSection(Ctx));
+    switchSection(Ctx.getAsmInfo()->getStackSection(Ctx, /*Exec=*/false));
 }
 
 void MCELFStreamer::emitLabel(MCSymbol *S, SMLoc Loc) {

llvm/lib/Target/BPF/MCTargetDesc/BPFMCAsmInfo.h

@@ -49,7 +49,7 @@ class BPFMCAsmInfo : public MCAsmInfoELF {
     DwarfUsesRelocationsAcrossSections = enable;
   }
 
-  MCSection *getNonexecutableStackSection(MCContext &Ctx) const override {
+  MCSection *getStackSection(MCContext &Ctx, bool Exec) const override {
     return nullptr;
   }
 };

llvm/test/CodeGen/AArch64/trampoline.ll

@@ -263,3 +263,9 @@ define i64 @func2() {
   %fp = call ptr @llvm.adjust.trampoline(ptr @trampg)
   ret i64 0
 }
+
+; Check for the explicitly emitted .note.GNU-stack section (ELF only) in the
+; presence of trampolines.
+; UTC_ARGS: --disable
+; CHECK-LINUX:         .section        ".note.GNU-stack","x",@progbits
+; UTC_ARGS: --enable

llvm/test/CodeGen/RISCV/rv64-trampoline.ll

@@ -78,3 +78,10 @@ define i64 @test0(i64 %n, ptr %p) nounwind {
   ret i64 %ret
 
 }
+
+; Check for the explicitly emitted .note.GNU-stack section (ELF only) in the
+; presence of trampolines.
+; UTC_ARGS: --disable
+; RV64-LINUX:         .section        ".note.GNU-stack","x",@progbits
+; RV64:               .section        ".note.GNU-stack","x",@progbits
+; UTC_ARGS: --enable

llvm/tools/llvm-mc/llvm-mc.cpp

@@ -642,7 +642,8 @@ int main(int argc, char **argv) {
                : MAB->createObjectWriter(*OS),
         std::unique_ptr<MCCodeEmitter>(CE), *STI));
     if (NoExecStack)
-      Str->switchSection(Ctx.getAsmInfo()->getNonexecutableStackSection(Ctx));
+      Str->switchSection(
+          Ctx.getAsmInfo()->getStackSection(Ctx, /*Exec=*/false));
     Str->emitVersionForTarget(TheTriple, VersionTuple(), nullptr,
                               VersionTuple());
   }