[MC][COFF][ELF] Reject instructions in IMAGE_SCN_CNT_UNINITIALIZED_DATA/SHT_NOBITS sections

For `.bss; nop`, MC inappropriately calls abort() (via report_fatal_error()) with a message
`cannot have fixups in virtual section!`
It is a bug to crash for invalid user input. Fix it by erroring out early in EmitInstToData().

Similarly, emitIntValue() in a virtual section (SHT_NOBITS in ELF) can crash with the mssage
`non-zero initializer found in section '.bss'` (see D4199)
It'd be nice to report the location but so many directives can call emitIntValue()
and it is difficult to track every location.
Note, COFF does not crash because MCAssembler::writeSectionData() is not
called for an IMAGE_SCN_CNT_UNINITIALIZED_DATA section.

Note, GNU as' arm64 backend reports ``Error: attempt to store non-zero value in section `.bss'``
for a non-zero .inst but fails to do so for other instructions.
We simply reject all instructions, even if the encoding is all zeros.

The Mach-O counterpart is D48517 (see `test/MC/MachO/zerofill-text.s`)

Reviewed By: rnk, skan

Differential Revision: https://reviews.llvm.org/D78138

include/llvm/MC/MCSection.h

@@ -189,6 +189,8 @@ public:
   /// file contents.
   virtual bool isVirtualSection() const = 0;
 
+  virtual StringRef getVirtualSectionKind() const;
+
   /// Add a pending label for the requested subsection. This label will be
   /// associated with a fragment in flushPendingLabels()
   void addPendingLabel(MCSymbol* label, unsigned Subsection = 0);

include/llvm/MC/MCSectionCOFF.h

@@ -74,6 +74,7 @@ public:
                             const MCExpr *Subsection) const override;
   bool UseCodeAlign() const override;
   bool isVirtualSection() const override;
+  StringRef getVirtualSectionKind() const override;
 
   unsigned getOrAssignWinCFISectionID(unsigned *NextID) const {
     if (WinCFISectionID == ~0U)

include/llvm/MC/MCSectionELF.h

@@ -78,6 +78,7 @@ public:
                             const MCExpr *Subsection) const override;
   bool UseCodeAlign() const override;
   bool isVirtualSection() const override;
+  StringRef getVirtualSectionKind() const override;
 
   bool isUnique() const { return UniqueID != NonUniqueID; }
   unsigned getUniqueID() const { return UniqueID; }

lib/MC/MCAssembler.cpp

@@ -683,11 +683,16 @@ void MCAssembler::writeSectionData(raw_ostream &OS, const MCSection *Sec,
         // directives to fill the contents of virtual sections.
         const MCDataFragment &DF = cast<MCDataFragment>(F);
         if (DF.fixup_begin() != DF.fixup_end())
-          report_fatal_error("cannot have fixups in virtual section!");
+          getContext().reportError(SMLoc(), Sec->getVirtualSectionKind() +
+                                                " section '" + Sec->getName() +
+                                                "' cannot have fixups");
         for (unsigned i = 0, e = DF.getContents().size(); i != e; ++i)
           if (DF.getContents()[i]) {
-            report_fatal_error("non-zero initializer found in section '" +
+            getContext().reportError(SMLoc(),
-                               Sec->getName() + "'");
+                                     Sec->getVirtualSectionKind() +
+                                         " section '" + Sec->getName() +
+                                         "' cannot have non-zero initializers");
+            break;
           }
         break;
       }

lib/MC/MCObjectStreamer.cpp

@@ -367,6 +367,13 @@ bool MCObjectStreamer::mayHaveInstructions(MCSection &Sec) const {
 
 void MCObjectStreamer::emitInstruction(const MCInst &Inst,
                                        const MCSubtargetInfo &STI) {
+  const MCSection &Sec = *getCurrentSectionOnly();
+  if (Sec.isVirtualSection()) {
+    getContext().reportError(Inst.getLoc(), Twine(Sec.getVirtualSectionKind()) +
+                                                " section '" + Sec.getName() +
+                                                "' cannot have instructions");
+    return;
+  }
   getAssembler().getBackend().emitInstructionBegin(*this, Inst);
   emitInstructionImpl(Inst, STI);
   getAssembler().getBackend().emitInstructionEnd(*this, Inst);

lib/MC/MCSection.cpp

@@ -87,6 +87,8 @@ MCSection::getSubsectionInsertionPoint(unsigned Subsection) {
   return IP;
 }
 
+StringRef MCSection::getVirtualSectionKind() const { return "virtual"; }
+
 void MCSection::addPendingLabel(MCSymbol *label, unsigned Subsection) {
   PendingLabels.push_back(PendingLabel(label, Subsection));
 }

lib/MC/MCSectionCOFF.cpp

@@ -111,3 +111,7 @@ bool MCSectionCOFF::UseCodeAlign() const {
 bool MCSectionCOFF::isVirtualSection() const {
   return getCharacteristics() & COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA;
 }
+
+StringRef MCSectionCOFF::getVirtualSectionKind() const {
+  return "IMAGE_SCN_CNT_UNINITIALIZED_DATA";
+}

lib/MC/MCSectionELF.cpp

@@ -196,3 +196,5 @@ bool MCSectionELF::UseCodeAlign() const {
 bool MCSectionELF::isVirtualSection() const {
   return getType() == ELF::SHT_NOBITS;
 }
+
+StringRef MCSectionELF::getVirtualSectionKind() const { return "SHT_NOBITS"; }

test/MC/COFF/bss-text.s

@@ -0,0 +1,13 @@
+# RUN: not llvm-mc -filetype=obj -triple=x86_64-pc-win32 %s -o /dev/null 2>&1 | FileCheck %s
+
+## -filetype=asm does not check the error.
+# RUN: llvm-mc -triple=x86_64-pc-win32 %s
+
+.section uninitialized,"b"
+# MCRelaxableFragment
+# CHECK: {{.*}}.s:[[#@LINE+1]]:3: error: IMAGE_SCN_CNT_UNINITIALIZED_DATA section 'uninitialized' cannot have instructions
+  jmp foo
+
+.bss
+# CHECK: {{.*}}.s:[[#@LINE+1]]:3: error: IMAGE_SCN_CNT_UNINITIALIZED_DATA section '.bss' cannot have instructions
+  addb %al,(%rax)

test/MC/ELF/ARM/bss-non-zero-value.s

@@ -1,9 +0,0 @@
-// RUN: not --crash llvm-mc -filetype=obj -triple arm-linux-gnu %s -o %t 2>%t.out
-// RUN: FileCheck --input-file=%t.out %s
-// CHECK: non-zero initializer found in section '.bss'
-	.bss
-	.globl	a
-	.align	2
-a:
-	.long	1
-	.size	a, 4

test/MC/ELF/nobits-non-zero-value.s

@@ -0,0 +1,16 @@
+# RUN: not llvm-mc -filetype=obj -triple=x86_64 %s -o /dev/null 2>&1 | FileCheck %s
+
+## -filetype=asm does not check the error.
+# RUN: llvm-mc -triple=x86_64 %s
+
+.section .tbss,"aw",@nobits
+# MCRelaxableFragment
+# CHECK: {{.*}}.s:[[#@LINE+1]]:3: error: SHT_NOBITS section '.tbss' cannot have instructions
+  jmp foo
+
+.bss
+# CHECK: {{.*}}.s:[[#@LINE+1]]:3: error: SHT_NOBITS section '.bss' cannot have instructions
+  addb %al,(%rax)
+
+# CHECK: <unknown>:0: error: SHT_NOBITS section '.bss' cannot have non-zero initializers
+  .long 1

test/MC/X86/reloc-bss.s

@@ -1,9 +0,0 @@
-# RUN: not --crash llvm-mc -filetype=obj -triple=x86_64-linux-gnu %s 2>&1 | FileCheck %s
-# CHECK: LLVM ERROR: cannot have fixups in virtual section!
-
-.section        .init_array,"awT",@nobits
-
-.hidden patatino
-.globl  patatino
-patatino:
-  movl __init_array_start, %eax