[lld/mac] Don't crash when dead-stripping removes all unwind info

[thakis]

Summary

If the input has compact unwind info but all of it is removed
after dead stripping, we would crash. Now we don't write any
__unwind_info section at all, like ld64.

This is a bit awkward to implement because we only know the final
state of unwind info after UnwindInfoSectionImpl<Ptr>::finalize(),
which is called after sections are added. So add a small amount of
bookkeeping to relocateCompactUnwind() instead (which runs earlier)
so that we can predict what finalize() will do before it runs.

Fixes PR51010.

[int3]

I'm planning to add pointers from InputSections to their corresponding CompactUnwindEntries. Maybe I can make this less awkward along the way...

[thakis]

Thanks!

lld/MachO/UnwindInfoSection.h

template <class Ptr> struct CompactUnwindEntry {
   compact_unwind_encoding_t encoding;
   Ptr personality;
   Ptr lsda;
 };
 
 class UnwindInfoSection : public SyntheticSection {
 public:
   bool isNeeded() const override {
-    return !compactUnwindSection->inputs.empty();
+    return !compactUnwindSection->inputs.empty() && !allEntriesAreOmitted;
   }
   uint64_t getSize() const override { return unwindInfoSize; }
   virtual void addInput(ConcatInputSection *) = 0;
   std::vector<ConcatInputSection *> getInputs() {
     return compactUnwindSection->inputs;
   }
   void prepareRelocations();
 
 protected:
   UnwindInfoSection();
   virtual void prepareRelocations(ConcatInputSection *) = 0;
 
   ConcatOutputSection *compactUnwindSection;
   uint64_t unwindInfoSize = 0;
+  bool allEntriesAreOmitted = true;
 };
 
 UnwindInfoSection *makeUnwindInfoSection();
 
 } // namespace macho
 } // namespace lld
 
 #endif

lld/MachO/UnwindInfoSection.cpp

void UnwindInfoSectionImpl<Ptr>::prepareRelocations(ConcatInputSection *isec) {
   // FIXME: Make this skip relocations for CompactUnwindEntries that
   // point to dead-stripped functions. That might save some amount of
   // work. But since there are usually just few personality functions
   // that are referenced from many places, at least some of them likely
   // live, it wouldn't reduce number of got entries.
   for (size_t i = 0; i < isec->relocs.size(); ++i) {
     Reloc &r = isec->relocs[i];
     assert(target->hasAttr(r.type, RelocAttrBits::UNSIGNED));
+
+    if (r.offset % sizeof(CompactUnwindEntry<Ptr>) == 0) {
+      if (auto *referentIsec = r.referent.dyn_cast<InputSection *>())
+        if (!cast<ConcatInputSection>(referentIsec)->shouldOmitFromOutput())
+          allEntriesAreOmitted = false;
+      continue;
+    }
+
     if (r.offset % sizeof(CompactUnwindEntry<Ptr>) !=
         offsetof(CompactUnwindEntry<Ptr>, personality))
       continue;
 
     if (auto *s = r.referent.dyn_cast<Symbol *>()) {
       if (auto *undefined = dyn_cast<Undefined>(s)) {
         treatUndefinedSymbol(*undefined);
         // treatUndefinedSymbol() can replace s with a DylibSymbol; re-check.
template <class Ptr> void UnwindInfoSectionImpl<Ptr>::finalize() {
   cuPtrVector.erase(
       std::lower_bound(cuPtrVector.begin(), cuPtrVector.end(), &tombstone,
                        [](const CompactUnwindEntry<Ptr> *a,
                           const CompactUnwindEntry<Ptr> *b) {
                          return a->functionAddress < b->functionAddress;
                        }),
       cuPtrVector.end());
 
+  // If there are no entries left after adding explicit "no unwind info"
+  // entries and removing entries for dead-stripped functions, don't write
+  // an __unwind_info section at all.
+  assert(allEntriesAreOmitted == cuPtrVector.empty());
+  if (cuPtrVector.empty())
+    return;
+
   // Fold adjacent entries with matching encoding+personality+lsda
   // We use three iterators on the same cuPtrVector to fold in-situ:
   // (1) `foldBegin` is the first of a potential sequence of matching entries
   // (2) `foldEnd` is the first non-matching entry after `foldBegin`.
   // The semi-open interval [ foldBegin .. foldEnd ) contains a range
   // entries that can be folded into a single entry and written to ...
   // (3) `foldWrite`
   auto foldWrite = cuPtrVector.begin();
template <class Ptr> void UnwindInfoSectionImpl<Ptr>::finalize() {
   unwindInfoSize =
       level2PagesOffset + secondLevelPages.size() * SECOND_LEVEL_PAGE_BYTES;
 }
 
 // All inputs are relocated and output addresses are known, so write!
 
 template <class Ptr>
 void UnwindInfoSectionImpl<Ptr>::writeTo(uint8_t *buf) const {
+  assert(!cuPtrVector.empty() && "call only if there is unwind info");
+
   // section header
   auto *uip = reinterpret_cast<unwind_info_section_header *>(buf);
   uip->version = 1;
   uip->commonEncodingsArraySectionOffset = sizeof(unwind_info_section_header);
   uip->commonEncodingsArrayCount = commonEncodings.size();
   uip->personalityArraySectionOffset =
       uip->commonEncodingsArraySectionOffset +
       (uip->commonEncodingsArrayCount * sizeof(uint32_t));

lld/test/MachO/compact-unwind.s

 # CHECK-DAG:     function offset=0x[[#%.8x,QUUX-BASE]], encoding{{.*}}=0x00000000
 
 ## Check that we do not add rebase opcodes to the compact unwind section.
 # CHECK:      Rebase table:
 # CHECK-NEXT: segment      section        address          type
 # CHECK-NEXT: __DATA_CONST __got          0x{{[0-9A-F]*}}  pointer
 # CHECK-NOT:  __TEXT
 
+## Check that we don't create an __unwind_info section if no unwind info
+## remains after dead-stripping.
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin19.0.0 \
+# RUN:   %t/empty-after-dead-strip.s -o %t/x86_64-empty-after-dead-strip.o
+# RUN: %lld -dylib -dead_strip -arch x86_64 -lSystem \
+# RUN:   %t/x86_64-empty-after-dead-strip.o -o %t/x86_64-empty-after-strip.dylib
+# RUN: llvm-objdump --macho --unwind-info %t/x86_64-empty-after-strip.dylib | \
+# RUN:   FileCheck %s --check-prefixes=NOUNWIND --allow-empty
+# NOUNWIND-NOT: Contents of __unwind_info section:
+
 #--- my-personality.s
 .globl _my_personality, _exception0
 .text
 .p2align 2
 _foo:
   .cfi_startproc
 ## This will generate a section relocation.
   .cfi_personality 155, _my_personality
_stripped:
   .cfi_endproc
 
 
 .section __TEXT,__gcc_except_tab
 _exception1:
   .space 1
 
 .subsections_via_symbols
+
+#--- empty-after-dead-strip.s
+.text
+
+## Local symbol with unwind info.
+## The symbol is removed by -dead_strip.
+_foo :
+  .cfi_startproc
+  .cfi_def_cfa_offset 16
+  retq
+  .cfi_endproc
+
+.subsections_via_symbols