[lld-macho] If export_size is zero, export_off must be zero

[int3]

Summary

Otherwise tools like codesign_allocate will choke. We were already
handling this correctly for the other DYLD_INFO sections.

Doing this correctly is a bit subtle: we don't know if export_size will
be zero until we have run ExportSection::finalizeContents(). However,
we must still add the ExportSection to the __LINKEDIT segment in order
that it gets sorted during sortSectionsAndSegments().

[int3]

lld/test/MachO/no-unneeded-dyld-info.s

16–17

I think we missed testing this the first time around because back then we couldn't emit anything other than executables, which would always have one exported symbol (i.e. main).

[oontvoo]

LGTM

optional : need tests for the case export_off is non-zero too?

[int3]

actual fix

[int3]

optional : need tests for the case export_off is non-zero too?

I think that's sufficiently covered via our other tests. Actually I'd forgotten to run them when uploading the diff the first time around :) when I did I saw that my initial fix actually broke most of them... but that's addressed now.

lld/MachO/SyntheticSections.h

 };
 
 // Stores a trie that describes the set of exported symbols.
 class ExportSection final : public LinkEditSection {
 public:
   ExportSection();
   void finalizeContents() override;
   uint64_t getRawSize() const override { return size; }
+  bool isNeeded() const override { return size; }
   void writeTo(uint8_t *buf) const override;
 
   bool hasWeakSymbol = false;
 
 private:
   TrieBuilder trieBuilder;
   size_t size = 0;
 };

lld/MachO/Writer.cpp

for (const auto &it : concatOutputSections) {
     ConcatOutputSection *osec = it.second;
     assert(segname != segment_names::ld);
     if (osec->isNeeded())
       getOrCreateOutputSegment(segname)->addOutputSection(osec);
   }
 
   for (SyntheticSection *ssec : syntheticSections) {
     auto it = concatOutputSections.find({ssec->segname, ssec->name});
-    if (ssec->isNeeded()) {
+    // We add all LinkEdit sections here because we don't know if they are
+    // needed until their finalizeContents() methods get called later. While
+    // this means that we add some redundant sections to __LINKEDIT, there is
+    // is no redundancy in the output, as we do not emit section headers for
+    // any LinkEdit sections.
+    if (ssec->isNeeded() || ssec->segname == segment_names::linkEdit) {
       if (it == concatOutputSections.end()) {
         getOrCreateOutputSegment(ssec->segname)->addOutputSection(ssec);
       } else {
         fatal("section from " +
               toString(it->second->firstSection()->getFile()) +
               " conflicts with synthetic section " + ssec->segname + "," +
               ssec->name);
       }

lld/test/MachO/no-unneeded-dyld-info.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
-# RUN: %lld -o %t %t.o
+# RUN: %lld -dylib -o %t %t.o
 # RUN: llvm-objdump --macho --all-headers %t | FileCheck %s
 
 # CHECK:                 cmd LC_DYLD_INFO_ONLY
 # CHECK-NEXT:        cmdsize 48
 # CHECK-NEXT:     rebase_off 0
 # CHECK-NEXT:    rebase_size 0
 # CHECK-NEXT:       bind_off 0
 # CHECK-NEXT:      bind_size 0
 # CHECK-NEXT:  weak_bind_off 0
 # CHECK-NEXT: weak_bind_size 0
 # CHECK-NEXT:  lazy_bind_off 0
 # CHECK-NEXT: lazy_bind_size 0
+# CHECK-NEXT: export_off     0
+# CHECK-NEXT: export_size    0
 
-.globl _main
-_main:
-  ret
+_not_exported:
+  .space 1