[lld][mach-o] Avoid segfaulting when handling an empty section list.

[mattd]

Summary

The following patch avoids segfaulting if the section list is empty when writing a mach-o MH_OBJECT. I ran into this case from a more complicated example trying to dead_strip while using '-r' in lld.

I'm not sure if having empty sections is a legal mach-o, but it does seem that other llvm-binutils tools can ingest such a boring object with out issue. Would it be better to emit an error, emit a warning, or do nothing? It seems that adding a warning diagnostic might be helpful to users, as I did not expect to have a section-less object when the linker was done.

[ruiu]

LGTM

I don't have a strong preference over silently accepting or reporting an error on an empty YAML, so this change seems fine. Thank you for doing htis.

[mattd]

In D67735#1682203, @ruiu wrote:

LGTM

I don't have a strong preference over silently accepting or reporting an error on an empty YAML, so this change seems fine. Thank you for doing htis.

Thanks @ruiu! Would you mind accepting this patch so that I can land it, or should I wait for others to accept it?

[ruiu]

Apologies, I didn't choose "accept" in the previous comment. Doing it now. Feel free to commit (but please run all unit tests before commiting.)

lld/trunk/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp

 template <typename T>
 llvm::Error MachOFileLayout::writeSingleSegmentLoadCommand(uint8_t *&lc) {
   typename T::command* seg = reinterpret_cast<typename T::command*>(lc);
   seg->cmd = T::LC;
   seg->cmdsize = sizeof(typename T::command)
                           + _file.sections.size() * sizeof(typename T::section);
   uint8_t *next = lc + seg->cmdsize;
   memset(seg->segname, 0, 16);
+  seg->flags = 0;
   seg->vmaddr = 0;
+  seg->fileoff = _endOfLoadCommands;
+  seg->maxprot = VM_PROT_READ|VM_PROT_WRITE|VM_PROT_EXECUTE;
+  seg->initprot = VM_PROT_READ|VM_PROT_WRITE|VM_PROT_EXECUTE;
+  seg->nsects = _file.sections.size();
+  if (seg->nsects) {
     seg->vmsize = _file.sections.back().address
                 + _file.sections.back().content.size();
-  seg->fileoff = _endOfLoadCommands;
     seg->filesize = _sectInfo[&_file.sections.back()].fileOffset +
                     _file.sections.back().content.size() -
                     _sectInfo[&_file.sections.front()].fileOffset;
-  seg->maxprot = VM_PROT_READ|VM_PROT_WRITE|VM_PROT_EXECUTE;
-  seg->initprot = VM_PROT_READ|VM_PROT_WRITE|VM_PROT_EXECUTE;
-  seg->nsects = _file.sections.size();
-  seg->flags = 0;
+  }
   if (_swap)
     swapStruct(*seg);
   typename T::section *sout = reinterpret_cast<typename T::section*>
                                               (lc+sizeof(typename T::command));
   uint32_t relOffset = _startOfRelocations;
   uint32_t indirectSymRunningIndex = 0;
   for (const Section &sin : _file.sections) {
     setString16(sin.sectionName, sout->sectname);

lld/trunk/test/mach-o/empty-sections.yaml

+# RUN: ld64.lld -arch x86_64 -r %s -o %t
+#
+# Test that writing empty mach-o sections does not segfault the linker.
+#
+
+--- !mach-o
+arch:            x86_64
+file-type:       MH_OBJECT
+...