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

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.

Reviewers: kledzik, ruiu

Subscribers: llvm-commits, jrm

Tags: #lld, #llvm

Differential Revision: https://reviews.llvm.org/D67735

llvm-svn: 372995

1 files changed, 8 insertions, 6 deletions

diff --git a/lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp b/lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
index da27c7cadf9..ab7ea7e07f2 100644
--- a/lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
+++ b/lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
@@ -626,17 +626,19 @@ llvm::Error MachOFileLayout::writeSingleSegmentLoadCommand(uint8_t *&lc) {
                           + _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->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 (seg->nsects) {
+    seg->vmsize = _file.sections.back().address
+                + _file.sections.back().content.size();
+    seg->filesize = _sectInfo[&_file.sections.back()].fileOffset +
+                    _file.sections.back().content.size() -
+                    _sectInfo[&_file.sections.front()].fileOffset;
+  }
   if (_swap)
     swapStruct(*seg);
   typename T::section *sout = reinterpret_cast<typename T::section*>