[PATCH] D67735: [lld][mach-o] Avoid segfaulting when handling an empty section list.

[Matt Davis via Phabricator via llvm-commits]

mattd created this revision.
mattd added reviewers: kledzik, ruiu.
mattd added a project: lld.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.
mattd retitled this revision from "[lld][mach-o] Avoid segfaulting when handling an empty segment list." to "[lld][mach-o] Avoid segfaulting when handling an empty section list.".

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.


https://reviews.llvm.org/D67735

Files:
  lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
  lld/test/mach-o/empty-sections.yaml


Index: lld/test/mach-o/empty-sections.yaml
===================================================================
--- /dev/null
+++ lld/test/mach-o/empty-sections.yaml
@@ -0,0 +1,9 @@
+# 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
+...
Index: lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
===================================================================
--- lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
+++ lld/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
@@ -626,17 +626,19 @@
                           + _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*>


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