[MachO] Add out-of-bounds check to MachOObjectFile.cpp

[sbc100]

Summary

This is a followup to rL333496.

[thegameg]

LGTM, thanks!

[JDevlieghere]

lib/Object/MachOObjectFile.cpp

110

Is the goal to ensure the caller did the check or should we make this a runtime check?

[thegameg]

lib/Object/MachOObjectFile.cpp

110

To ensure the caller does the check and didn't rely on the behavior of StringRef::substr from https://reviews.llvm.org/rL333496. I don't think it's worth making it a runtime check.

[JDevlieghere]

lib/Object/MachOObjectFile.cpp

110

Alright, I was worried this might trigger in OSS-fuzz but it sounds like an assertion is what we want here. Thanks for confirming!

lib/Object/MachOObjectFile.cpp

getSectionPtr(const MachOObjectFile &O, MachOObjectFile::LoadCommandInfo L,
   unsigned SectionSize = Is64 ? sizeof(MachO::section_64) :
                                 sizeof(MachO::section);
 
   uintptr_t SectionAddr = CommandAddr + SegmentLoadSize + Sec * SectionSize;
   return reinterpret_cast<const char*>(SectionAddr);
 }
 
 static const char *getPtr(const MachOObjectFile &O, size_t Offset) {
+  assert(Offset <= O.getData().size());
   return O.getData().data() + Offset;
 }
 
 static MachO::nlist_base
 getSymbolTableEntryBase(const MachOObjectFile &O, DataRefImpl DRI) {
   const char *P = reinterpret_cast<const char *>(DRI.p);
   return getStruct<MachO::nlist_base>(O, P);
 }