[MachO] Fix detecting malformed DWARF.

[ikudrin]

Summary

This fixes an invalid constant used to detect the reserved range when reading the compilation unit header.

See also: D64622 and D65039.

[probinson]

LGTM

[dblaikie]

Was this fixing an existing buildbot/test failure? (if not, it should probably have a test? If it did fix an existing failure - could you mention what that failure was?)

[ikudrin]

In D71546#1797665, @dblaikie wrote:

Was this fixing an existing buildbot/test failure? (if not, it should probably have a test? If it did fix an existing failure - could you mention what that failure was?)

The main issue was that some valid length values were wrongly interpreted as reserved, which come from inconsistency in the DWARF3 documentation which was fixed in DWARF4. See D64622 and http://lists.dwarfstd.org/pipermail/dwarf-discuss-dwarfstd.org/2008-June/003343.html for more details.

I have created D72695 for the test.

lld/lib/ReaderWriter/MachO/MachONormalizedFileToAtoms.cpp

readCompUnit(const NormalizedFile &normalizedFile,
              const Section &strings,
              StringRef path) {
   // FIXME: Cribbed from llvm-dwp -- should share "lightweight CU DIE
   //        inspection" code if possible.
   uint64_t offset = 0;
   llvm::dwarf::DwarfFormat Format = llvm::dwarf::DwarfFormat::DWARF32;
   auto infoData = dataExtractorFromSection(normalizedFile, info);
   uint32_t length = infoData.getU32(&offset);
-  if (length == 0xffffffff) {
+  if (length == llvm::dwarf::DW_LENGTH_DWARF64) {
     Format = llvm::dwarf::DwarfFormat::DWARF64;
     infoData.getU64(&offset);
   }
-  else if (length > 0xffffff00)
+  else if (length >= llvm::dwarf::DW_LENGTH_lo_reserved)
     return llvm::make_error<GenericError>("Malformed DWARF in " + path);
 
   uint16_t version = infoData.getU16(&offset);
 
   if (version < 2 || version > 4)
     return llvm::make_error<GenericError>("Unsupported DWARF version in " +
                                           path);