[Coverage] Ensure that coverage mapping data has an expected alignment in 'covmapping' files.

[ikudrin]

Summary

Coverage mapping data is organized in a sequence of blocks, each of which is expected
to be aligned by 8 bytes. This feature is used when reading those blocks, see
VersionedCovMapFuncRecordReader::readFunctionRecords(). If a misaligned covearge
mapping data has more than one block, it causes llvm-cov to fail.

[vsk]

This lgtm with a nit.

lib/ProfileData/Coverage/CoverageMappingReader.cpp

496 ↗

(On Diff #57341)

Could you use the truncated error for this (to match line 475)?

llvm/trunk/lib/ProfileData/Coverage/CoverageMappingReader.cpp

uint64_t Address =
       decodeULEB128(reinterpret_cast<const uint8_t *>(Data.data()), &N);
   if (N > Data.size())
     return coveragemap_error::malformed;
   Data = Data.substr(N);
   if (Data.size() < ProfileNamesSize)
     return coveragemap_error::malformed;
   ProfileNames.create(Data.substr(0, ProfileNamesSize), Address);
   CoverageMapping = Data.substr(ProfileNamesSize);
+  // Skip the padding bytes because coverage map data has an alignment of 8.
+  if (CoverageMapping.size() < 1)
+    return coveragemap_error::truncated;
+  size_t Pad = alignmentAdjustment(CoverageMapping.data(), 8);
+  if (CoverageMapping.size() < Pad)
+    return coveragemap_error::malformed;
+  CoverageMapping = CoverageMapping.substr(Pad);
   return std::error_code();
 }
 
 static ErrorOr<SectionRef> lookupSection(ObjectFile &OF, StringRef Name) {
   StringRef FoundName;
   for (const auto &Section : OF.sections()) {
     if (auto EC = Section.getName(FoundName))
       return EC;

llvm/trunk/tools/llvm-cov/TestingSupport.cpp

if (auto Err =
     errs() << "error: " << Err.message() << "\n";
     return 1;
   }
 
   raw_fd_ostream OS(FD, true);
   OS << "llvmcovmtestdata";
   encodeULEB128(ProfileNamesData.size(), OS);
   encodeULEB128(ProfileNamesAddress, OS);
-  OS << ProfileNamesData << CoverageMappingData;
+  OS << ProfileNamesData;
+  // Coverage mapping data is expected to have an alignment of 8.
+  for (unsigned Pad = OffsetToAlignment(OS.tell(), 8); Pad; --Pad)
+    OS.write(uint8_t(0));
+  OS << CoverageMappingData;
 
   return 0;
 }