Bitstream reader: Fix undefined behavior seen after rL364464

[bjope]

Summary

After rL364464 the following tests started to fail when
running the clang-doc tests with an ubsan instrumented
build of clang-doc:

Clang Tools :: clang-doc/single-file-public.cpp
Extra Tools Unit Tests :: clang-doc/./ClangDocTests/BitcodeTest.emitEnumInfoBitcode
Extra Tools Unit Tests :: clang-doc/./ClangDocTests/BitcodeTest.emitMethodInfoBitcode
Extra Tools Unit Tests :: clang-doc/./ClangDocTests/BitcodeTest.emitRecordInfoBitcode
Extra Tools Unit Tests :: clang-doc/./ClangDocTests/SerializeTest.emitInfoWithCommentBitcode

We need to check that the read value is in range for being
casted to the llvm::bitc::FixedAbbrevIDs enum, before the
cast in ClangDocBitcodeReader::skipUntilRecordOrBlock.

SerializedDiagnosticReader::skipUntilRecordOrBlock was updated
in the same way.

[jfb]

Thanks for catching this!

clang-tools-extra/clang-doc/BitcodeReader.cpp

ClangDocBitcodeReader::skipUntilRecordOrBlock(unsigned &BlockOrRecordID) {
   while (!Stream.AtEndOfStream()) {
     Expected<unsigned> MaybeCode = Stream.ReadCode();
     if (!MaybeCode) {
       // FIXME this drops the error on the floor.
       consumeError(MaybeCode.takeError());
       return Cursor::BadBlock;
     }
 
-    // FIXME check that the enum is in range.
-    auto Code = static_cast<llvm::bitc::FixedAbbrevIDs>(MaybeCode.get());
-
-    switch (Code) {
+    unsigned Code = MaybeCode.get();
+    if (Code >= static_cast<unsigned>(llvm::bitc::FIRST_APPLICATION_ABBREV)) {
+      BlockOrRecordID = Code;
+      return Cursor::Record;
+    }
+    switch (static_cast<llvm::bitc::FixedAbbrevIDs>(Code)) {
     case llvm::bitc::ENTER_SUBBLOCK:
       if (Expected<unsigned> MaybeID = Stream.ReadSubBlockID())
         BlockOrRecordID = MaybeID.get();
       else {
         // FIXME this drops the error on the floor.
         consumeError(MaybeID.takeError());
       }
       return Cursor::BlockBegin;
     case llvm::bitc::END_BLOCK:
       if (Stream.ReadBlockEnd())
         return Cursor::BadBlock;
       return Cursor::BlockEnd;
     case llvm::bitc::DEFINE_ABBREV:
       if (llvm::Error Err = Stream.ReadAbbrevRecord()) {
         // FIXME this drops the error on the floor.
         consumeError(std::move(Err));
       }
       continue;
     case llvm::bitc::UNABBREV_RECORD:
       return Cursor::BadBlock;
-    default:
-      BlockOrRecordID = Code;
-      return Cursor::Record;
+    case llvm::bitc::FIRST_APPLICATION_ABBREV:
+      llvm_unreachable("Unexpected abbrev id.");
     }
   }
   llvm_unreachable("Premature stream end.");
 }
 
 llvm::Error ClangDocBitcodeReader::validateStream() {
   if (Stream.AtEndOfStream())
     return llvm::make_error<llvm::StringError>("Premature end of stream.\n",

clang/lib/Frontend/SerializedDiagnosticReader.cpp

SerializedDiagnosticReader::skipUntilRecordOrBlock(
 
   while (!Stream.AtEndOfStream()) {
     unsigned Code;
     if (Expected<unsigned> Res = Stream.ReadCode())
       Code = Res.get();
     else
       return llvm::errorToErrorCode(Res.takeError());
 
-    switch ((llvm::bitc::FixedAbbrevIDs)Code) {
+    if (Code >= static_cast<unsigned>(llvm::bitc::FIRST_APPLICATION_ABBREV)) {
+      // We found a record.
+      BlockOrRecordID = Code;
+      return Cursor::Record;
+    }
+    switch (static_cast<llvm::bitc::FixedAbbrevIDs>(Code)) {
     case llvm::bitc::ENTER_SUBBLOCK:
       if (Expected<unsigned> Res = Stream.ReadSubBlockID())
         BlockOrRecordID = Res.get();
       else
         return llvm::errorToErrorCode(Res.takeError());
       return Cursor::BlockBegin;
 
     case llvm::bitc::END_BLOCK:
       if (Stream.ReadBlockEnd())
         return SDError::InvalidDiagnostics;
       return Cursor::BlockEnd;
 
     case llvm::bitc::DEFINE_ABBREV:
       if (llvm::Error Err = Stream.ReadAbbrevRecord())
         return llvm::errorToErrorCode(std::move(Err));
       continue;
 
     case llvm::bitc::UNABBREV_RECORD:
       return SDError::UnsupportedConstruct;
 
-    default:
-      // We found a record.
-      BlockOrRecordID = Code;
-      return Cursor::Record;
+    case llvm::bitc::FIRST_APPLICATION_ABBREV:
+      llvm_unreachable("Unexpected abbrev id.");
     }
   }
 
   return SDError::InvalidDiagnostics;
 }
 
 std::error_code
 SerializedDiagnosticReader::readMetaBlock(llvm::BitstreamCursor &Stream) {