Support unknown types in looksLikeFloatCastOverflowDataV1 heuristic.

[m.ostapenko]

Summary

Hi,

trying to merge recent sanitizer library into GCC, I've run to the issue: looksLikeFloatCastOverflowDataV1 heuristic failed to work if one of the types is unknown (GCC's _Decimal32 was my case, AFAIK UBSan doesn't support decimal floating types). I see, this is a GCC - specific issue, but since we just make heuristic more precise... . I wonder if it's possible to change heuristic in a such way to support both LLVM and GCC.

Another approach would be to add new entry function (say, __ubsan_handle_float_cast_overflow_old), but I'm not sure what approach is more desirable here.

Thanks.

[filcab]

LGTM. Creating another entry function was what we were avoiding. ;-)

lib/ubsan/ubsan_handlers.cc

320

Minor nit: s/probably/most likely/

[m.ostapenko]

Thanks! Addressing Filipe's nit.

[m.ostapenko]

I'm sorry, this issue blocks my work on merging sanitizer library into GCC. Is it OK to commit? If not, could someone proceed the review please?

[filcab]

Ah, sorry. I thought you had already comitted, but with the nit fixed.
Go ahead and commit. Thank you!

lib/ubsan/ubsan_handlers.cc

 static bool looksLikeFloatCastOverflowDataV1(void *Data) {
   // First field is either a pointer to filename or a pointer to a
   // TypeDescriptor.
   u8 *FilenameOrTypeDescriptor;
   internal_memcpy(&FilenameOrTypeDescriptor, Data,
                   sizeof(FilenameOrTypeDescriptor));
 
   // Heuristic: For float_cast_overflow, the TypeKind will be either TK_Integer
-  // (0x0) or TK_Float (0x1). Adding both bytes will be 0 or 1 (for BE or LE).
-  // If it were a filename, adding two printable characters will not yield such
-  // a value.
+  // (0x0), TK_Float (0x1) or TK_Unknown (0xff). If both types are known,
+  // adding both bytes will be 0 or 1 (for BE or LE). If it were a filename,
+  // adding two printable characters will not yield such a value. Otherwise,
+  // if one of them is 0xff, this is probably TK_Unknown type descriptor.
   u16 MaybeFromTypeKind =
       FilenameOrTypeDescriptor[0] + FilenameOrTypeDescriptor[1];
-  return MaybeFromTypeKind < 2;
+  return MaybeFromTypeKind < 2 || FilenameOrTypeDescriptor[0] == 0xff ||
+         FilenameOrTypeDescriptor[1] == 0xff;
 }
 
 static void handleFloatCastOverflow(void *DataPtr, ValueHandle From,
                                     ReportOptions Opts) {
   SymbolizedStackHolder CallerLoc;
   Location Loc;
   const TypeDescriptor *FromType, *ToType;