[objects] Take into account the LSB bit in purecap ABI in Code::GetOffsetFromInstructionStart

When the PCC is sealed in purecap ABI, the LSB bit is set, leading
to an off-by-one error in the computation of the offset from the
instruction start to the current PCC address. This can cause errors
such as in OptimizedFrame::LookupExceptionHandlerInTable, where the
computed offset is used to look up an exception handler table.

Author: RoundofThree

src/objects/code-inl.h

@@ -171,7 +171,14 @@ Address Code::InstructionEnd(Isolate* isolate, Address pc) const {
 }
 
 int Code::GetOffsetFromInstructionStart(Isolate* isolate, Address pc) const {
+#if defined(__CHERI_PURE_CAPABILITY__) && defined(__aarch64__)
+  // NOTE(zyj20): Take into account that the LSB of sealed PCC is set.
+  // The resulting capability could be invalid but it won't be
+  // used as a pointer anyways.
+  const Address offset = (pc & ~1) - InstructionStart(isolate, pc);
+#else
   const Address offset = pc - InstructionStart(isolate, pc);
+#endif
   DCHECK_LE(offset, instruction_size());
   return static_cast<int>(offset);
 }