[hwasan] Don't check code model if there are no globals (#131152)

Currently, the code model check is always performed even if there are no
globals, because:
1) the HWASan compiler pass always leaves a note
2) the HWASan runtime always performs the check if there is a HWASan
globals note.
This unnecessarily adds a 2**32 byte size limit.

This patch elides the check if the globals note doesn't actually contain
globals, thus allowing larger libraries to be successfully instrumented
without globals.

Sent from my iPhone

Author: thurstond

compiler-rt/lib/hwasan/hwasan_globals.cpp

@@ -73,16 +73,21 @@ ArrayRef<const hwasan_global> HwasanGlobalsFor(ElfW(Addr) base,
         continue;
       }
 
-      // Only libraries with instrumented globals need to be checked against the
-      // code model since they use relocations that aren't checked at link time.
-      CheckCodeModel(base, phdr, phnum);
-
       auto *global_note = reinterpret_cast<const hwasan_global_note *>(desc);
       auto *globals_begin = reinterpret_cast<const hwasan_global *>(
           note + global_note->begin_relptr);
       auto *globals_end = reinterpret_cast<const hwasan_global *>(
           note + global_note->end_relptr);
 
+      // Only libraries with instrumented globals need to be checked against the
+      // code model since they use relocations that aren't checked at link time.
+      //
+      // There is always a HWASan globals note ("Create the note even if we
+      // aren't instrumenting globals." - HWAddressSanitizer.cpp), but we can
+      // elide the code model check if there are no globals.
+      if (globals_begin != globals_end)
+        CheckCodeModel(base, phdr, phnum);
+
       return {globals_begin, globals_end};
     }
   }