[compiler-rt] [hwasan] Don't check code model if there are no real globals (PR #131152)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Thurston Dang (thurstond)

<details>
<summary>Changes</summary>

Currently, the code model check is always performed even if there are no real globals, because the HWASan compiler pass always leaves a 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

---
Full diff: https://github.com/llvm/llvm-project/pull/131152.diff


1 Files Affected:

- (modified) compiler-rt/lib/hwasan/hwasan_globals.cpp (+9-4) 


``````````diff
diff --git a/compiler-rt/lib/hwasan/hwasan_globals.cpp b/compiler-rt/lib/hwasan/hwasan_globals.cpp
index 7e0f3df20dd05..e2723450e26ec 100644
--- a/compiler-rt/lib/hwasan/hwasan_globals.cpp
+++ b/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 real globals.
+      if (globals_begin != globals_end)
+        CheckCodeModel(base, phdr, phnum);
+
       return {globals_begin, globals_end};
     }
   }

``````````

</details>


https://github.com/llvm/llvm-project/pull/131152