[hwasan] Don't check code model if there are no globals

[thurstond]

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

[llvmbot]

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

Author: Thurston Dang (thurstond)

Changes

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 --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};
     }
   }

[fmayer]

What makes a global real vs unreal?

[thurstond]

What makes a global real vs unreal?

  // Create a zero-length global in hwasan_globals so that the linker will
  // always create start and stop symbols.

Zero-length globals aren't real, they can't hurt you

except they can hurt because the runtime will still perform the code model check.

[fmayer]

A real good change.

nit: you still have "real" in the commit message