[compiler-rt] Poison last word of FakeFrame (PR #133689)

[Vitaly Buka via llvm-commits]

================
@@ -231,6 +232,13 @@ static ALWAYS_INLINE uptr OnMalloc(uptr class_id, uptr size) {
     return 0;  // Out of fake stack.
   uptr ptr = reinterpret_cast<uptr>(ff);
   SetShadow(ptr, size, class_id, 0);
+
+  // Poison everything beyond user size, use kNumberOfSizeClasses to prevent
+  // SetShadow from inlining PoisonShadow
+  SetShadow(reinterpret_cast<uptr>(ptr + size),
----------------
vitalybuka wrote:

```
ALWAYS_INLINE void SetShadow(uptr ptr, uptr size, uptr class_id, u64 magic) {
  u64 *shadow = reinterpret_cast<u64*>(MemToShadow(ptr));
  if (ASAN_SHADOW_SCALE == 3 && class_id <= 6) {
    // This code expects ASAN_SHADOW_SCALE=3.
    for (uptr i = 0; i < (((uptr)1) << class_id); i++) {
      shadow[i] = magic;
      // Make sure this does not become memset.
      SanitizerBreakOptimization(nullptr);
    }
  } else {
    // The size class is too big, it's cheaper to poison only size bytes.
    PoisonShadow(ptr, size, static_cast<u8>(magic));
  }

  if (magic != 0) {
    PoisonShadow(ptr + size , ???, kAsanStackRightRedzoneMagic);
  }
}
```

https://github.com/llvm/llvm-project/pull/133689