[HWASan] Apply kTagMask on kFallbackFreeTag

[hjl.tools]

Summary

Apply kTagMask, which is 0x3F for Intel LAM, on kFallbackFreeTag (0xBC)
if not in alias mode. Apply kTagMask in alias mode will lead to

hwasan_allocator.cpp:286:38: error: static assertion failed: fallback tag must n
ot be a short granule tag.

286 |       static_assert(kFallbackFreeTag >= kShadowAlignment,

due to

constexpr unsigned kTagBits = 3;
...
constexpr uptr kTagMask = (1UL << kTagBits) - 1; 0x7
...
constexpr uptr kShadowScale = 4;
constexpr uptr kShadowAlignment = 1ULL << kShadowScale; 0x10
...
static constexpr tag_t kFallbackFreeTag = 0xBC & kTagMask; // 0x4

[morehouse]

I deliberately chose not to mask the fallback tag.

It's never valid to access freed memory anyway, so we can keep a full 8 bit tag to reduce the chance of a false negative.

[hjl.tools]

In D110998#3039753, @morehouse wrote:

I deliberately chose not to mask the fallback tag.

It's never valid to access freed memory anyway, so we can keep a full 8 bit tag to reduce the chance of a false negative.

Drop this patch then.

compiler-rt/lib/hwasan/hwasan_allocator.cpp

 namespace __hwasan {
 
 static Allocator allocator;
 static AllocatorCache fallback_allocator_cache;
 static SpinMutex fallback_mutex;
 static atomic_uint8_t hwasan_allocator_tagging_enabled;
 
 static constexpr tag_t kFallbackAllocTag = 0xBB & kTagMask;
+#if defined(HWASAN_ALIASING_MODE)
 static constexpr tag_t kFallbackFreeTag = 0xBC;
+#else
+static constexpr tag_t kFallbackFreeTag = 0xBC & kTagMask;
+#endif
 
 enum RightAlignMode {
   kRightAlignNever,
   kRightAlignSometimes,
   kRightAlignAlways
 };
 
 // Initialized in HwasanAllocatorInit, an never changed.