[HWASan][NFC] Introduce constants for tag bits and masks.

[morehouse]

Summary

x86_64 aliasing mode will use fewer than 8 bits for tags, so refactor
existing code to remove hard-coded 0xff and 8 values.

Also introduces kAddressUntagMask, which in aliasing mode will only
untag the alias bits and not fixed bits in the tag.

[vitalybuka]

compiler-rt/lib/hwasan/hwasan.h

43

should this be kTagMask for consistency?

[vitalybuka]

compiler-rt/lib/hwasan/hwasan.h

40–47

If we keep this as constexpr, we likely will not be able to control these at runtime.
As-is it does not matter, but we may have to move it into flags later.

[morehouse]

• Add clarifying comments.

compiler-rt/lib/hwasan/hwasan.h

40–47

Yes. For now I prefer to keep it simple. We can make it a flag in a later change.

43

I think it's best to keep the current naming. Added some comments for clarity.

[vitalybuka]

compiler-rt/lib/hwasan/hwasan.h

47

Sorry, I asked about the following

[morehouse]

compiler-rt/lib/hwasan/hwasan.h

47

That works for the current implementation, but it won't work when we introduce x86 aliasing mode.

Then we'll still want to extract all 8 bits when checking the tag in memory, but we only want to mask out 3 bits when untagging the pointer. This is why I left the 0xFF here.

[eugenis]

LGTM

[morehouse]

• Remove kAddressUntagMask. It's unneeded with short tags.
• Add num_bits param to GenerateRandomTag() for use in aliasing mode free().

[morehouse]

@eugenis @vitalybuka I've made some changes to this patch to accommodate changes in the following patches. Please take another look.

[vitalybuka]

LGTM

[eugenis]

LGTM

compiler-rt/lib/hwasan/hwasan.h

 #ifndef HWASAN_REPLACE_OPERATORS_NEW_AND_DELETE
 #define HWASAN_REPLACE_OPERATORS_NEW_AND_DELETE HWASAN_WITH_INTERCEPTORS
 #endif
 
 typedef u8 tag_t;
 
 // TBI (Top Byte Ignore) feature of AArch64: bits [63:56] are ignored in address
 // translation and can be used to store a tag.
-const unsigned kAddressTagShift = 56;
-const uptr kAddressTagMask = 0xFFUL << kAddressTagShift;
+constexpr unsigned kAddressTagShift = 56;
+constexpr unsigned kTagBits = 8;
+
+// Mask for extracting tag bits from the lower 8 bits.
+constexpr uptr kTagMask = (1UL << kTagBits) - 1;
+
+// Masks for extracting and removing tags from full pointers.
+constexpr uptr kAddressTagMask = kTagMask << kAddressTagShift;
 
 // Minimal alignment of the shadow base address. Determines the space available
 // for threads and stack histories. This is an ABI constant.
 const unsigned kShadowBaseAlignment = 32;
 
 const unsigned kRecordAddrBaseTagShift = 3;
 const unsigned kRecordFPShift = 48;
 const unsigned kRecordFPLShift = 4;
 const unsigned kRecordFPModulus = 1 << (64 - kRecordFPShift + kRecordFPLShift);
 
 static inline tag_t GetTagFromPointer(uptr p) {
-  return p >> kAddressTagShift;
+  return (p >> kAddressTagShift) & kTagMask;
 }
 
 static inline uptr UntagAddr(uptr tagged_addr) {
   return tagged_addr & ~kAddressTagMask;
 }
 
 static inline void *UntagPtr(const void *tagged_ptr) {
   return reinterpret_cast<void *>(

compiler-rt/lib/hwasan/hwasan_thread.h

public:
   bool AddrIsInStack(uptr addr) {
     return addr >= stack_bottom_ && addr < stack_top_;
   }
 
   AllocatorCache *allocator_cache() { return &allocator_cache_; }
   HeapAllocationsRingBuffer *heap_allocations() { return heap_allocations_; }
   StackAllocationsRingBuffer *stack_allocations() { return stack_allocations_; }
 
-  tag_t GenerateRandomTag();
+  tag_t GenerateRandomTag(uptr num_bits = kTagBits);
 
   void DisableTagging() { tagging_disabled_++; }
   void EnableTagging() { tagging_disabled_--; }
 
   u64 unique_id() const { return unique_id_; }
   void Announce() {
     if (announced_) return;
     announced_ = true;

compiler-rt/lib/hwasan/hwasan_thread.cpp

 static u32 xorshift(u32 state) {
   state ^= state << 13;
   state ^= state >> 17;
   state ^= state << 5;
   return state;
 }
 
 // Generate a (pseudo-)random non-zero tag.
-tag_t Thread::GenerateRandomTag() {
+tag_t Thread::GenerateRandomTag(uptr num_bits) {
+  DCHECK_GT(num_bits, 0);
   if (tagging_disabled_) return 0;
   tag_t tag;
+  const uptr tag_mask = (1ULL << num_bits) - 1;
   do {
     if (flags()->random_tags) {
       if (!random_buffer_)
         random_buffer_ = random_state_ = xorshift(random_state_);
       CHECK(random_buffer_);
-      tag = random_buffer_ & 0xFF;
-      random_buffer_ >>= 8;
+      tag = random_buffer_ & tag_mask;
+      random_buffer_ >>= num_bits;
     } else {
-      tag = random_state_ = (random_state_ + 1) & 0xFF;
+      random_state_ += 1;
+      tag = random_state_ & tag_mask;
     }
   } while (!tag);
   return tag;
 }
 
 } // namespace __hwasan