[hwasan] An option to disable tag randomization.

[eugenis]

Summary

Avoid flaky test failures by by using a monotonic number sequence of
heap tags.

Does not affect stack tags: the way we generate those guarantees
uniqueness for at least 30-something first allocas in any function,
as well as the UAR tag.

[kcc]

LGTM

compiler-rt/trunk/lib/hwasan/hwasan_flags.inc

 HWASAN_FLAG(bool, retag_in_realloc, true, "")
 HWASAN_FLAG(bool, print_stats, false, "")
 HWASAN_FLAG(bool, halt_on_error, true, "")
 HWASAN_FLAG(bool, atexit, false, "")
 
 // Test only flag to disable malloc/realloc/free memory tagging on startup.
 // Tagging can be reenabled with __hwasan_enable_allocator_tagging().
 HWASAN_FLAG(bool, disable_allocator_tagging, false, "")
+
+// If false, use simple increment of a thread local counter to generate new
+// tags.
+HWASAN_FLAG(bool, random_tags, true, "")

compiler-rt/trunk/lib/hwasan/hwasan_thread.cc

 HwasanThread *HwasanThread::Create(thread_callback_t start_routine,
                                void *arg) {
   uptr PageSize = GetPageSizeCached();
   uptr size = RoundUpTo(sizeof(HwasanThread), PageSize);
   HwasanThread *thread = (HwasanThread*)MmapOrDie(size, __func__);
   thread->start_routine_ = start_routine;
   thread->arg_ = arg;
   thread->destructor_iterations_ = GetPthreadDestructorIterations();
-  thread->random_state_ = RandomSeed();
+  thread->random_state_ = flags()->random_tags ? RandomSeed() : 0;
 
   return thread;
 }
 
 void HwasanThread::SetThreadStackAndTls() {
   uptr tls_size = 0;
   uptr stack_size = 0;
   GetThreadStackAndTls(IsMainThread(), &stack_bottom_, &stack_size,
static u32 xorshift(u32 state) {
   state ^= state << 5;
   return state;
 }
 
 // Generate a (pseudo-)random non-zero tag.
 tag_t HwasanThread::GenerateRandomTag() {
   tag_t tag;
   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;
+    } else {
+      tag = random_state_ = (random_state_ + 1) & 0xFF;
+    }
   } while (!tag);
   return tag;
 }
 
 } // namespace __hwasan

compiler-rt/trunk/test/hwasan/lit.cfg

 clang_hwasan_cxxflags = config.cxx_mode_flags + clang_hwasan_cflags
 
 def build_invocation(compile_flags):
   return " " + " ".join([config.clang] + compile_flags) + " "
 
 config.substitutions.append( ("%clang_hwasan ", build_invocation(clang_hwasan_cflags)) )
 config.substitutions.append( ("%clangxx_hwasan ", build_invocation(clang_hwasan_cxxflags)) )
 
-default_hwasan_opts_str = ':'.join(['disable_allocator_tagging=1'] + config.default_sanitizer_opts)
+default_hwasan_opts_str = ':'.join(['disable_allocator_tagging=1', 'random_tags=0'] + config.default_sanitizer_opts)
 if default_hwasan_opts_str:
   config.environment['HWASAN_OPTIONS'] = default_hwasan_opts_str
   default_hwasan_opts_str += ':'
 config.substitutions.append(('%env_hwasan_opts=',
                              'env HWASAN_OPTIONS=' + default_hwasan_opts_str))
 
 # Default test suffixes.
 config.suffixes = ['.c', '.cc', '.cpp']
 
 if config.host_os not in ['Linux', 'Android']:
   config.unsupported = True