Index: lib/asan/asan_flags.cc
===================================================================
--- lib/asan/asan_flags.cc
+++ lib/asan/asan_flags.cc
@@ -176,13 +176,15 @@
 }
 
 void InitializeFlags(Flags *f) {
-  CommonFlags *cf = common_flags();
   SetCommonFlagsDefaults();
-  cf->detect_leaks = CAN_SANITIZE_LEAKS;
-  cf->external_symbolizer_path = GetEnv("ASAN_SYMBOLIZER_PATH");
-  cf->malloc_context_size = kDefaultMallocContextSize;
-  cf->intercept_tls_get_addr = true;
-  cf->coverage = false;
+  {
+    CommonFlags cf = *common_flags();
+    cf.detect_leaks = CAN_SANITIZE_LEAKS;
+    cf.external_symbolizer_path = GetEnv("ASAN_SYMBOLIZER_PATH");
+    cf.malloc_context_size = kDefaultMallocContextSize;
+    cf.intercept_tls_get_addr = true;
+    OverrideCommonFlags(cf);
+  }
 
   internal_memset(f, 0, sizeof(*f));
   f->quarantine_size = (ASAN_LOW_MEMORY) ? 1UL << 26 : 1UL << 28;
@@ -263,17 +265,17 @@
   }
 
   // Flag validation:
-  if (!CAN_SANITIZE_LEAKS && cf->detect_leaks) {
+  if (!CAN_SANITIZE_LEAKS && common_flags()->detect_leaks) {
     Report("%s: detect_leaks is not supported on this platform.\n",
            SanitizerToolName);
-    cf->detect_leaks = false;
+    Die();
   }
   // Make "strict_init_order" imply "check_initialization_order".
   // TODO(samsonov): Use a single runtime flag for an init-order checker.
   if (f->strict_init_order) {
     f->check_initialization_order = true;
   }
-  CHECK_LE((uptr)cf->malloc_context_size, kStackTraceMax);
+  CHECK_LE((uptr)common_flags()->malloc_context_size, kStackTraceMax);
   CHECK_LE(f->min_uar_stack_size_log, f->max_uar_stack_size_log);
   CHECK_GE(f->redzone, 16);
   CHECK_GE(f->max_redzone, f->redzone);
Index: lib/lsan/lsan_common.cc
===================================================================
--- lib/lsan/lsan_common.cc
+++ lib/lsan/lsan_common.cc
@@ -74,12 +74,13 @@
 
   // Set defaults for common flags (only in standalone mode) and parse
   // them from LSAN_OPTIONS.
-  CommonFlags *cf = common_flags();
   if (standalone) {
     SetCommonFlagsDefaults();
-    cf->external_symbolizer_path = GetEnv("LSAN_SYMBOLIZER_PATH");
-    cf->malloc_context_size = 30;
-    cf->detect_leaks = true;
+    CommonFlags cf = *common_flags();
+    cf.external_symbolizer_path = GetEnv("LSAN_SYMBOLIZER_PATH");
+    cf.malloc_context_size = 30;
+    cf.detect_leaks = true;
+    OverrideCommonFlags(cf);
   }
   ParseCommonFlagsFromString(options);
 }
Index: lib/msan/msan.cc
===================================================================
--- lib/msan/msan.cc
+++ lib/msan/msan.cc
@@ -144,14 +144,17 @@
 }
 
 static void InitializeFlags(Flags *f, const char *options) {
-  CommonFlags *cf = common_flags();
   SetCommonFlagsDefaults();
-  cf->external_symbolizer_path = GetEnv("MSAN_SYMBOLIZER_PATH");
-  cf->malloc_context_size = 20;
-  cf->handle_ioctl = true;
-  // FIXME: test and enable.
-  cf->check_printf = false;
-  cf->intercept_tls_get_addr = true;
+  {
+    CommonFlags cf = *common_flags();
+    cf.external_symbolizer_path = GetEnv("MSAN_SYMBOLIZER_PATH");
+    cf.malloc_context_size = 20;
+    cf.handle_ioctl = true;
+    // FIXME: test and enable.
+    cf.check_printf = false;
+    cf.intercept_tls_get_addr = true;
+    OverrideCommonFlags(cf);
+  }
 
   internal_memset(f, 0, sizeof(*f));
   f->poison_heap_with_zeroes = false;
Index: lib/sanitizer_common/sanitizer_flags.h
===================================================================
--- lib/sanitizer_common/sanitizer_flags.h
+++ lib/sanitizer_common/sanitizer_flags.h
@@ -71,7 +71,7 @@
 
 // Functions to get/set global CommonFlags shared by all sanitizer runtimes:
 extern CommonFlags common_flags_dont_use;
-inline CommonFlags *common_flags() {
+inline const CommonFlags *common_flags() {
   return &common_flags_dont_use;
 }
 
@@ -82,6 +82,16 @@
 inline void ParseCommonFlagsFromString(const char *str) {
   common_flags_dont_use.ParseFromString(str);
 }
+
+// This function can only be used to setup tool-specific overrides for
+// CommonFlags defaults. Generally, it should only be used right after
+// SetCommonFlagsDefaults(), but before ParseCommonFlagsFromString(), and
+// only during the flags initialization (i.e. before they are used for
+// the first time).
+inline void OverrideCommonFlags(const CommonFlags &cf) {
+  common_flags_dont_use = cf;
+}
+
 void PrintFlagDescriptions();
 
 }  // namespace __sanitizer
Index: lib/tsan/dd/dd_rtl.cc
===================================================================
--- lib/tsan/dd/dd_rtl.cc
+++ lib/tsan/dd/dd_rtl.cc
@@ -70,10 +70,13 @@
   // Default values.
   f->second_deadlock_stack = false;
 
-  CommonFlags *cf = common_flags();
   SetCommonFlagsDefaults();
-  // Override some common flags defaults.
-  cf->allow_addr2line = true;
+  {
+    // Override some common flags defaults.
+    CommonFlags cf = *common_flags();
+    cf.allow_addr2line = true;
+    OverrideCommonFlags(cf);
+  }
 
   // Override from command line.
   ParseFlag(env, &f->second_deadlock_stack, "second_deadlock_stack", "");
Index: lib/tsan/rtl/tsan_flags.cc
===================================================================
--- lib/tsan/rtl/tsan_flags.cc
+++ lib/tsan/rtl/tsan_flags.cc
@@ -93,13 +93,16 @@
   // DDFlags
   f->second_deadlock_stack = false;
 
-  CommonFlags *cf = common_flags();
   SetCommonFlagsDefaults();
-  // Override some common flags defaults.
-  cf->allow_addr2line = true;
-  cf->detect_deadlocks = true;
-  cf->print_suppressions = false;
-  cf->stack_trace_format = "    #%n %f %S %M";
+  {
+    // Override some common flags defaults.
+    CommonFlags cf = *common_flags();
+    cf.allow_addr2line = true;
+    cf.detect_deadlocks = true;
+    cf.print_suppressions = false;
+    cf.stack_trace_format = "    #%n %f %S %M";
+    OverrideCommonFlags(cf);
+  }
 
   // Let a frontend override.
   ParseFlags(f, __tsan_default_options());
@@ -115,7 +118,8 @@
     f->report_signal_unsafe = false;
   }
 
-  if (cf->help) PrintFlagDescriptions();
+  if (common_flags()->help)
+    PrintFlagDescriptions();
 
   if (f->history_size < 0 || f->history_size > 7) {
     Printf("ThreadSanitizer: incorrect value for history_size"
Index: lib/ubsan/ubsan_flags.cc
===================================================================
--- lib/ubsan/ubsan_flags.cc
+++ lib/ubsan/ubsan_flags.cc
@@ -22,9 +22,10 @@
 }
 
 void InitializeCommonFlags() {
-  CommonFlags *cf = common_flags();
   SetCommonFlagsDefaults();
-  cf->print_summary = false;
+  CommonFlags cf = *common_flags();
+  cf.print_summary = false;
+  OverrideCommonFlags(cf);
   // Override from user-specified string.
   ParseCommonFlagsFromString(MaybeCallUbsanDefaultOptions());
   // Override from environment variable.